Anatomy

| DEFINING COMPARATIVE ANATOMYThe title of this course might be more adequately be called "Comparative Vertebrate Functional Anatomy". Why emphasis on function' Integration of form and function is a fundamental necessity in studying an organism. We need to realize that anatomy of an animal is a certain way because an organism needs to function. The most successful structures are those that are going to be able to persist in the population; less successful structures will die out with their owners. Example - Activity of wing muscles of the bird and the bat. Dissection of these animals can give an idea of what the muscular and skeletal structures look like, but you still need to correspond the structures to what actually goes on during flight. What we will see this semester are variations on themes - all organisms must be able to perform certain functions to survive. They all must perform locomotion (or not, in the case of sessile organisms), respiration, circulation, excretion, digestion, and reproduction. The diversity of life that we see is a product of animals finding different ways of performing these functions. A BRIEF HISTORY OF ANATOMYAnatomy comes from the Greek word that means "to cut up" or "dissect" Much of the early work in anatomy was based mostly on descriptions of organ systems, muscle systems, and usually was conducted on domestic animals such as livestock and chickens. Aristotle - 4th century BC Made detailed observations of animal anatomy. For example, he described watching the heart of a chick develop from the liquid mass of an egg. Also established groupings of animals based on their structural form, which is now known as the field of taxonomyGalen - 2nd century AD • Greek physician known for his comparative study of animals • Carried out experiments to understand kidney function, movement of blood through arteries. • Book on Anatomical Preparations was accepted for nearly 1400 years as the Western World’s most authoritative reference on medical anatomy • Downside to Galen’s work - he had little concern for pain and suffering of his animal subjects and often dissected and examined animals while they were still alive.Very few advancements in anatomical study during the Middle Ages, primarily because advancements in biological thought were also relatively infrequent during this time. After the Middle Ages, work moved to include the study of functional anatomy, or the study of how structures within organisms, such as cells, tissues, organs, organ systems, and other complex functional units, perform specific functions. Leonardo da Vinci (15th century AD) • His studies of anatomy, design and mechanics are well-known, as are his sketches and work on the anatomy of flight.William Harvey (17th century) • Studied circulation of blood in the body, including the function of valves in the veins from the limbs.Giovanni Borelli (17th century) • Applied the concept of levers (originally studied by Archimedes) to the locomotion of animalsCarl von Linne (Carolus Linnaeus; 1707 - 1778) • Devised the binomial system for naming plants and animal which forms the basis of modern taxonomy • Philosophically argued that species were unchangeable, created originally as we find them today (based on creation as described in Genesis)Jean-Baptiste de Lamarck (19th century) • Philosophie Zoologique (1809) spoke to three issues of evolution: 1) fact: species change through time - the simplest arise through spontaneous generation from inanimate matter but thereafter evolved onward and upward into higher forms 2) course: progressive changes in species along an ascending scale, from the lowest/simplest to most complex/"perfect" (humans) 3) mechanism: need itself produces heritable evolutionary changes - when environments or behaviors changed, an animal developed new needs to meet the demands the environment placed on it* Summarized as: "Evolution by means of the inheritance of acquired characteristics"Georges Cuvier (19th century) • Compared organs of various vertebrates and studied functional relationships among the organs • Extremely knowledgeable in the skeletal structure of animals, and could infer the shapes of bones that would connect to neighboring bones.Karl Ernst von Baer, Ernst Haeckel • Both studied comparative developmental anatomy • von Baer noted that all early vertebrate embryos look "fishlike" and diverge anatomically as development proceeds • Haeckel proposed the "biogenetic law", or "ontogeny recapitulates phylogeny", which implied that during the embryonic stages (ontogeny) all higher animals progressively change morphologically and resemble the evolutionary stages that preceded (phylogeny). The theory has since been re-evaluated and given less emphasis in comparative studies.Charles Darwin (19th century) • In his books On the Origin of Species (1859) and The Descent of Man (1871) he helped to establish the evolutionary basis of our modern synthesis of comparative, functional and adaptive morphology and anatomy • Proposed three conditions for and mechanisms of evolutionary change: 1) If left unchecked, members of any species will increase naturally in number because all species posess a high reproductive potential. 2) Competition for resources. 3) Survival of the few - natural selection - nature weeding out the less fit. Superior adaptations would, on average, fare better and survive to pass on their successful adaptations.Alfred Wallace (19th century) • Idependently developed the concept of "survival of the fittest" from the observation that the human population increases faster than food to correspond with Darwin’s "survival of the few"Comparative anatomy as an interdisciplinary field: As with many fields, comparative anatomy must be interdisciplinary to incorporate the full range of factors influencing animal morphology. We now find people from all fields involved in the study, including the following fields: Zoology - study of animals Physiology - study of function Histology - study of cell and tissue structure Genetics - study of our genetic blueprint and its effects Ecology - study of the relationship between organisms and their environment Developmental Biology - study of the ontogeny of individuals from fertilization to parturition Evolutionary biology - study of natural selection and adaptation of organisms to their environment Phylogeny - comparative study of evolutionary relationships between organismsResearch focuses on a seemingly simple question - evolution of animal body plans, and how long ago different body plans diverged. Still, two centuries of comparative anatomy have not yielded a consensus about the separation of different phyla. Although we know that a swordfish, a fly, and Marilyn Monroe all have different body plans (representative of their phyla), we still do not know how the characteristics unique to each arose. Several different fields have been involved in this study to resolve the phylogenetic tree: Paleontologists - look at the fossil record to come up with possible relationships based on morphology Paleoecologists - try to imagine how each organism may have functioned in the environment, either as a filter feeder, predator or grazer Developmental biologists, molecular biologists and geneticists - have looked at homeobox genes (which regulate the expression of other genes and determine the features characteristic of each body segment) to see how they are different, and may give some indication of where each body plan diverged.Without the cooperative efforts between these diverse fields, the exact relationships between the different body plans might be more speculative. This example should give you some idea of the different directions that the field of comparative anatomy can take you. Using comparative anatomy in the real world - medicine and beyond At this point, you may begin thinking about how you will use this course in the future, to get you thinking about how the concepts can be applied - most limit their thinking of the medical applications of this class! As a small sample of what you can potentially do: Medicine - the techniques you learn in this class, including memorization and integration of concepts, as well as the terminology, will certainly help in a medical career. Athletics and physical therapy - requires knowing how bones and muscles interact as lever systems for maximum benefit of training exercises as well as to minimize possibility of stress or injury to body tissues. Physical anthropology - understanding how stature, body shape and limb proportions relate to environment, and apply to human origin, distribution and ecology. Animal behavior - how anatomical structures are used in behavior. Examples: horns and antlers used in combat displays are related to acquiring mating opportunities. Evolutionary biology - anatomy helps us to understand organismal phylogenetic relationships Forensic anthropology - analysis of physical remains of humans to determine identity and circumstances of death Biological and medical illustrationAny other areas'MORPHOLOGICAL CONCEPTSTo analyze design, concepts of form, function, and evolution have developed which address similarity, symmetry, and segmentation. Similarities - corresponding parts may be considered similar to each other by: • Homology - two or more features that share a common ancestry: bird’s wing and mole’s arm may be traced back to common ancestral reptile - serial homology - special case with similarities between successively repreated elements in the same organism: vertebral collumn, muscle segments• Analogy - features with a similar function: wings of bats and bees similar in function but of different ancestral structural origin • Homoplasy - features that simply look alike; may or may not be homologous or analogous: turtle and dolphin flippers; insect wings which look like leaves but cannot photosynthesizeSymmetry - how the body meets the surrounding environment: • radial symmetry - the body is laid out equally from a central axis; any of several planes passing through the center divids the animal into equal halves • bilateral symmetry - only the midsagittal section divides the body into two equal halves • body regions are described by basic terms of: - anterior = head end (cranial/superior) - posterior = tail (caudal/inferior) - dorsal = back - ventral = front - the midline is medial; the sides lateral - attached appendages have a distal (farther away) and proximal (closer) portion - the pectoral region or chest supports the forelimbs - the pelvis region refers to the hips which support the hindlimbs - a frontal plane divides the body into dorsal and ventral sections, sagittal plane into left and right, and transverse plane into anterior and posterior portionsSegmentation - a body built of repeated or duplicated segments (metameres) separated by a series of septa.In addition to these three, Cephalization is the pronounced tendency for the anterior end of the body to become more and more distinctly separated and differentiated from the rest of the body as a head. During cephalization, the brain and sense organs become centralized at the head, and there forms a greater elaboration of the feeding apparatus, which includes jaws, musculature, teeth, beaks, tongues and glands. EVOLUTIONARY MORPHOLOGYEvolution and morphology have not always been happy companions - cooperation between disciplines has led to concepts of design and change in design. The concept of function covers both how a part works and how it serves adaptively in the environment - cheek muscles of a mouse function both within an organism (chewing) and by meeting environmental demands (resource processing), which are defined by: • function: the action or property of a part as it works in an organism • biological role: how the part is used in the environment during the course of the organism’s life historyPreadaptation: a structure or behavior posesses the necessary form and function before the biological role arises that it eventually serves - feathers in birds probably served as insulation to conserve body heat prior to development of flight (thermoregulation now a secondary function) Evolutionary change involves continuous renovations - old parts are altered but new parts rarely addedComparisons among characters require careful use of terminology defining relationships - traits may be Primitive/Generalized/Derived/Specialized An important distinction to make is among the terms. They are not necessarily interchangeable, and should be used carefully when describing morphology. Primitive and derived are antonyms - • Primitive - structures that are similar to that of the ancestors or shared by all living groups • Derived - structures that are different from that of the ancestorsas are generalized and specialized • Generalized - modified to perform a variety of functions • Specialized - modified to perform restricted functionsAs an example: In mammals, the pentadactyl (five phalanges) condition is primitive, in that it is found in all living groups. However, there is a derived condition in some mammals, such as the bat wing, in which the first digit is elongated, or in the horse foot, which is reduced completely to a single digit. In contrast, our anterior phalanges (fingers) are generalized, in that they can perform a number of different functions, from playing the piano to carving a sculpture. However, our posterior phalanges (toes) are specialized, and can usually only perform the function of balance and walking. PHYLOGENYThe course of evolution (phylogeny) is often summarized in dendrograms (schematic diagrams) that depict treelike branched connections between groups Phylogenies serve as a graphical representation of the evolutionary relationships of organisms. They may show: • which organisms branched off first from a common ancestor; i.e. the major stages of evolution of fishes (Fig. 3.6 and 3.13 in text) • may also give information on the relative abundance of these taxa; i.e. Fig. 1.24 in text.Each branch in the dichotomous branching pattern signifies a point at which two taxa diverge based on some morphological or other character trait. All extant species usually listed in a line at the top. Extinct species’ lines do not meet up with those of extant species. PALEONTOLOGYVertebrate evolution was once referred to as the "Vertebrate Story" by paleontologist Alfred Romer - unfolds across 590 million years with roughly 99.9% of all species which ever to have evolved now extinct All that survives are their remnants, the fossils and scetchy vignettes they tell of the structure and early history of vertebrates Fossil remnants may include bones, teeth, eggs, small boney elements (embryos, diet'), feces, DNA traces - fossil dating, restoration, and reconstruction lead to an improved understanding of the past CHARACTERISTICS OF CHORDATESFour distinctive derived characteristics of chordates distinguish them from their ancestors: A. Notochord, or a rod of vacuolated cells, encased by a firm sheath that lies ventral to the neural tube in vertebrate embryos and some adults. B. Hollow nerve cord that lies dorsal to the notochord C. Pharyngeal pouches D. Endostyle - elongated groove in the pharynx floor of protochordates that may develop as the thyroid gland in chordatesIn the subphylum Vertebrata, all members possess the four chordate characteristics at some time in development, but often these structures are altered significantly in adult animals. These four characteristics may be found in some of the ancestors of chordates and are commonly placed in an informal grouping called Protochordates. These serve as living representations of the missing fossils in vertebrate evolution. PROTOCHORDATESPhylum Hemichordata - acorn worms and pterobranchs Hemichordates are a group of organisms that show an affinity to the chordates, but are lacking some key characteristics of chordates. They include two groups • Enteropneusta (acorn worms). These are 2 cm to 1.5 m long; marine in shallow waters, solitary, live in mud or vegetation; filter-feeders. They have well-developed gill slits, and a stomochord, at one time thought to be homologous with the notochord. They also have a dorsal strand of nerve cells, believed to be the precursor to the dorsal hollow nerve cord. Example genera: Saccoglossus, Dolichoglossus. • Pterobranchia (pterobranchs). These are tiny, deep-sea, colonial, moss-like animals. There is no trace of dorsal nerve cord or notochord, and only one pair of gill slits in species of the genus Cephalodiscus.Balanoglossus (Fig. 2.9 in text) has some characteristics in common with chordates, such as gill slits and a dorsal nerve cord; however, this species also has a ventral nerve cord, and the nerve cords in general are not hollow like most chordates, but instead are solid. This particular species has a worldwide distribution, lives in shallow sea water, and can range between a few centimeters to up to two meters (6' 6" !!). In addition, this species also lacks a notochord. It does have a structure called the stomochord , or a diverticulum (blind sac) that is made up of cells that resemble those found in the notochord, but has a different developmental pathway. So, it should be clear why these species are called Hemichordates, and are not included with the true chordates. Subphylum Urochordata (Phylum Chordata) - tunicates/sea squirts Urochordates are all marine, and are enclosed in a tough cellulose-like tunic (hence the common name tunicate Tunicates are members of the true chordates, and represent some of the most primitive ancestors of the Subphylum Vertebrata (see Fig, 2.14 - 2.18 in text). Most of the 2000 species belong to the taxon Ascidiacea (sea squirts). This group undergoes complete metamorphosis from a mobile larva to a sessile adult, resorbing the tail and notochord. Some are solitary, most are colonial. The few remaining species of tunicates belong to the taxa Thaliacea and Appendicularia (larvaceans). Thaliaceans lack a tail and notochord; they have no known larval stage. They are small, free-swimming, pelagic barrel-shaped animals that use jet propulsion. Appendicularians do not metamorphose, and are able to reproduce as free-swimming larvae. Tunicates differ strongly in appearance between the adult and larva. • The larval form possesses more of the chordate characteristics than the adult form (see figure). Adult is sessile (and sometimes colonial), and must obtain food by siphoning sea water through its body and trapping food particles in the endostyle. • Larvae are tadpole-like and free-living, and have an endostyle, gill slits, dorsal nerve cord, and notochord. The larval stage lasts only a few days, and ends when the larva attaches to a substrate and metamorphoses into an adult.***This example should illustrate to you that although chordates are said to have four basic things in common, these characteristics need not be retained throughout life for an animal to be considered a chordate. Rather, they must only be present in an individual at some time during development.*** Subphylum Cephalochordata (Phylum Chordata) - Amphioxus The last group are the cephalochordates, which are usually represented by one organism - Branchiostoma lanceolatum , commonly called Amphioxus (which means "sharp at both ends"). See Fig. 2.22 - 2.24 in text. Amphioxus are 2-3 inches in length, and live on seashores throughout the temperate zone. Fish-like in appearance, it has a laterally compressed dorsal fin, but it does not have complete organs, or any bony structures. Amphioxus shows some cephalization, in that the primary feeding structures are concentrated at the anterior end, and it has a pigment spot on the anterior end that may be used for orienting toward light.  Origin of Free-Swimming Vertebrates In contrast to protochordates (hemichordates, urochordates, and cephalochordates), vertebrates are actively-feeding, predatory organisms that move by lateral undulation of an elongate body. • Cephalochordates are like vertebrates in having the derived feature of an elongate body as adults, but are still (primitively) filter feeders; that is, they feed while motionless, moving food-laden water by means of cilia on their gill bars. • Hemichordates and most urochordates are also filter-feeders, moving water through their gill slits, but are sessile as adults. When ascidian tunicates metamorphose, the notochord is resorbed. • Note, however, that ascidian and larvacean urochordates have a free-swimming larval stage (with a notochord); ascidians metamorphose to sessile adults, but larvaceans become sexually mature as mobile "larvae."These observations have led workers to suggest that the freely-swimming mode of locomotion of vertebrates (and cephalochordates) evolved by retaining the form of the larvae of the "ancestors" (hemichordates and urochordates) as the form of the adults of the descendants (cephalochordates and vertebrates). This general phenomenon is called paedomorphosis: the evolutionary retention of larval features of the ancestors as the adult features of the descendants.Diversity of VertebratesThis chapter will be a veritable "parade of taxa", as we start tracing the evolution of vertebrates and the derived characters that distinguish them from the chordates that we discussed in the last lecture. General characteristics of vertebrates Vertebrates may be characterized by 12 general derived characteristics. You should become very familiar with these traits, and identify how they are expressed in the vertebrates you will see in lab. 1. Bilateral symmetry 2. Two pairs of jointed locomotor appendages, which can include fins (pectoral and anal/dorsal fins, as well as the forelimbs and hindlimbs). 3. Outer covering of protective cellular skin, which can be modified into special structures such as scales, hair and feathers 4. Metamerism found in skeletal, muscular and nervous system. This was described in a previous lecture - structures can include ribs, vertebrae, muscles and ganglia/peripheral nerves. 5. Well-developed coelom, or body cavity completely lined with epithelium (cellular tissue), that may be divided into 2 to 4 compartments. 6. Well-developed internal skeleton of cartilage and bone, separated into axial skeleton (skull, vertebrae, ribs, sternum) and appendicular skeleton (girdles and appendages). 7. Highly developed brain enclosed by skull, and nerve cord enclosed by vertebrae. This provides advanced neural structures that are highly protected from damage. 8. Well-developed sense organs (eyes, ears, nostrils) located on the head (cephalization). 9. Respiratory system, including either gills or lungs, and located closely to the pharynx or throat. 10. Closed circulatory system with ventral heart and median dorsal artery. 11. Genital and excretory systems closely related, utilizing common ducts and pathways. 12. Digestive tracts with two major digestive glands (liver and pancreas) that secrete into it.Grouping the vertebrate classes Within the vertebrate classes, there are further ways of subdividing the groups based on derived characteristics. There are eight recognized extant classes of vertebrates(Figure 3.1, p. 81 in text): Myxini - hagfishes Cephalaspidomorpha - lampreys Chondrichthyes - cartilagenous fishes Osteichthyes - bony fishes Amphibia - frogs, toads, salamanders, and caecilians Reptilia - turtles, snakes, lizards, crocodilians Aves - birds Mammalia - mammalsThese can be grouped based on their general habitat requirements: Pisces - collective term for all fishes; includes Myxini, Cephalaspidomorpha, Chondrichthyes, Osteichthyes Tetrapoda - collective term for the terrestrial vertebrates; they have four feet unless some have been secondarily lost or converted to other uses. Includes Amphibia, Reptilia, Aves, MammaliaOr based on their feeding habits: Agnatha - jawless vertebrates, including Myxini and Cephalaspidomorpha Gnathostomes - vertebrates with jaws derived from the mandibular arch, which may have (in primitive vertebrates) supported gills. Includes Chondrichthyes, Osteichthyes, Amphibia, Reptilia, Aves, MammaliaOr based on their embryonic characteristics Anamniotes - vertebrates that lack an amnion, or extraembryonic membrane that surrounds the embryo and encases it in amniotic fluid. Includes Myxini, Cephalaspidomorpha, Chondrichthyes, Osteichthyes, Amphibia. Amniotes - vertebrates that possess an amnion. Includes Reptilia, Aves, Mammalia.Don't let these different terms confuse you! They are all ways of distinguishing taxa based on primitive versus derived traits. Use them to help you memorize and classify the taxa we will be talking about. Tracing vertebrate evolution through the fossil record Keep in mind that the evolutionary relationships among the different taxa that we are discussing have been determined from the fossil record. Vertebrates, among all species of animals in the world, have the best fossil record. Reasons include the presence of hard parts, such as bones and scales. The exception to this are the cartilaginous fish, because cartilage does not fossilize well, and the birds, due to the fact that they have hollow bones that can be crushed and lost. In contrast, the best fossil record among the vertebrates exists among the large mammals, whose bones are preserved well as fossils. Superclass Agnatha The Agnatha are in some texts referred to as a class, and in others as a superclass. In general this group shares the common characteristics of: • no jaws • no paired appendages • a completely cartilagenous skeleton • a single nostril • 6 - 14 external or concealed gill slits • a persistent notochord • a two-chambered heartBecause the fossil record is very poor for these species, it is unclear whether the two Agnathan groups should be described as classes or orders. Ancestral forms of this class were the Ostracoderms, which are extinct, but were heavily-armored on their heads and trunk. The ostracoderms were believed to be detritus feeders, because of their jawless mouths. Extant Agnathans include two groups, called cyclostomes, because of their circular mouths: Class Myxini, Order Myxiniformes - hagfishes (Figure 3-8, p. 88 in text) • temperate, marine deep water • feed on detritus and carrion, as well as polychaete worms • use sensitive tentacles around their mouths in locating prey • single nostril opens into pharynxClass Cephalaspidomorpha, Order Petromyzontiformes - lampreys (Figure 3-10, p. 90 in text) • temperate, anadromous (hatch/breed in fresh water, mature in marine) and freshwater • parasitic as adults - attach to other fishes with their suction-like mouths and rasp a hole in the skin • buccal glands secrete an anticoagulant to ensure free-flowing food source • larvae are called ammocoetes, resembling the amphioxus - primarily detritus feeders until they metamorphose into adults, sometimes after 6 or 7 years as a larva.The remaining vertebrate orders are Gnathostomes (possess true jaws). Evolutionary studies have shown that in most cases the jaw is modified from one of the gill arches that were used to support gills in more primitive species. Evolution of jaws represents an advancement in morphology, expanding the function of the mouth to a wider range of potential prey types. Thus, the jaws are an example of a derived structure that is more generalized than its ancestral form. Class Chondrichthyes - cartilagenous fishes characterized by: • paired nostrils • skeleton completely cartilagenous with no endoskeletal bone • no swim bladder • scales dermal placoid when present • gill arches internal to gills • freshwater and marine speciesContains three main groups: Subclass Elasmobranchii • 5 - 7 gill openings plus spiracle anterior to first gill • upper jaw not attached to braincase • teeth derived from placoid scales, deciduous and continually replaced • claspers present in males, internal fertilization, ovoviviparous (egg contained within the uterus, where the young develop and then hatch as miniature adults) or viviparous (embryos develop internally and then emerge as a miniature adult) • modern species present by end of Mesozoic Order Squaliformes - true sharks • almost purely predaceous/marine • heterocercal tailfin - caudal fin is longer on the dorsal side than on the ventral sideOrder Rajiformes - Rays, skates, sawfishes • greatly flattened bottom dwellers • scales not over entire body • pectoral fins winglike • crushing teeth - mollusk eaters • spiracles greatly enlarged • oviparous - produce an egg pouch covered in a very tough shellSubclass Holocephali - Chimaeras • upper jaw fused to braincase • flat, bony plates instead of teeth • operculum covering gillslits • strictly marine feeding on mollusksClass Osteichthyes - bony fishes • endoskeleton made up of bone • jaws and paired appendages • gill arches internal to gills • gills covered by bony operculum • dermal scales not placoid • many forms have swim bladder • appeared in Devonian - dominant vertebrates since mid Devonian • arose in freshwater, moved into saltwaterSubclass Actinopterygii: ray-finned fish • fin rays attach directly to girdles • internal nostrils - nares absent • single gas bladder • known from DevonianSuperorder Chondrostei (sturgeons and paddlefish) • general primitive form • typically small • skeleton primarily cartilage • heterocercal tail • ganoid scales • most died out by end of MesozoicSuperorder Neopterygii (most bony fishes) Division Ginglymodi - garpike • moderate ossification of skeleton • heterocercal tail • elongated jaws • ganoid scales • dominant during MesozoicDivision Halecostomi Subdivision Halecomorphi - bowfin • represented by single freshwater species Amia calva • cycloid scales • almost homocercal tail Subdivision Teleostei - true bony fish • skeleton mostly bony • tail typically homocercal • no spiracle • scales ctenoid or cycloid • known from marine forms first, originating from Holosteans during Mesozoic • major radiation associated with modifications in locomotor or feeding mechanisms and high fecundity, i.e. - maxilla and premaxilla independently mobile from rest of skull - pelvic and pectoral fins adapted for speed and braking giving maneuverability - fusiform bodies streamlined for speedSubclass Sarcopterygii: species previously believed to be extinct, such as the coelacanths and lungfish. • fleshy lobed fins so that fin rays do not articulate directly to girdles • internal and external nares • many retain the heterocercal tail • the coelacanth is represented by a single species that lives off the Comoro Islands near MadagascarThe rise of the Tetrapoda classes and the movement from water to land represents one of the major evolutionary events in the history of vertebrates. New structural designs were required to make the transition to land in order to cope with increased oxygen levels, decreased water supply, more fluctuating ambient temperature, and slight changes in the way sensory information is obtained. Class Amphibia - Amphibians • arose from Crossopterygian, Rhipidistian ancestors • three extant orders, two extinct subclasses • lungs and skin used as adult respiratory organs • gills present in larvae, retained into adulthood in some neotinic forms (salamanders) • heart with two atria and one ventricle - "three chambered" • skin is naked or with bony dermal elements • ectothermic - must regulate body temperature by moving to different microclimates within its environment • group includes smallest terrestrial vertebrates up to some 5’ in length • name implies continued tie to water - eggs must be laid in water or at least in very moist environment; young develop as gill breathing, water-dwelling tadpoles • embryos lack an amnion, but eggs are laid in a jelly-like protective coatingOrder Urodela (Caudata) - salamanders • tail maintained throughout life • limbs 1 -2 "normal" pairs • elongated trunk and long tail • can retain larval characteristics (flattened, shovel-shaped head, fleshy tail, external gills) in adult forms (paedomorphic) - the result is a sexually mature individual with many other body parts in the larval or juvenile condition (neoteny)Order Salientia (Anura) - frogs and toads • loose tail as adults • caudal vertebrae fuse to form long inflexible urostyle - relates to saltatorial locomotion • long hind limbs developed for saltatorial locomotion • vocal cords well developed • ear modified for reception of airborne sound wavesOrder Gymnophiona (Apoda) - caecilians • elongated, snake-like, with no limbs or girdles • no vocal cords or airborne sound detection • some retain scales embedded in skin • notochord persists • minute eyes, lack lids • chemosensory tentacle on headClass Reptilia • first fully terrestrial vertebrates • development of cleidoic (closed/self-contained) egg; embryo with extra-embryonic membrane and relatively impermeable shell • lungs for respiration • heart with two atria and ventricle partially or totally (Crocodilians) divided • one occipital condyle • skin with epidermal scales or bony plates • ectothermic, sometimes called heliotherms because they can regulate body temperature by using solar radiation • first appeared in late Paleozoic, so numerous by Mesozoic known as "Age of Reptiles"Subclass Anapsida ("no opening"); Order Testudinata (turtles) • ribs modified along with epidermal plates to form shell - carapace and plastron • girdles inside ribs • jaws covered with horny epidermal plates, no teeth • little change since TriassicSubclass Diapsida ("two openings") Order Squamata - lizards and snakes • contains most modern reptiles • lizards known from Cretaceous, snakes in Cenozoic • skull has lost one or both temporal regions • vertebrae usually procoelous • abdominal ribs usually greatly reduced or absent • body covered with horny epidermal scales • quadrate bone moveable • teeth set in socketsSubclass Archosauria - Ruling Reptiles • diapsid skull • contains dinosaurs and ancestors to birdsOrder Crocodilia - crocodiles • quadrate fixed • bony plates embedded in epidermis • teeth set in sockets • abdominal ribs present in Gastralia • ventricles completely separated • developed secondary palate • "crop" similar to birdsClass Aves Subclass Neornithes - modern birds • endothermic rather than ectothermic • the reptile scale into a feather which is the only unique characteristic of this class • four-chambered heart • epidermal scales on bill, legs, feet • bill instead of teeth; teeth absent in modern forms • modifications for flight include hollow bones, pectoral appendages modified as wings, air sacs, large eyes and large cerebellum • modifications for vocalizationClass Mammalia • possess hair/fur • mammary glands to nourish young • endothermic • viviparous (oviparous in one order) • two occipital condyles • zygomatic arch and secondary palate • single dentary bone in lower jaw • dentary-squamosal jaw articulation • muscular diaphragm • arose from synapsid reptiles which branched off at base of reptilian treeSubclass Prototheria - egg-laying mammals • oviparous • mammary glands without nipples • cloaca still present • pectoral girdle with separate precoracoid, coracoid, and interclavicle bonesSubclass Theria Infraclass Metatheria - marsupial mammals • viviparous, young born extremely altricial • abdominal skin pouch (marsupium) supported by epipubic bones • lack typical chorioallantoic placenta, have yolk-type • vagina doubled, no cloaca • mammary glands located inside marsupium • restricted to New World tropics and AustraliaInfraclass Eutheria - "true" or placental mammals • viviparous • chorioallantoic placenta • vagina single • mammary glands with external nipples • precoracoid and interclavicle gone • arose in Cretaceous, great radiation of insectivore-like ancestors during CenozoicFinal note: We will be using the terminology and phylogenies developed in this lecture as we go through the different systems. Try to become comfortably familiar with this phylogeny, as I will be referring to it later. Because this is comparative anatomy, the focus of this course will be on how different systems developed in different taxa, without emphasis on any single taxonomic group (such as humans). General Definitions: Adaptive radiation - evolutionary process in which descendants from an ancestral species multiply and diverge to occupy many different habitats and modes of life Agnatha - jawless vertebrates, including Myxini, Cephalaspidomorpha Amniotes - vertebrates that possess an amnion or extraembryonic membrane that surrounds the embryo and encases it in amniotic fluid, including Reptilia, Aves, and Mammalia Anamniotes - vertebrates that lack an amnion . Includes Myxini, Cephalaspidomorpha, Chondrichthyes, Osteichthyes, Amphibia Cleidoic egg - self-contained egg that allows animal to bypass larval stage Cloaca - posterior chamber of most fishes, nonmammalian tetrapods, and monotreme mammals into which the digestive tract and urogenital passages discharge). Coelom - body cavity completely lined with epithelium (cellular tissue Gnathostomes - vertebrates with jaws derived from the mandibular arch, which may have (in primitive vertebrates) supported gills. Includes Chondrichthyes, Osteichthyes, Amphibia, Reptilia, Aves, Mammalia Heliotherm - regulates body temperature by using solar radiation . Marsupium - skin pouch in which immature young are carried Neoteny - paedomorphosis that results from the slowing down of somatic development relative to reproductive development. Oviparous - eggs are laid and the young develop outside of the mother Ovoviviparous - egg contained within the uterus, where the young develop and then hatch as miniature adults Paedomorphosis - retention of juvenile characters into the adult stage Pisces - collective term for all fishes; includes Myxini, Cephalaspidomorpha, Chondrichthyes, Osteichthyes Placenta - composed of parts of the uterine lining and fetal extraembryonic membranes through which exchanges between mother and embryo occur. Swim bladder - sac of air that acts as a hydrostatic mechanism, allowing a fish to control its vertical position in the water Tetrapoda - collective term for the terrestrial vertebrates; they have four feet unless some have been secondarily lost or converted to other uses. Includes Amphibia, Reptilia, Aves, Mammalia Viviparous - embryos develop internally and then emerge as a miniature adultBIOLOGICAL DESIGN "You can drop a mouse down a thousand-yard mine shaft and, on arriving at the bottom, its gets a slight shock and walks away. A rat is killed, and man is broken, a horse splashes." Haldane (1928). Animals are equipped to address biological demands: • long neck of a giraffe to access treetop vegetation • cats’ claws to hook prey • bison’s fur to protect from coldbut an animal’s design must also  address physical demands of gravity and motion Organisms have an extensive range of sizes from small bacteria (0.3 microns long) to a large whale (~30 m long).     • range of size is about 8 orders of magnitude, or 100,000,000-fold - humans are relatively large; only an order of magnitude distinguishes us from the largest living things, but we are six or seven orders larger than the smallest.     • range of organism size is not trivial - the force of gravity is equal anywhere on the earth, but the effects are very different depending on the size of the organism.     • the range of sizes among vertebrates is about four orders of magnitude, half that of living organisms, and vertebrates occupy the top half of the range - fishes range in length from 10-2 to 10 m, and mammals from 10 cm to 30 m. Differences in size necessitate differences in body design and performance - a grasshopper enlarged to the size of a human would not be capable of jumping as it could at its normal small size The study of size and its consequences is known as scaling - elephants and shrews share the same fundamental skeletal architecture, organs, biochemical pathways, and body temperature, but an elephant is not simply a very large shrew As body size changes, demands on various body parts change disproportionally.  Size and shape are necessarily linked, and the consequences affect everything from metabolism to body design. Surface to Volume ratio The relationship of surface area to volume is the most important factor that determines how size influences the design of organisms     • contact between an organism and its environment occurs at the surface area, whereas the internal workings are related to its volume or mass     • the two quantities are not related in a simple proportionality - you cannot simultaneously double the surface area and volume of an organism unless you change its shape drastically. As an example, the length of a side of a cube is L.  The area of one side of the cube is L2, and the formulas for total surface area (S) and volume (V) are     S = 6L2 and V = L3 For a sphere, the formulas are     S = 4¼r2 and V = 4(¼)r3/3 In both cases, the formula for area is the length of something squared, and the formula for volume is the length cubed.  In general, for isometric objects areas are proportional to L2 and volumes proportional to L3.  Isometric objects are those that vary in size while retaining the same shape. So, if you double the length of a solid, you increase the surface by 22, or fourfold, and the volume by 23, or eightfold.  Therefore,     Surfaces are proportional to the squares of lengths.     Volumes are proportional to the cubes of lengths. And,     Surface/Volume is proportional to L2/L3 (= 1/L), or     Surface/Volume is inversely proportional to Length. This means that a larger object will have less surface relative to its volume than will a smaller isometric object of the same shape.  A whale is big inside with little outside, and a bacteria is big outside with little inside. Scaling and Allometry In nature, there must be an increasing deviation in larger organisms from being spherical if processes that use surfaces must be held constant.  In other words, the surface area must increase allometrically (out of proportion) to maintain the surface/volume ratio.  Organisms generally grow allometrically rather than isometrically and shape ends up being a function of size (see Figs. 4.9 and 4.10 in text). For quantifying allometry, body mass is often measured, and the cube root taken as an indication of length or size (since volume is proportional to the cube of length) • if one plots surface area or volume against length, one gets a plot in which the line is a curve • if we re-label the axes using geometric rather than arithmetic scales, the same data yield a straight line - the intervals on the axes represent not arithmetic differences, but rather factors of increase.  (Another equivalent way of getting a straight line is to take the logarithm of each data point value.) Generally, the relative sizes of two parts, x and y, can be expressed mathematically in the allometric equation     y = bxa where a and b are constants.  This equation is often called the allometric equation.  Ignoring b, which is a constant, this can be written as     (y-interval factor) = (x-interval factor)a, or     a = log (y-interval factor)/log(x-interval factor) The use of graphs with geometric axes is to determine the slope a, also called the scaling factor or exponent of proportionality between two measurements     • a scaling factor of 1 means that a measurement scales in proportion to length     • a factor of 2 means scaling with area, and that area is proportional to the square of the measurement     • a factor of 3 means scaling with volume, and that volume is proportional to the cube of the measurement     •  scaling factor of 0 means that the measurement does not scale with size; i.e., it is size-independent. As examples: • data from mice to cattle indicate that the scaling factor ranges from 1.9 to 2.0 - this slight negative allometry means that surface area increases a little more slowly than size than our isometric expectation.  In other words, larger animals are more rotund, and smaller ones are more elongate. • the observed scaling factor for skeletal mass of terrestrial mammals (from mice to elephants) is 3.25 - one possible explanation is that you could interpret this number as positive allometry, meaning that larger mammals have relatively more skeletal mass than expected for their weight.  A shrew is about 4% skeleton, a human about 8.5%, a horse 10%, and an elephant nearly 13%.  Because the mass of the bones is relatively greater in larger animals, one might conclude that they have compensated for increased weight with relatively stronger bones.  To compensate fir increased body mass and the effects of gravity, the bones of big animals must be disproportionately thick to support their weight. Biomechanics - physical forces are a permanent part of an animal’s environment and analysis of biological design requires an understanding of how these forces affect an animal’s experiences. Length is a concept of distance Time is a concept of the flow of events Force describes the effects of one body acting on another Mass is a fundamental property of matter where weight a measure of force Velocity is the rate of change in an object’s position Acceleration is the rate of change in its velocity Center of mass is the single point at which the mass of a body is evenly balanced Basic Force Laws 1. First law of inertia: Because of its inertia, every body continues in a state of rest or in a uniform path of motion until a new force acts on it to set it in motion or change its direction. 2. Second law of motion: The change in an object’s motion is proportional to the force acting on it (Force = mass x acceleration) 3. Third law of action/reaction: Between two objects in contact, there is for each action an opposite and equal reaction. The effects of forces on a body may be expressed using simple levers (bones) and torques (muscles).  Modifications in basic bone/muscle lever/torque systems results in limbs designed for speed and/or strength (see Fig. 4.25 and 4.26). • adaptation for flight requires modifications to overcome drag and gravity • modifications in the skull and jaw articulation for food handling and chewing Biophysics - concerned with the exchange of energy, particularly the exchange of heat and  diffusion of molecules     • concurrent exchange - transfer of heat/molecules between conduits where flow is in the same direction     • countercurrent exchange - transfer of heat/molecules between conduits where flow is in the opposite direction     • crosscurrent exchange - transfer of heat/molecules between conduits where flow is at a 90° angle     • Monocular vision - no overlap in the visual fields - common in prey species     • Binocular vision - overlap in the visual fields which allows varying degrees of depth perception in the zone of overlap - stereoscopic image  EARLY EMBRYOLOGYWhy do we need knowledge of embryology to study anatomy' • The fact is that all of those structures in all organisms are derived from a single cell that was formed by the union of two gametes. • Every organism you see came originally from a single cell, which divided and differentiated to form more complex structures. Thus, everything that we will study during the remaining part of the semester is dependent on the complicated embryological process.Reproduction starts with two cells, the sperm and egg - haploid cells formed through the process of meiosis which are specially designed for their own specific purpose. Sperm are: • extremely small cells that lack most of their cytoplasm • designed to travel through an aquatic medium (either internal or external) to reach the egg cell • travel by movements of one or more flagella that propel it toward the egg • all sperm consist of three basic pieces:     - the head, which contains the genetic material and is capped by the acrosome (cap) at the apex that contains enzymes needed for the sperm to penetrate the egg     - the middle piece contains the primary power source of mitochondria that fuel the movements of the tail piece. Eggs are: • designed more for providing nutrient sources to the developing young than for movement • contains yolk that consists of lipids and protein for nutrients, along with enzymes needed to initiate development • primarily classified by the amount and the distribution of yolk in the egg:     - microlecithal eggs (characteristic of the protochordates and eutherian mammals) have a very small amount of yolk, and the young hatch quickly as a result     - mesolecithal eggs (characteristic of lampreys and amphibians) have an intermediate amount of yolk, and the young hatch at a later stage of development     - macrolecithal eggs (characteristic of fishes, reptiles, birds and monotremes) have a large amount of yolk, and the young hatch at an even later stage     - isolecithal: distribution of yolk can be even through the egg     - telolecithal: yolk concentrated in one part of the egg - the area with less yolk and prominent haploid nucleus as the animal pole and the area with more yolk as the vegetal poleAt fertilization, enzymes in the acrosome of the sperm help to penetrate the egg • requires that the sperm break through the plasma and vitelline membrane surrounding the egg • to prevent more than one sperm from penetrating the egg (polyspermy), the egg undergoes a cortical reaction to bring the sperm head into the interior of the egg and change the vitelline envelope to form the fertilization membraneJust after fertilization the zygote (fertilized egg) undergoes cleavage (mitotic cell divisions) and becomes subdivided into smaller cells - the gross arrangement of cells differs greatly among vertebrates, depending on the amount of yolk in the egg: Holoblastic cleavage occurs when the cleavage furrows pass through the entire egg • cleavage can either be equal, where the resulting cells contain the same amount of yolk, or unequal, in which some cells contain more yolk than others:     - equal cleavage occurs in microlecithal eggs     - unequal cleavage occurs in mesolecithal eggs • cleavage results in the formation of a ball of cells (blastomeres) surrounding an internal cavity (blastocoel) Meroblastic cleavage occurs more in macrolecithal eggs • cleavage takes place only in a disk at the animal pole • the cleavage furrows do not extend into the yolk • results in the formation of the blastodisk that lies on the top of the yolkGastrulation is characterized by cell movement and reorganization within the embryo (morphogenetic movements) to the interior of the embryo, forming three primary germ layers: ectoderm, mesoderm, and endoderm. The cells migrate inward at the blastopore, which forms, or is close to, the location of the anus in the adult • the ectoderm forms the outer tube of the embryo • the endoderm is an inner tube that forms the alimentary canal and all its derivative organs • the mesoderm lies between these two layers. At the end of gastrulation, the embryo is bilaterally symmetrical, with three discrete cell layers, and rudiments of the notochord and neural tube. This blastopore-to-anus developmental pathway is found in Chordata, Hemichordata, Echinodermata (starfish, sea urchins, sea cucumbers, etc.), uniting these groups into a monophyletic group called the Deuterostomes. The plesiomorphic condition, found in the Protostomes, is for the blastopore to become the mouth. Gastrulation in Amphioxus • begins by the flattening of the blastula, loss of the blastocoel, and formation of the archenteron - the embryonic gut cavity that is lined with endoderm. After flattening, two cell layers can be distinguished - ectoderm and endoderm. • chordamesoderm, or the longitudinal mid-dorsal group of mesoderm cells, moves into the roof of the archenteron during gastrulation and gives rise to the notochord • after flattening, the process of folding continues to further form the archenteron as well as the blastopore, or external opening of the gastrula • further differentiation of cells occurs through the process of budding off of mesodermal cells to:     - form pouches that will later become organs     - within these pouches are spaces that will become the body cavity or coelom     - notochord formation proceeds with the condensing of the chordamesoderm into the notochord     - the neural tube then forms from pinching of the ectoderm over the notochord *Note: One important thing to realize at this point is that in your text, as well as in other diagrams of embryonic development, cells are color-coded to help you recognize where they are from:     • ectoderm is generally blue     • endoderm yellow     • mesoderm pink     • cells of the notochord are usually represented as green Gastrulation in amphibians • amphibian gastrulation is changed slightly due to the larger amount of yolk contained within the egg - the cells at the animal pole are retained for the formation of the embryo while the yolk-filled cells at the vegetal pole are used more by the embryo as an energy source • gastrulation is initiated by invagination of cells to form the dorsal lip of the blastopore • cell movements cause the pushing out of the ectoderm and inward movement of the endoderm and yolk-filled cells • the archenteron is formed and the blastocoel is slowly filled with cells and lost • the yolk-filled cells of the vegetal pole remain for a short time to fill the space between the dorsal and ventral lip of the blastopore, thus forming the yolk plug • the mesoderm gradually differentiates from the rest of the cells of the gastrula, as do the chordamesoderm cells which go on to form the notochord • the gastrula then progresses into the neural tube formation stage, called neurulation Gastrulation in birds • with birds, we begin to see the meroblastic type of development of the embryos, significantly more yolk than in the previous examples, and the movement of cells is different as the cells lie more in sheets rather than in a ball. • two processes lead to cell movement in the chick embryo:     - delamination: sheets of cells split into separate layers     - ingression: individual surface cells migrate to the interior of the embryo • during delamination, two layers of cells form (the hypoblast and the epiblast) with a cavity in between that is comparable to the blastocoel in amphibians - separation of these two layers results in the formation of two regions of the blastodisk, the area opaca and the area pellucida • during ingression the primitive streak forms (a longitudinal thickening of cells along the blastoderm of large-yolked eggs) through which prospective chordamesoderm and mesoderm cells move inward - cells of the hypoblast are replaced by endodermal cells • the primitive streak lengthens along the surface of the yolk through ingression - the embryo grows longer and occupies more of the area pellucida • after gastrulation, the process of neurulation, or formation of the neural tube and associated structures, occurs Neurulation • occurs at or near the end of gastrulation and transforms the gastrula into a neurula by establishing the central nervous system • the ectoderm gives rise to neural folds flanking a neural groove along an axis from the blastopore toward the future head - these sink into the dorsum of the embryo and the folds meet mid-dorsally, forming a neural tube, of which the anterior part becomes the brain and the rest, the spinal cord • a population of mesodermal cells called chordamesoderm aggregates to form the notochord - chordamesoderm generally induces (embryonic induction) the neural tube to form (if the chordamesoderm is removed experimentally, the neural tube will not form) During neurulation • the chordamesoderm that will go to form the notochord induces neural plate formation, which is the first stage in the formation of the neural tube. • characterized in most vertebrates by three stages     - during the neural plate stage, the ectoderm on the dorsal side of the embryo overlying the notochord thickens to form the neural plate     - at the neural fold stage, the thickened ectoderm folds, leaving an elevated area along the neural groove. The neural fold is wider in the anterior portion of the vertebrate embryo, which is the region that will form the brain.     - during the neural tube stage, the neural folds move closer together and fuse - the neural groove becomes the cavity within the neural tube, which will later be capable of circulating cerebrospinal fluid that aids in the function of the central nervous system. One derived characteristic found in vertebrates is the formation of neural crest cells • ectodermally derived • develop along the top of the neural tube as the neural folds close • most neural crest cells change into mesenchyme, an embryonic tissue that consists of star-shaped cells from all three germ layers • develop into the visceral skeleton (i.e. gill arches, some of which will develop into jaws), pigment cells, sensory and postganglionic neurons, the dentine-producing cells of teeth, Schwann cells that help protect neurons, and bony scales Differentiation and derivation - Organogenesis: After the production of the nerve tube, differentiation of the germ layers occurs rapidly, and organogenesis begins, in which the primary tissues differentiate into specific organs and tissues (Fig. 5.17). * Endoderm - Endoderm gives rise to the epithelium of the alimentary tract, to structures derived from the pharyngeal pouches such as parathyroid glands, thymus gland, Eustachian tube and middle ear cavity (not the ossicles), and to structures that develop as an evagination of the gut, such as the thyroid gland, lungs or swim bladder, liver, gall bladder, pancreas, and urinary bladder. * Mesoderm - becomes organized into three regions: the epimere (dorsal mesoderm), mesomere (intermediate mesoderm), and hypomere (lateral mesoderm).     - Epimere: The somites constitute most of the dorsal mesoderm and have three regions: • dermatome - forms the dermis of the mid-dorsal skin • sclerotome gives rise to the vertebrae • myotome forms skeletal muscles other than those of the gill arches    - Mesomere: gives rise to the kidney tubules, excretory organs, and reproductive ducts.     - Hypomere: lateral-plate mesoderm is confined to the trunk and is divided into somatic mesoderm (parietal peritoneum) and splanchnic mesoderm (visceral peritoneum, mesenteries, heart and associated structures, lymphatic system, gonads and visceral muscles) * Ectoderm - gives rise to: • Neural tube • Epidermis and associated glands • Neural crest and its derivatives: migrate through the embryo, giving rise to a diversity of structures • Ectodermal placodes: localized thickenings that sink below the surface and give rise to sensory neurons and sensory structures: olfactory placodes, forming the olfactory sacs; lens placodes, for the lens of the eye; otic placodes, to become the membranous labyrinth; a group of placodes that contributes neurons to the sensory ganglia of cranial nerves V, VII, VIII, IX, and X; and last, placodes that form the neuromasts of the cephalic and lateral line canals Sources of energy during development and extraembryonic membranes The cleidoic egg is an important derived trait among many vertebrates and enabled tetrapods to be independent of water. In just considering the macro- and mesolecithal species, we know that both contain a moderate to large amount of yolk for the embryo to use as an energy source. In the cleidoic egg, water and oxygen are obtained through diffusion. Extraembryonic membranes vary in complexity among the vertebrates: Yolk sac - forms around the yolk, and connects to the embryo via the yolk stalk to provide nutritional support during development - amniotes and anamniotes differ in their yolk sacs     fish have a trilaminar yolk sac, with an extraembryonic coelom that surrounds the yolk     birds and reptiles possess a bilaminar yolk sac (consisting only of endoderm and splanchnic mesoderm). Shell membrane/shell - is formed only in the cleidoic egg of amniotes - surrounds the embryo, yolk and albumins (egg white) and protects it - provides a surface for diffusion of oxygen     Allantois - acts as a compartment for storage of nitrogenous excretory products such as uric acid, and may remain after birth or hatching as the urinary bladder.     Amnion - surrounds the embryo, and is filled with amniotic fluid to cushion the embryo     Chorion - surrounds the amnion and yolk sac Mammalian developmental modifications • the mammalian egg does contain some yolk, but it is microlecithal and isolecithal - requires that the embryo implant quickly in order to obtain more nutrients from the mother. • early cleavage in mammalian embryos followed by the blastocyst stage, of which the outer layer of cells is called the trophoblast. The inner cell mass of the blastocyst will go on to form the embryo. • during implantation in the uterus, the placenta is formed, which is a structure for physiological exchange between the fetus and the mother. The placenta consists of both a maternal contribution (endometrium of the uterus) and fetal contribution (trophoblast), which is believed to be used as an immunological barrier that prevents rejection of the fetus (and its paternal chromosomes) by the mother. The shape of the placenta varies depending on the species. • the inner cell mass of the blastocyst develops into the blastodisk (similar to that in chickens). Early stages of development of the mammalian embryo, such as primitive streak stage, neurulation and germ layer differentiation, are similar to that occurring in chickens and reptiles. • the primary difference found in mammals is the development of the umbilical cord - contains allantois and yolk sac as well as circulatory system structures that connect the embryo to the placenta. Ontogeny and Phylogeny The notion of a parallel between the stages of development (ontogeny) and the evolutionary history of adults (phylogeny) predates the acceptance of evolution. It was thought that there was a Scala Naturae, a "Ladder of Nature" or "Scale of Being" for living things, which could be arranged in a sequence, as if on the rungs of a ladder. The highest rung was viewed as a stage of perfection. Likewise, it was generally noted that the ontogeny of an individual consisted of a series of stepwise stages, and it was natural to assume a connection between the two. Carl Von Baer: made a number of general conclusions about development called Von Baer's laws: 1. In development from the egg the general characters appear before the special characters. 2. From the more general characters the less general and finally the special characters are developed. 3. During its development an animal departs more and more from the form of other animals. 4. The young stages in the development of an animal are not like the adult stages of other animals lower down on the scale, but are like the young stages of those animals.In other words, a chick embryo would be recognizable at an early stage as Vertebrata, but not any particular subtaxon. Later, it would be recognizable as Aves, and finally, it would be recognizable as a Gallus domesticus. Therefore, the ontogenetic stages do not run parallel to the sequence of taxa on the scale of being. Ernst Haeckel: believed that the adult stages of the chain of ancestors are repeated during the ontogeny of the descendants, but that these stages are crowded back into the earlier stages of ontogeny. Thus, ontogeny is an abbreviated version of phylogeny. Haeckel claimed that the gill slits of human embryos were literally the same structures of ancestral adult fishes, that were pushed back into the early ontogeny of humans by an acceleration of development in lineages. In other words, the sequence of ontogenies was condensed, and new features were added by terminal addition. Von Baer, in contrast, argued that the gills slits are not the adult stages of ancestors; rather they are simply a stage common to the early ontogeny of all vertebrates. That is, evolution proceeds from "undifferentiated homogeneity to differentiated heterogeneity"; from the general to the specific. Von Baer's theory requires only that organisms differentiate; Haeckel's requires a change in developmental timing. Biogenetic Law Both of these ideas preceded Darwin's theory of evolution and were re-read in the light of Darwin. Although Darwin favored Von Baer, Haeckel's ideas became more accepted. Haeckel's theory came to be known as the theory of recapitulation or the biogenetic law - Ontogeny recapitulates Phylogeny. This was an attractive idea, because it gave biologists a way of reading phylogeny directly from ontogeny. The biogenetic law eventually lost popularity with the rise of experimental embryology and Mendelian genetics. Embryology showed that many varieties of change in developmental timing were possible, and that different parts of the organism might differ in rates of development; Mendelian genetics showed that genes could effect changes at any stage of development, and that terminal addition was not the only possibility.     Definitions  Acrosome - cap at the apex of a sperm head that contains enzymes needed for the sperm to penetrate the egg Allantois - extraembryonic membrane that develops as an outgrowth of the hindgut. Serves for respiration and excretion in reptile and bird embryos, contributes to the placenta in eutherians, and forms the urinary bladder and part of the urethra in adult amniotes Archenteron - the embryonic gut cavity that is lined with endoderm Blastocoel - a cavity of the blastula that becomes obliterated during gastrulation and mesoderm formation Blastodisk - the disk of cells formed during cleavage that lies on top of the yolk of large-yolked eggs of fishes, reptiles and birds, and on the top of the yolk sac of mammals Blastopore - external opening of the gastrula Blastula - ball of cells formed during cleavage, usually containing a blastocoel Chordamesoderm - the longitudinal middorsal group of meosdermal cells that moves into the roof of the archenteron during gastrulation and gives rise to the notochord Delamination - downward movement of cells to form a new layer near the yolk Dermatome - the lateral portion of a somite which will form the dermis of the skin Fate map - shows the cell areas of blastulas that will subsequently give rise to particular kinds of cells Holoblastic cleavage - cleavage furrows pass through the entire egg Ingression - the longitudinal movement of cells along the surface of the yolk Mesenchyme - an embryonic tissue that consists of star-shaped wandering cells that gives rise to most adult tissues Myotome - a muscle segment, usually applied to embryonic segments Nephric ridge - the region of the mesoderm between the somite and lateral plate that gives rise to the kidneys and gonads Neural crest - a pair of ridges of ectodermal cells that develop along the top of the neural tube as the neural folds close; this derived character of vertebrates gives rise to many of their distinctive features, including visceral skeleton, pigment cells, sensory and postganglionic neurons, the dentine-producing cells of teeth, and certain bony scales Neural tube - the tube formed in the embryo by the joining of the pair of neural folds; the precursor of the brain and spinal cord Neuroectoderm - the portion of the ectoderm that gives rise to the neural tube and neural crest. Primitive streak - a longitudinal thickening of cells along the blastoderm of large-yolked eggs through which prospective chordamesoderm and mesoderm cells move inward Sclerotome - the medial portion of a somite that forms the vertebrae Somatic - descriptive of structures that develop into the body wall or appendages as opposed to those in the gut tube, such as the somatic muscles, somatic skeleton Splanchnic - descriptive of structures that supply the gut Trophoblast - outer layer of the mammalian blastocyst; initiates placenta formation INTEGUMENTThere is nothing more conspicuous about an organism than its skin.  It is our primary means of identifying the organism, and is what defines the boundary of its body.  Skin is also the primary means through which an organism interacts with its environment. Because of its importance as the primary interface between an organism and its environment, the skin is designed to perform many functions.  These functions include: • support and protect soft tissues against abrasion, microbes • reception and transduction of external stimuli - i.e. heat, chemical, tactile • transport of materials involved in excretion, secretion, resorption, dehydration, rehydration • heat regulation • respiration • nutrition/nutrient storage - i.e. storage of vitamins, synthesis of Vitamin D • locomotion • coloration - cryptic or displayDifferent vertebrate taxa have very diverse ways of performing these functions and have evolved many different structures derived from the integument. Basic structure of the integument: The integument consists primarily of the skin and its derivatives.  Skin is a functional unit composed layers of fairly distincy epidermis (derived from ectoderm) and dermis (derived from the dermatome of somites) that are separated by the basement membrane (Fig. 6.1, p. 199). • Epidermis     - is relatively thin in most animals     - the upper layer composed of mostly dead, differentiated cells (stratum corneum) with a lot of keratin which helps the skin maintain some protection against water loss and bacteria     - continually produced by the most basal layer of the epidermis (stratum germinativum) and consists of cuboidal cells that are generalized and move toward the upper layer as they differentiate     - as the cells move outward, most synthesize keratin, a water-insoluble protein, the cells become flattened, die, and are sloughed off.  Other epidermal cells form multicellular glands or isolated glandular cells. • Dermis     - is more of a connective tissue than protective     - irregularly-shaped connective tissue cells that produce the extracellular matrix, including collagen and elastic fibers     - the upper layer (stratum laxum) lies directly below the basement membrane and is mostly loosely-packed cells     - the stratum compactum lies below and contains more tightly-packed cells     - the presence of elastin in the dermis is a synapomorphy of Gnathostomata - in part, the dermis anchors the skin to the underlying musculature     - also includes dermal scales, blood vessels, nerves, pigment cells, the bases of feathers and hairs, and their associated erector muscles.Integument of the vertebrate classes If we again tour through the different taxa that we discussed previously, we find many different forms of integument, based on the different environment that each organism inhabits. Amphioxus has an epidermis with a single layer of cells.  A synapomorphy of Craniata is the presence of a stratified (multilayered) epidermis.  The horny teeth of lampreys are keratin - most other fishes have little or no keratin in the skin. There are three major types of hard tissue associated with skin: Enamel     - the hardest tissue in the body     - made of hydroxyapatite and has no cells or tubules within it; only about 3% of it is organic     - ectodermal in origin and is produced by accretion of layers     - generally it is the most superficial of hard tissues and is found on teeth and the outer layers of denticles, scales and dermal armor - one type of enamel is ganoine Dentine     - is softer than enamel and has about 25% organic fibers     - usually contains tubules occupied by the processes of the mesodermal cells     - found on the same structures as enamel, but is always deep to the enamel layer     - some types of dentine are osteodentine, orthodentine, and cosmine, the last of these has characteristic types of canals Bone     - has about the same level of organic component as dentine     - may have osteons (Haversian systems) as does osteodentine, or may be deposited in layers like orthodentine     - unlike enamel and dentine, bone may undergo drastic reorganizationAgnathans The skin of living agnathans lacks dermal bone or scales, but the earliest craniate fossils (Ostracoderms) are known from tiny scales of dermal bone found in the Cambrian period.  These scales had     • a deepest, thin layer of lamellar bone,     • a thick layer of spongy (vascular) bone,     • another layer called dentine, and     • a surface coat of enamel-like material, often called ganoine. There was a pore-canal system that likely functioned in electroreception Chondrichthyes The skin is covered with denticles or placoid scales with layers of dense lamellar bone, dentine, and enamel Teeth are modified placoid scales Bony fishes Integument of fish is characterized by structures that help the organism maintain its water balance     • generally characterized by thin epidermis, with little or no keratinized cells at the stratum corneum     • mucus secreted from fish’s skin which seals out water and also prevents invasion by ectoparasites and fungus     • glands are unicellular - derived from a single epidermal cell Structures associated most with the fishes are scales:     • composed of three basic compounds: bone, dentine and enamel (moving from inside to outside); the outside layer, enamel, is the hardest tissue in the body, and therefore can be very protective     • because they contain compounds that are similar to those in teeth, scales are often compared to teeth     • basal types of scales include (Fig. 6.11): cycloid scale - thin bony scale having a smooth surface and rounded margins ctenoid scale - thin bony scale having comblike processes on its outer part and a serrate margin placoid scale - scaly outgrowth of the skin, that is thicker and more embedded in the skin cosmoid scale - thick bony plates that are embedded into the skin, that act more like a bony armor    • perform a more protective function, although the protectiveness of the scale is determined by the thickness of the bone Amphibians The earliest tetrapods had dermal scales, which probably functioned as armor.  Among living amphibians, caecilians have tiny dermal scales called osteoderms.  Their homology with dermal armor is not clear. Amphibians mark the transition between the aquatic and terrestrial environment.  Skin remains similar to its aquatic roots and resembles the skin of the fish; however, scales are not present. To prevent water loss, amphibians utilize mucus, which is a similar mechanism that fish use to prevent taking on additional water.  However, the mucus in amphibians is secreted by multicellular glands rather than the unicellular glands in fish. Because the integument of amphibians makes them somewhat vulnerable, many amphibians also secrete toxins that prevent them from being eaten by other organisms.  The primary gland responsible for the secretion is the parotid gland, located behind the ear of amphibians. Reptiles Reptiles show more advanced integumental adaptations to the terrestrial environment because they are more far-removed from the water.  In contrast, the cells are more highly keratinized. The integument is modified into horny scales in snakes and lizards.  In snakes, the scales on the ventral surface can be further modified into scutes, which can be used in locomotion.  In turtles the epidermis is strongly modified into plates that cover the shell, and because they increase in diameter each year, they can be used to age the animals. Birds The integument of birds reflects some reptilian ancestry and some new developments of the class.  Scales are present on the legs and feet of most birds, and the bill is covered in a tough skin that is highly keratinized.  The remaining skin is relatively thin. The defining characteristic of bird integument is feathers: - derived originally from scales, so that scales and feathers are homologous - function in flight (flight feathers) as well as temperature regulation (contour feathers) - basic structure of feather calamus, rachis and vane, which are derived from a feather follicle (Fig. 6.16).  The vane is composed of barbs that help to hold the shape of the feather and can be put back into place during preening.Birds are not always completely covered in feathers - instead, feathers usually grow along tracts called pterylae, and bare spots are called aptera Some feathers are modified to perform different functions     - down feathers are softer feathers because they lack all the barbs of flight feathers     - bristles and filoplumes are specially modified feathers that are used in catching prey (e.g., bristles around the bill of swallows and flycatchers) and display (filoplumes of grouse) Mammals Mammals generally have skin that conforms to the basic structure described previously, with the epidermal layers of the skin being especially thick in areas such as the soles and the palms of the feet, where proection is needed. Hair is the distinctive characteristic of mammals, and it provides insulation as well as some additional protection to the animal     - grow in folllicles derived from the stratum germinativum of the epidermus but are rooted in the dermis (Fig. 6.20)     - hair growth continues until the mitosis in the root stops - individuals in which mitosis completely stops at the hair root are usually the ones that go bald. The fine structure of an individual hair consists of three layers: medulla, cortex and cuticular scale (which contain a lot of keratin).  (Fig. 6.20).  Softer hairs (such as our fine body hairs) lack a medulla, whereas our scalp hair contains a medulla and is usually very strong. Modifications of hair include guard hairs (that protect the undercoat hair), quills (such as in hedgehogs and porcupines) and vibrissae (the tactile whiskers on the snouts of mammals). Other modifications of mammalian skin includes blubber, which is found in many cetaceans and marine mammals.  Blubber is a highly thickened subcutaneous fat layer that adds to the insulation of marine mammals and also acts as a food source for the body. Glands of the skin:  Glands associated with the skin that help to protect the skin and its associatedd structures, aid in heat regulation, and give off scent.  Include:     - sebaceous glands which lubricate and waterproof hairs - special case in birds the uropygial gland located at the base of the tail which secretes a waxy substance that is used to waterproof and clean feathers.     - two types of sweat glands in mammals aid in heat regulation: eccrine and apocrine sweat glands     - eccrine sweat glands secrete a watery solution that assists in evaporative cooling on the entire body     - apocrine sweat glands have thicker secretions that contain more odor, and are sometimes modified into scent glands in some species to use for scent marking (dogs) or defense (skunks); also the wax gland, which secretes the wax in mammalian ears.     - the mammary gland (related to sebaceous glands) which contain fatty tissue in addition to secretory cells that produce milk; usually only become active under hormonal influences, such as the secretion of prolactin by the body that occurs in females during pregnancy and lactation. Nails, claws, hoofs, horns and antlers: all are integumental derivatives.     - nails grow from the nail bed located in the epidermis at the distal part of the phalanges; the nail is higly cornified in ungulates whereas in clawed animals the nail is elongated and thickened for defense or predation     - horns are supported by a bony structure growing out from the skull; surrounding the bony core is a highly keratinized layer of the epidermis which is generally permanent     - antlers are not present throughout the year, and are shed during the non-breeding season; develop under a protective covering of skin (velvet), which is lost as the antlers mature     - rhinoceros horns are simply hairlike keratin fibers that are woven together without a bony core - similar to baleen in whales that is used for feeding Integument coloration - Pigment cells Pigment cells (chromatophores) are derived from neural crest cells that break off from the ectoderm during neural tube formation and are usually found in the dermis     - in the epidermis of mammals and birds, the pigment cells are usually melanophores which contain the pigment melanin.  Melanin is red or blackish brown.  Melanophores in the epidermis are usually responsible for slow color change, such as that related to aging or seasonal changes.     - in groups other than mammals and birds the chromatophores are mostly in the dermis:     - melanophores are like those of the epidermis      iridophores have organelles that contain platelets of guanine pigment, which reflects or scatters light     - xanthophores and erythrophores have yellowish pteridine pigments and reddish carotenoid pigments     - dermal chromatophores are responsible for rapid, physiological color change. Coloration can be of many types, including cryptic (providing blend into the environment) and aposematic (warning coloriation, that occurs in some snakes)   Definitions: Aposematic coloration - a form of coloration that serves to advertise the presence of dangerous, venomous or distasteful species Chromatophore - a vertebrate cell of neural crest origin that carries pigment or reflective granules Cosmoid scale - thick bony plates that are embedded into the skin, that act more like a bony armor Ctenoid scale - thin bony scale having comblike processes on its outer part and a serrate margin Cycloid scale - thin bony scale having a smooth surface and rounded margins Erythrophores - pigment cells that contain red pigments Fibroblast - irregularly-shaped connective tissue cell that produces the extracellular matrix, including collagen fibers Iridophores - pigment cells that confer a silvery appearance Keratin - a horny protein synthesized by the epidermal cells of many vertebrates Macrophages - large cells that phagacytose, or ingest, foreign material Placoid scale - scaly outgrowth of the skin, that is thicker and more embedded in the skin Sebaceous gland - branched alveolar gland that produces oily and waxy secretions Uropygial gland - an oil-secreting gland of birds located dorsal to the tail base Vibrissae - long tactile whiskers found on the snouts of mammals Xanthophores - pigment cells that contain yellowish pigments    INTRODUCTION TO THE SKELETAL SYSTEM We will begin our discussion of the skeletal system by talking about the tissues that will go into the construction of skeletal elements. The two primary components are cartilage and bone, with additional support coming from fibrous materials such as ligaments and tendons. These structural materials must be able to • withstand tremendous forces that affect an organism • support the mass of the body and all of the muscles and organs that are part of the body • remain strong under the stresses of locomotion, such as when the feet strike the ground, sending the force of the impact through the body frame • be strong at the junction where two bones meet, where stress is applied and felt • protect against impact to soft tissues, such as the skull protects the brainAltogether, bones and other skeletal materials must be resistant to such stresses, or they may break or distort. The types of forces experienced on different parts of the body will influence the structural material that is used. Overall strength of the skeleton arises from a composite assemblage of elements • heterogeneous materials composed of many different elements, that is generally much stronger than homogenous materials • composite materials dissipates fractures and breakage, such that the overall material is much stronger - cartilage without associated supportive materials would be mushy and flatten rapidlyCartilage: • defined as a firm but elastic skeletal tissue whose matrix contains chondroitin sulfate (ground substance) and collagen or elastic protein (fibers) molecules that bind with water • the cellular elements of cartilage are called chondrocytes which lie in spaces called lacunae (Fig. 5.20, p. 173) surrounded by the perichondrium, fibrous connective tissue that lies on the outside of cartilaginous tissue • because cartilage is very watery, it is highly flexible and can change drastically under stress but snaps back into its original shape.• may be found in several forms (ranked from least dense to most dense): Hyaline cartilage - cartilage with a clear translucent matrix; found primarily on the ends of ribs and on the trachea Elastic cartilage - cartilage containing elastin fibers that appears yellowish; found primarily on external ear and epiglottis Fibrocartilage - cartilage containing collagen fibers; found in the intervertebral disks and pubic symphysis Calcified cartilage - cartilage containing deposited calcium salts; found in the vertebrae of cartilaginous fishBone: • also a composite tissue similar to cartilage, but with a greater mineral component, primarily consisting of calcium (Fig. 5.21, p. 174) • bone consists of calcium phosphate and other organic salts deposited in a matrix • may be deposited in highly ordered units called osteons (Haversian canal system) - each a series of concentric rings made up of bone cells and layers of bone matric surrounding a central canal through which nervess and blood and lymphatic vessels travel • bone cells are identified based on their activity: - osteoblasts produce new bone - osteoclasts remove and resorb existing bone by secreting acid to break down mineral component of bone and enzymes to break down the collagen component of bone - osteocytes maintain equilibrium in fully formed bone• bones that bear more weight, such as the femur, have about 67 percent mineral, whereas antlers contain only 59 percent mineralSkeletal Development During embryonic development, the skeleton remains primarily cartilagenous to form the basic structural components and framework of the body After the basic structure of the embryo is formed, bone begins to be deposited in one of two ways: 1) Membrane/dermal bone • formed through the deposition of calcium salts and osteoblasts within the connective tissue located near skin surfaces • formation of membrane bone begins with the formation of trabeculae - small rods or tubes through connective tissue that provide reinforcement • trabeculae then fuse together and then bony tissue forms around them2) Cartilage replacement bone • formed in and around the cartilage of the embryonic endoskeleton • begins forming in two regions - around the cartilage (endochondral bone), such as occurs on the diaphysis of the bone - within the cartilage (perichondral bone)• trabeculae are formed (as they are in membrane bone formation) and act as an interconnected structural network within the bone - the spaces within and between trabeculae form the bone marrowAs ossification proceeds, the replacement of cartilage by bony tissue forms from the diaphysis to the epiphysis (end of bone) • in anamniotes the epiphysis remains cartilagenous until adult size is reached • in mammals and birds, epiphyseal plates (space between the diaphysis and epiphysis) are formed which remain cartilagenous to allow for room to grow until the organism has reached full adult sizeTo grow in girth, bones must continually be rebuilt through the combination of osteoblasts and osteoclasts (Fig. 5.24, p. 176). • the resulting structure is columnar, with each osteocyte surrounded by bone to form the osteon , with a vascularized center • from the continued production of osteons, and the ossification of the areas around trabeculae, two types of bone form: - cancellous bone - spongelike tissue that lies at the interior part of bone and at the ends - compact bone - dense peripheral bone tissueIn addition to structural support, bone • can be mobilized for other uses by the body - i.e. calcium source for egg shell production in birds • is responsible for red and white blood cell production - red bone marrow is the hemopoietic tissue of bone which is replaced by yellow bone marrow later in life, containing mostly fat cellsJoints, tendons, and ligaments: Each independent part of the skeletal system must articulate with another part, or with other parts of the body (such as muscles) The joint is a point of articulation between elements, including: • synarthroses - joints at which there is limited movement but room for growth - sutures where two dermal bones meet, such as in the skull or in the shell of turtles - synchondroses where sheets of cartilage ossify, but remain separatedby a plate of cartilage (such as in long bone production and the epiphyseal plateor ribs) - symphyses where two individual halves of bone meet and are separated by cartilage (such as in the jaw or pubic symphysis)• diarthroses are joints at which there is considerable movement, i.e. hinge joints (elbow), ball and socket joints (shoulder, hip), saddle joints (thumb). - diarthroses are connected by strong ligaments, or sheets of connective tissue composed of collagen - between bones is a viscous synovial fluid surrounded by a synovial membrane, as well as articular cartilages that help to maintain the fluid movement of the jointTendons: • are tightly packed bundles of parallel collagen fibers that connect muscle to bone • those that move against adjacent tissues often have fluid-filled sheaths surrounding them • concentrate the force of a muscle onto a relatively small area of the skeleton • the narrowness of tendons allows many of them to be packed in a small space • those that must pass around corners behave much like a pulley system, either a bony tunnel or process, or a ligament loop - generally the forces are tensile - if shearing forces occur, as when a tendon passes over a bending joint, there may be a sesamoid bone within the tendon to withstand the shearing forces (example the patella or kneecap)• collagen fibers of a tendon merges with the perimysium and endomysium of the muscle - perimysium of muscles that attach to large areas of bone, such as the supraspinatus muscle, merges with the periosteum of the boneLigaments: • join bone to bone, binding the skeleton together in a passive fashion • are similar to tendons, but may be more variable, having more irregularly arranged fibers, and some elasticity • nuchal ligaments, for example, are prominent in animals with large heads and/or long necks - hold the head and neck in a normal posture without muscular effort - lowering the neck to the ground, in contrast, requires muscular contraction    Breakdown of the skeletal system Two general parts of the skeleton are the:   Axial skeleton: Skull Mandible Hyoid Vertebral column Ribs Sternum | Appendicular skeleton: Pectoral girdle Forelimbs (or fins) Pelvic girdle Hindlimbs (or fins) |   The Skull and Visceral Skeleton General function of the skull and visceral skeleton: When describing the characteristics of vertebrates, one important characteristic was cephalization, or the evolution of a head region distinct from the body that acted as a centralized location of sensory, nervous and trophic (feeding) function • these structures require associated protective structures to prevent them from being damaged • other structures can be derived from the skull (i.e. antlers and horns) for combat or for mating displays • teeth and tusks are also derived from structures related to the skullGenerally when describing the skull, we divide it into three parts: 1. Chondrocranium - anterior part of the axial skeleton that encases the special sense organs and contributes to the skeletal elements encasing the brain. 2. Splanchnocranium - or visceral arches that support and move the gills and contribute to production of the jaws in gnathostomes. 3. Dermatocranium - dermal bones that encase the chondrocranium and splanchnocranium and contribute to the braincase, jaws, and skeletal elements of the mouth (teeth)The Chondrocranium: The chondrocranium is sometimes called the neurocranium and covers the ventral, lateral and posterior parts of the brain as well as the ear and nose Development: • the chondrocranium is formed by a combination of mesodermal sclerotome and neural crest cells • during development, cartilage forms around the brain beginning at the notochord • starts with the development of parachordals (cartilagenous rods) that run anteriorly and meet to form the basal plate • sense organs are then surrounded by cartilage to form the optic capsules (for the eyes), nasal capsules (for the olfactory organs) and auditory capsules (for the ear) (Fig. 7.1, p. 206 in text). • in the posterior region of the chondrocranium the occipital arch develops, which is perforated by the foramen magnum to allow for passage of the spinal cord to the developing brainParts of the chondrocranium: • the chondrocranium is most visible in more primitive species, such as the cartilagenous fishes • specific regions, such as the rostrum, denotes the anterior portion of the cranium • the occipital condyle is in the posterior region of the chondrocranium and articulates with the notochord/vertebral column • in more advanced vertebrates, the chondrocranium is later ossified and becomes a more minor part of the skullThe Splanchnocranium The splanchnocranium consists of the visceral arches composed of cartilage or cartilage replacement bone • visceral arches are the most visible in agnathans • generally seven visceral arches grow to support the developing pharyngeal pouches that are formed during early embryonic development • each visceral arch is a > shaped structure composed of two parts, the epibranchial cartilage and a ceratobranchial cartilage (Fig. 7.5, p. 227) • in more advanced vertebrates, parts of the splanchnocranium are modified to form derived structures such as jaws, ears and parts of the hyoid apparatus and pharyngeal cartilage (Fig. 7.7, p. 228)Origin of jaws and jaw suspension • jaws are primarily derived from the cartilage of the first visceral arch • in primitive species such as lampreys, food was moved through the pharynx by moving the first visceral arch to create a pump-like action • jaw formation probably evolved from selection favoring fish that utilized the first visceral arch to help it seize preyIn the first jawed fishes (Chondrichthyes) the first visceral arch was renamed the mandibular arch, which consisted of two cartilages: the palatoquadrate cartilage (upper jaw) and mandibular or Meckel’s cartilage (lower jaw) (Fig. 7.18, p. 239) • the dorsal part of the second gill arch (hyoid arch) - called the hyomandibular cartilage - articulates with the chondrocranium • these arches work together in different ways to create jaw movement, based on the ways in which these arches articulate with the chondrocranium (Fig. 7.8, p. 229): Amphistylic (primitive cartilaginous fishes) - jaw is supported both by the hyomandibular and by a direct connection between the jaw and the chondrocranium Hyostylic (elasmobranchs and most bony fishes) - upper jaw loses any major direct connection with the chondrocranium and the upper and lower jaws are supported solely by the hyomandibular Autostylic (lungfishes and in tetrapod ancestors) - upper jaw (pterygoquadrate cartilage) articulates or is fused with the chondrocranium, lower jaw forms from the mandibular cartilage, and the jaw remains unsupported by the hyomandibularIn mammals, the pterygoquadrate cartilage is modified further to form the incus and malleus - the hyomandibular becomes part of the hyoid and the stapes     The fate of the branchial arches is shown in Table 7.2 (p. 228) - LEARN THIS   The Dermatocranium The dermatocranium is composed of plates of dermal bone that cover the head and protect the brain and gills Six basic groups of dermal bones make up the dermatocranium (Table 7.3 and Fig. 7.10, p. 230): 1. Facial series - encircles the external nares and collectively form the snout - the maxilla and premaxilla (incisive) define the margins of the snout and usually bear teeth - the nasal lies medially to the naris - the septomaxilla a small dermal bone which, when present, sinks below the surface bones and aids in forming the nasal cavity2. Orbital series - encircles the eye and superficially defines the orbit - the lacrimal is associated with the nasolacrimal duct - the prefrontals, postfrontals, and postorbital ring above and behind the orbit - the jugal complete the lower margin3. Temporal series - lies behind the orbit and completes the posterior wall of the braincase - the otic (temporal) notch suspends the tympanic membrane - intertemporal, sipratemporal, and tabular make up the medial part of the series - usually lost in advanced species - squamosal and quadratojugal complete the lateral margins4. Vault series - the roofing bones that run across the top of the skull and cover the brain beneath - include the frontal (anteriorly), parietal (medially) and postparietal or interparietal (posteriorly)5. Palatal series - form the roof of the mouth - largest and most medial pterygoid - lateral elements vomer, palatine, and ectopterygoid - teeth may be present on any or all of these bones6. Mandibular series - Encases mandibular cartilage - Meckel’s cartilage is usually encased in dermal bones of this series - laterally, the wall includes the tooth-bearing dentary and one or two splenials, angular, and surangular - many wrap around to the medial side of the mandible and meet the prearticular and coronoids - left and right mandibles usually meet anteriorly at the midline in a mandibular symphasis    An Overview of Skull Morphology Agnathans: Early ostracoderms posessed a flattened head shield formed from a single piece of of arched dermal bone, two close-set eyes dorsally set and a pineal opening between them - formed the roof over the pharynx and held the sequential branchial arches - a ventral plate (cartilage') streched across the floor acted as a suctioning device to pull water into the mouth and across the gillsLamprey and hagfish as heirs to ostracoderms lost all bone and specialized for parasitic or scavenging lifestyleswith a rasping tongue - the braincase is entirely cartilage and branchial arches form an unjointed branchial basketAll vertebrates, with the exception of the agnathans have jaws and form the group Gnathostomes ("jaw mouth") - modification of the branchial arches into biting or grasping devices Fish: Much of the information describing the three regions of the skull is generally characteristic of the primitive fishes and Chondricthyes, and is the precursor for skull and visceral skeletal formation in tetrapods In Chondrichthyans, the braincase is an elaborate cartilagenous case around the brain. The only modification in the Osteicthyes is that the skull region and its associated cartilagenous structures are ossified. - the dermatochranium is absent, reflecting a loss of all bone from the skeleton - the otic capsue rests on the posterior part of the endocranium and encloses the sensory organs of the ear - the upper jaw consists of the fully functional endoskeletal palatoquadrate - makes limited contributions to higher vertebrates as the epipterygoid (fuses to the cranium) and quadrate (suspends the lower jaw); replaced by maxilla and premaxilla as the upper jaw - the lower jaw (mandible) consists only of Meckel’s cartilage - encased in exoskeletal bone of the dermatocranium and supports teeth; may be ossified as the mental (anterior) and articular (posterior) - the hyoid apparatus derives ventrally from the splanchnocranium as a support for the floor of the mouth and functional gillsIn the movement from fishes to tetrapods, several general changes in the skull may be noted: 1. A gradual reduction in the number of the separate bony elements by elimination and fusion - there may be as many as 180 bones in a fish’s skull whereas the human skull contains only 28 2. Autostylic method of jaw attachment - creates changes in the articulation of the jaw and the evolution of the hard palate 3. Shift in gas exchange mechanism from gills to lungs - requires the evolution of a pair of internal nostrils, or choanae that pass from the external nares to the lungs 4. Creates shift in the function of the visceral arches - no longer used to support gills and are often modified to perform different functions (Table 7.2, p. 228) - hyobranchial apparatus evolves to support the tongue and larynx - arch V form the cartilage of the larynx, or cricoid cartilage - other arches become the auditory ossicles or other cartilages5. Movement of the dermatocranium from a close relationship with the integument to a deeper position in the head where it articulates more closely with the chondrocraniumStages of dermatocranium development include the formation and fusion of the different cranial bones to surround the endocranium • include a stage in which there are temporary gaps between bones (fontanelles) and fusion to form sutures • sometimes results in the production of intermediary Wormian bones that are small islands of bone that are between suturesAmphibians General characteristics of the amphibian skull are strong deviations from the generalized tetrapod skull - only partial ossification of the skull occurs, with much of the chondrocranium remaining cartilagenous - the skull is broad and flat (Figure 7.30, p. 246) - in aquatic salamanders, as with fish, there is a unidirectional flow of food and water into the mouth and out the gill slits; in metamorphosed salamanders and adult frogs, this is replaced by bidirectional flow or a sticky tongueReptiles and Birds - Share common feature of a completely ossified skull and openings (fenestrae) in the outer dermatocranium in the temporal region 1. Diapsida: Superior and inferior temporal fenestrae, above and below the postorbital-squamosal bar. The condition in squamates and birds is highly modified. The lower arch is lost in squamates and in snakes the postorbital bar is also lost. 2. Synapsida: Inferior temporal fenestra only. Found in Mammalia and modified in that the postorbital bar is lost. 3. Euryapsida: Superior temporal fenestra only (Plesiosaurs and ichthyosaurs, both extinct groups) 4. Anapsida: No opening: found in turtles and related fossil forms emarginations of the posterior margin of the skull roof in recent turtles replace fenestrae to allow temporal muscle expansion - in reptiles the dermatocranium tends to be heavier than that of birds, which is thinner and has air spaces - birds also have more highly developed vision, which results in reinforcement of the eyeball with a ring of bones (sclerotic bones) that ring the orbit, but do not articulate with it (Figure 7-14, p. 233). - birds tend to have larger brain-to-body size ratios, requiring increased braincase size - birds have modification of the jaws into the beak and loss of articulated teethMammals - marked by a loss of the sclerotic bones and an increase in other structures associated with sense organs and change in the feeding apparatus - increased dependence on the senses of smell and hearing; evolution of turbinate bones (also called nasal conchae) that increase the surface area available for olfaction; further modification of branchial arches into the auditory ossicles: malleus, incus and stapes - formation of the hard palate or roof of the mouth that continues as the soft palate that allow for feeding while still being able to breathe; epiglottis also forms to deflect food away from larynx entrance and into esophagus - a hard palate also leads to shift in dentition; teeth located at jaw margins and socketed in the jaw itself; primitive condition of teeth is homodont thecodont, which is undifferentiated tooth structure in single alveoli (scokets); derived condition is heterodont, where teeth are modified for different functions: incisors used for nipping, canines for grasping or tearing, and premolars and molars used for cutting and grinding    Definitions Acoelous - centrum flat on posterior and anterior surface, characteristic of some reptiles and mammals Amphicoelous - centrum hollowed at anterior and posterior end, characteristic of teleosts and early reptiles Amphistylic - jaw is supported both by the hyomandibular and by a direct connection between the jaw and the chondrocranium; found in primitive cartilaginous fishes Atlas - the first cervical vertebra of terrestrial vertebrates, which articulates with the skull; nodding movements of the head occur between the atlas and the skull Autostylic - upper jaw (pterygoquadrate cartilage) articulates or is fused with the chondrocranium, lower jaw forms from the mandibular cartilage, and the jaw remains unsupported by the hyomandibular found in lungfishes, and in the tetrapod ancestors Axis - the second cervical vertebra of mammals; rotary movements of the head occur between the atlas and axis Calcified cartilage - cartilage containing deposited calcium salts. Found in the vertebrae of cartilaginous fish Carinate - having a massively enlarged sternum to support flight muscles Cartilage - a firm but elastic skeletal tissue whose matrix contains proteoglycan molecules that bind with water. Choanae - internal nostrils that pass from the external nares to the lungs Chondrocranium - anterior part of the axial skeleton that encases the special sense organs and contributes to the skeletal elements encasing the brain Chondrocyte - a mature cartilage cell Composite materials - heterogeneous materials composed of many different elements, that is generally much stronger than homogenous materials. Cricoid cartilage - cartilage of the larynx that develops from the hyobranchial apparatus Dermatocranium - consists of dermal bones that encase the chondrocranium and splanchnocranium and contribute to the braincase, jaws, and skeletal elements of the mouth (teeth) Elastic cartilage - cartilage containing elastin fibers that appears yellowish. Found primarily on external ear and epiglottis. Fibrocartilage - cartilage containing collagen fibers. Found in the intervertebral disks and pubic symphysis Fontanelle - temporary gaps between bones, such as those that occur during the formation and fusion of the cranial bones Heterodont - teeth modified for different functions Hyaline cartilage - cartilage with a clear translucent matrix. Found primarily on the ends of ribs and on the trachea Hyomandibular cartilage - dorsal part of the hyoid arch that articulates with the chondrocranium Hyostylic - upper jaw loses any major direct connection with the chondrocranium and the upper and lower jaws are supported solely by the hyomandibular; found in elasmobranchs and most bony fishes Lacunae - small cavities, such as those in bone or cartilage, that contain osteocytes or chondrocytes Membrane bone - superficial bones that lie in or just beneath the skin and develop from the direct deposition of bone in connective tissue Opisthocoelous - centrum convex on anterior surface, concave on posterior surface, characteristic of some of the vertebrae of reptiles and mammals Perichondrium - the connective tissue that lies on the outside of cartilaginous tissue Procoelous - centrum concave on anterior surface, convex on posterior surface, characteristic of some reptiles and amphibians Pygostyle - the fused caudal vertebrae of a bird that support the tail feathers Ratite - lacking a distinctive keel or sternum for attachment of breast muscles Sacrum - the union of two or more vertebrae and their ribs, by which the pelvis articulates with the vertebral column Sclerotic bones - bones that surround the orbit but do not articulate with it, and provide reinforcement for the eye Splanchnocranium - or visceral arches that support and move the gills and contribute to production of the jaws in gnathostomes Thecodont - also called homodont, or undifferentiated tooth structure Turbinate bones - bones of the nasal cavity that increase the surface area available for olfaction Wormian bones - intermediary bones, or small islands of bone that occur between sutures in the skull Zygaphophysis - articular processes that extend forward and backward of neural arches and help to strengthen union between vertebrae THE POSTCRANIAL SKELETAL The primary function of the cranial skeleton was to protect and support the brain and associated sensory organs. In contrast, the postcranial skeleton is used less for protection but more for support of the body and for locomotion The postcranial skeleton can be divided into two regions: Trunk - includes the vertebral column, ribs and sternum (part of axial skeleton) Appendicular skeleton - limbs and girdlesNotochord vs. the Vertebral Column In early vertebrates the notochord is a non-bony skeletal support for swimming by lateral undulation - consists of vacuolated cells surrounded by a thick fibrous sheath that maintains rigidity, but is also flexible - ventral and parallel to the spinal cord (= dorsal hollow nerve cord) in vertebrates - absent in hemichordates, present in the tail of most larval urochordates but lost in the adults - present in the body and tail of cephalochordates and vertebrates - present in the embryo of all vertebrates but, as the vertebrae form around it, it becomes constricted and in many groups, such as mammals, there is almost nothing that remains.The vertebral column is a basic support structure that is developed from individual metameric units called vertebrae which replaces the notochord The basic units of an individual vertebra are:  Vertebral/neural arch - surrounds and protects the spinal cord  Hemal arch - surrounds and protects the caudal artery and vein in fishes  Neural spine - spinous process that projects dorsally from the neural arch  Centrum - body of the vertebra which replaces the notochord; shape depends on the vertebrate class (Fig. 8.4, p. 277): * Amphicoelous - centrum hollowed at anterior and posterior end, characteristic of teleosts and early reptiles * Opisthocoelous - centrum convex on anterior surface, concave on posterior surface, characteristic of some of the vertebrae of reptiles and mammals * Procoelous - centrum concave on anterior surface, convex on posterior surface, characteristic of some reptiles and amphibians * Acoelous - centrum flat on posterior and anterior surface, characteristic of some reptiles and mammals Transverse process - spinous process that projects laterally from the centrum  Zygaphophysis - articular processes that extend forward and backward from neural arches and help to strengthen the union between vertebrae  Intervertebral disks - pads derived from part of the notochord and composed of fibrocartilage and connective tissue that lie between adjacent centra that help to cushion the connection In tetrapods, two general anatomical relationships occur between centra and neural arches:  aspidospondyly - all arch elements (intercentrum, pleurocentrum, and neural arch) remain as separate ossified elements  holospondyly - all vertebral elements are fused into a single piece For most vertebrates (except for most fishes), regional variation in the appearance of the vertebrae are recognized (Fig. 8.2, p. 277):  Cervical - vertebrae of neck (not found in fishes)  Atlas - the first cervical vertebra of terrestrial vertebrates which articulates with the skull; nodding movements of the head occur between the atlas and the skull  Axis - the second cervical vertebra of mammals; rotary movements of the head occur between the atlas and axis  Thoracic - vertebrae of chest region, articulate with ribs  Lumbar - vertebrae of lower back  Sacral - lower vertebrae that are fused (three fused in dog and cat, five fused in human) as the sacrum  Caudal - vertebrae of tail (pygostyle in birds forms attachment for tail feathers) Ribs and Sternum The ribs and sternum serve several general purposes: - help to strengthen the body wall and lend support to the thorax - protect the organs of the thoracic region (circulatory and respiratory) - used as the site of muscle attachment - in amniotes they are used to assist in breathingThere are two general ways in which ribs can develop in different vertebrate classes: Intermuscular (dorsal) ribs - develop in the myosepta between the myomeres and attach to centra of vertebrae, between the dorsal and ventral muscle masses Subperitoneal (ventral) ribs - form between the ventral muscles and the lining of the coelomWith the exception of the agnathans that have no ribs at all, fishes generally show both types of rib morphology - may be found throughout the trunk, as is the case in salamanders and reptiles, or only in the thoracic region, as is the case for mammals and birdsMammalian ribs articulate with the vertebrae at two points: - the dorsal point of articulation tuberculum - the ventral point of articulation capitulum - also articulate differently with the sternum, and can be either true ribs (directly attached to the sternum via costal cartilages - 7 pairs in human), false ribs (attached indirectly to the sternum through the costal cartilages of the other ribs - 5 pairs in human) and floating ribs (do not attach to the sternum - 2 pairs in humans).The sternum forms a complete enclosure of the chest region in conjunction with the ribs and is connected closely to the shoulder girdle and ribs. - possessed only by tetrapods, with the exceptions of snakes and turtles (completely lacking) - acts to protect the thoracic region - serves as a site for attachment of the pectoral limbs - aids in rib movementsIn birds, the sternum is completely ossified: • ratite - lacking a distinctive keel or sternum for attachment of breast muscles • carinate - having a massively enlarged sternum to support flight musclesIn mammals, the sternum is divided into three regions (anterior to posterior): • manubrium • sternebrae (ossified bony elements) • xiphisternum and xiphoid cartilageSeveral vertebrates, including lizards and crocodiles also posess gastralia: - ribs of dermal origin restricted to the sides of the ventral body wall - do not articulate with the vertebrae but act as an accessory skeletal system for muscle attachment and support for the abdomen Definitions Acoelous - centrum flat on posterior and anterior surface, characteristic of some reptiles and mammals Amphicoelous - centrum hollowed at anterior and posterior end, characteristic of teleosts and early reptiles Amphistylic - jaw is supported both by the hyomandibular and by a direct connection between the jaw and the chondrocranium; found in primitive cartilaginous fishes Atlas - the first cervical vertebra of terrestrial vertebrates, which articulates with the skull; nodding movements of the head occur between the atlas and the skull Autostylic - upper jaw (pterygoquadrate cartilage) articulates or is fused with the chondrocranium, lower jaw forms from the mandibular cartilage, and the jaw remains unsupported by the hyomandibular found in lungfishes, and in the tetrapod ancestors Axis - the second cervical vertebra of mammals; rotary movements of the head occur between the atlas and axis Calcified cartilage - cartilage containing deposited calcium salts. Found in the vertebrae of cartilaginous fish Carinate - having a massively enlarged sternum to support flight muscles Cartilage - a firm but elastic skeletal tissue whose matrix contains proteoglycan molecules that bind with water. Choanae - internal nostrils that pass from the external nares to the lungs Chondrocranium - anterior part of the axial skeleton that encases the special sense organs and contributes to the skeletal elements encasing the brain Chondrocyte - a mature cartilage cell Composite materials - heterogeneous materials composed of many different elements, that is generally much stronger than homogenous materials. Cricoid cartilage - cartilage of the larynx that develops from the hyobranchial apparatus Dermatocranium - consists of dermal bones that encase the chondrocranium and splanchnocranium and contribute to the braincase, jaws, and skeletal elements of the mouth (teeth) Elastic cartilage - cartilage containing elastin fibers that appears yellowish. Found primarily on external ear and epiglottis. Fibrocartilage - cartilage containing collagen fibers. Found in the intervertebral disks and pubic symphysis Fontanelle - temporary gaps between bones, such as those that occur during the formation and fusion of the cranial bones Heterodont - teeth modified for different functions Hyaline cartilage - cartilage with a clear translucent matrix. Found primarily on the ends of ribs and on the trachea Hyomandibular cartilage - dorsal part of the hyoid arch that articulates with the chondrocranium Hyostylic - upper jaw loses any major direct connection with the chondrocranium and the upper and lower jaws are supported solely by the hyomandibular; found in elasmobranchs and most bony fishes Lacunae - small cavities, such as those in bone or cartilage, that contain osteocytes or chondrocytes Membrane bone - superficial bones that lie in or just beneath the skin and develop from the direct deposition of bone in connective tissue Opisthocoelous - centrum convex on anterior surface, concave on posterior surface, characteristic of some of the vertebrae of reptiles and mammals Perichondrium - the connective tissue that lies on the outside of cartilaginous tissue Procoelous - centrum concave on anterior surface, convex on posterior surface, characteristic of some reptiles and amphibians Pygostyle - the fused caudal vertebrae of a bird that support the tail feathers Ratite - lacking a distinctive keel or sternum for attachment of breast muscles Sacrum - the union of two or more vertebrae and their ribs, by which the pelvis articulates with the vertebral column Sclerotic bones - bones that surround the orbit but do not articulate with it, and provide reinforcement for the eye Splanchnocranium - or visceral arches that support and move the gills and contribute to production of the jaws in gnathostomes Thecodont - also called homodont, or undifferentiated tooth structure Turbinate bones - bones of the nasal cavity that increase the surface area available for olfaction Wormian bones - intermediary bones, or small islands of bone that occur between sutures in the skull Zygaphophysis - articular processes that extend forward and backward of neural arches and help to strengthen union between vertebrae     |