Subphylum Vertebrata

1. Subphylum Vertebrata • The vertebrates are a large and diverse group including the fishes and tetrapods (animals with 4 limbs). • Vertebrates possess the basic chordate characteristics, but also a number of novel homologous structures. • An alternative name for the group Craniata is actually a better descriptor for the entire group because all members possess a cranium, but some jawless fishes lack vertebrae.

2. Important developments of the Vertebrates • Musculoskeletal system. Vertebrates possess an endoskeleton, which is much more economical in materials than the exoskeleton of invertebrates. • It forms a jointed scaffolding for the attachment of muscles. Initially the endoskeleton probably was cartilaginous (it still is in jawless fishes and sharks) and later became bony in many groups.

3. Vertebrae • The term vertebrate is derived from the vertebrae, which are arranged in series to from the vertebral column or backbone. • During embryonic development the vertebrae develop around the notochord, which degenerates to form part of the intervertebral disks. The vertebrae also enclose the hollow dorsal nerve cord, protecting it from damage. • The vertebrae, ribs and skull form the axial skeleton, while the limbs, pelvic and pectoral girdles form what is referred to as the appendicular skeleton.

5. Important developments of the Vertebrates: Bone • Bone is a uniquely vertebrate characteristic. • Bone is stronger than cartilage, which makes it a better material to use for muscle attachment in places where mechanical stress may be high. • Bone may have evolved initially as a means of storing minerals and was later adapted for use in the skeleton. In many early vertebrates bone also provided protection against predators.

6. Connective tissue • Cartilage and bone are mineralized connective tissues (blood is also a connective tissue). • Connective tissues are generally characterized by a distinctive cell type surrounded or embedded in a comparatively abundant extracellular matrix.

7. Cartilage • Cartilage has lots of collagen (a type of protein) fibers embedded in a rubbery matrix of chondroitin sulfate. • The chondroitin sulfate and collagen are secreted by chondrocytes (cartilage cells) that are scattered in spaces throughout the matrix.

9. Cartilage • Collagen fibers have high tensile strength so they do not easily tear when stretched. • Together the composite of collagen fibers and chondroitin sulfate makes cartilage a strong, but somewhat flexible material.

10. Cartilage • There are several different types of cartilage that differ in structure depending on their function. • The more tensile strength required the more collagen fibers are present. If the cartilage must be springy and flexible more elastic fibers are present.

11. Hyaline cartilage • Hyaline cartilage is the most widespread. In embryos the skeleton is made of hyaline cartilage before bone formation occurs. • In adults hyaline cartilage occurs at the articulating surfaces of bones. It is smooth in appearance because the matrix contains relatively few collagen fibers. The smooth matrix however is very good at resisting compression.

12. Fibrocartilage and elastic cartilage • Fibrocartilage contains lots of collagen fibers, which gives it the ability to better resist tensile forces. It’s found where stretching is likely to occur and so occurs in the intervertebral disks and the pubic symphysis. • Elastic cartilage contains lots of elastic fibers and is springy. Your epiglottis and ear are good examples.

13. Cartilage • Cartilage, unlike bone, does not have a blood supply that penetrates into it to supply the chondrocytes. • Instead the chondrocytes are supplied by long-range diffusion of nutrients and gases through the matrix.

14. Bone • The skeleton of sharks is made of cartilage. • Other vertebrates have an embryonic skeleton that is made of cartilage, but most of the cartilage is replaced by bone as embryonic development proceeds.

15. Bone • Bone, like cartilage, is a mineralized connective tissue. • Bone differs from cartilage in cell type (osteocytes), the composition of the matrix (calcium phosphate in the form of hydroxyapatite) and in that it is vascularized. • Bone is also generally much more highly organized than cartilage.

16. Bone • In embryonic development hyaline cartilage is replaced by bone. • During this process of ossification calcium salts are deposited that seal off the chondrocytes, which then die. • Next blood vessels invade the calcified matrix producing channels and ultimately the marrow cavity.

17. Bone • Finally osteoblasts (bone cells) are introduced and these lay down the calcium phosphate matrix that finishes the conversion of cartilage to bone.

18. Intramembranous bone development • Not all bone develops via a cartilage precursor. • Many bones are formed directly from mesenchyme (a tissue which consists of loosely organized cells derived from the mesoderm).

19. Intramembranous bone development • As mesenchyme cells condense they are supplied with a dense supply of blood vessels and produce a gel-like matrix into which bars of bone matrix are deposited as osteoblasts are introduced. • Over time the bars of bone matrix are enlarged as new layers are laid down and eventually the gel matrix is entirely replaced.

20. Intramembranous bone development • Among the intramembranous bones are dermal bones and sesamoid bones. • Dermal bones develop within the dermis of the skin. Many bones of the skull, pectoral girdle and integument are formed in this way. • Sesamoid bones form within tendons. Examples include the patella and the pisiform bone of the wrist.

21. Pisiform bone (D) From http://en.wikipedia.org/wiki/File:Carpus.png

22. Structure of bone • Human bone is characterized by structures called osteons. These are concentric rings of osteocytes and the matrix laid down by them. • Nerves and blood vessels pass through a central canal in each osteon and there are also diagonal connections between osteons (Volkmann’s canals) which allow blood vessels to interconnect.

24. Structure of bone • Bone made up of osteons is found in many different vertebrates, but it is not the only pattern of bone structure. • In many teleost fishes bone is acellular (there are no osteocytes within the calcium phosphate matrix). Bone in these fish is laid down by osteocytes on the surface of the bone that do not become encased in their own secretions

25. Structure of bone • In amphibians and reptiles bone is often laid down seasonally so that growth rings may be apparent. • It is important to remember that bones are dynamic, not static. Thus, they respond to the daily stresses imposed on them and to other changes in the body. Forensic anthropologists can infer a lot about an individual’s life and experiences simply by examining bones.

26. Evolution of bone • Bone occurs only in vertebrates and it has been suggested that it may have originated as a way of storing calcium or phosphate. • Vertebrates evolved in the oceans and would naturally have tended to accumulate calcium and phosphate because of their high concentration in seawater.

27. Evolution of bone • Calcium and phosphate are important in cellular metabolic pathways so selection could have favored storing them. • Large stores in the skin would have produced a protective hard surface that provided protection against predators and selection could have then led to the armor found in early fishes.

28. 15.10 Ostracoderms

29. Evolution of bone • Only later did a bony endoskeleton develop presumably under selection for increased mechanical support. • The hard inorganic component of bone is calcium phosphate (in the form of hydroxyapatite) rather than the calcium carbonate found in invertebrates.

30. Evolution of bone • This difference in the form of calcium used is believed to be due to the fact that vertebrates have always engaged in intense bursts of anaerobic activity. • Anaerobic metabolism produces lactic acid which lowers blood pH. Under acid conditions calcium carbonate dissolves, which is not a desirable trait for a skeleton.

31. Evolution of bone • By using a skeleton of calcium phosphate rather than calcium carbonate vertebrates gained mechanical protection while avoiding the problem of bone dissolution.

32. Other mineralized tissues • These include enamel and dentine which are the hard components of teeth. Enamel forms the grinding surface and dentine is below. • Enamel is about 99% and dentine 90% mineralized. Bone and cartilage in contrast are both only about 70% mineralized. Because enamel is so hard, teeth are the most common fossils. • A third important mineralized tissue is cementum. This is a bone-like substance that holds teeth in their sockets e.g. in mammals.

33. Important developments of the Vertebrates: gas exchange • Various aspects of vertebrate physiology have been upgraded to meet increased metabolic demands. • For example the pharynx, which was used for filter feeding in primitive chordates has had muscles added that make it a powerful water pumping organ. • With the conversion of the pharyngeal slits to highly vascularized gills the pharynx has become specialized for gas exchange.

34. Gas exchange • Ancestral chordates probably depended on diffusion for gas exchange (as does Amphioxus). • Vertebrates being more active need specialized gas exchange structures and gills and lungs evolved to satisfy the need.

35. Salmon Gills

36. Gas exchange • Both gills and lungs have very large surface areas to increase the area available for diffusion of gases. • Gills are not self supporting and so are ineffective out of water. The conquest of the land required the elaboration of lungs from simple vascularized sacs to much more complex structures.

37. Gas exchange • Elaborations of the lungs include selection for increased surface area to maximize the rate of gas exchange and in birds a one-way flow of air through the lung (facilitated by a series of air sacs) that increases the efficiency of oxygen extraction from the air.

38. Cardiovascular system • Vertebrates possess a closed circulatory system and this enables higher pressures to be maintained in the system than is possible in the open circulatory system of non vertebrates chordates and invertebrates. • Blood carries oxygen and nutrients to all of the body’s tissues and removes carbon dioxide and other wastes.

39. Cardiovascular system • Blood vessels consist of arteries, veins and capillaries. • Arteries have thick muscular walls to enable them to withstand the pressure generated by the heart. Arteries carry oxygenated blood to the tissues.

40. Cardiovascular system • Capillaries are thin-walled (one cell thick) vessels where gases are exchanged with the tissues. • They are very narrow but very numerous and blood pressure drops as the blood passes through the capillary beds because of the overall increased diameter of the tubes.

41. Cardiovascular system • Veins have thinner, elastic walls and possess valves to prevent the backflow of blood. • They return deoxygenated blood from the tissues.

42. Cardiovascular system • Circulatory systems vary among vertebrates from single circulation systems (as in fish) where a single heart beat is used to propel blood around the entire circuit to the double circulatory system in mammals in which blood goes to the heart twice once to be pumped to the lungs and once to the tissues.

43. Cardiovascular system • Double circulatory systems have much higher pressure and are more efficient. • In addition, they keep oxygenated and deoxygenated blood from mixing. • Circulatory systems of different groups will be discussed in detail later.

44. Vertebrate circulatory systems Gill capillaries Lung and skin capillaries Lung capillaries Lung capillaries AMPHIBIANS REPTILES (EXCEPT BIRDS) MAMMALS AND BIRDS FISHES Right systemicaorta Pulmonarycircuit Artery Pulmocutaneouscircuit Pulmonarycircuit Gillcirculation Heart:ventricle (V) Left Systemicaorta A A A A A A Atrium (A) V V V V V Left Right Left Left Right Right Systemiccirculation Systemic circuit Systemic circuit Vein Systemic capillaries Systemic capillaries Systemic capillaries Systemic capillaries