F. AXIAL SKELETON – VERTEBRAE, RIBS, AND STERNUM
1. Discuss the overall function of vertebrae.
Vertebrae functions in locomotion, protects the spinal cord, and suspends the trunk.
2. Describe the structure of a typical vertebrae, including the following: centrum, neural arch, neural spine, hemal arch, hemal spine, chevron bone, neural (vertebral) canal, diapophyses, zygapophyses, prezygapophyses, and postzygapophyses.
Centrum – principal part of a vertebra; below spinal cord; may consist of one or two elements.
If two, the more anterior called an intercentrum, the more posterior (sometimes paired) called a pleurocentrum. If one, it may be the intercentrum (some extinct amphibians) or the pleurocentrum (amniotes)
Neural arches – arches over the spinal cord
Neural spine – projection from neural arch
Hemal arch – extends ventrally from centrum to surround blood vessels
Hemal spine – ventral projection from hemal arch
Chevron bone – hemal arch in amniotes
Diapophysis – transverse process that supports the tuberculum of a rib
Zygapophysis–process on the neural arch or spine; vertebral articulation
Pre–face upward or inward
Post–face downward or outward
3. List the four main types of intercentra articulations.
Neural arch, neural canal, transverse process, zygapophyses
4. Address the development of vertebrae. What is the fate of the notochord?
The somites develop a central cavity polulated by core cells, before rupturing on their medial sides, forming: the sclerotome medially, that surrounds the developing notocord and neural tube and the dermomyotome that is displaced laterally, and will itself split into a dermotome and a myotome. The sclerotome surrounding the notochord ventrally will develop into vertebral bodies, while the dorsal portion surrounding the developing neural tube will form vertebral arches, under the influence of inducing substances. Each sclerotome then splits into cranial and caudal segments, with spinal segmental nerves emerging between this split, and the vertebrae are formed from the fusion of the caudal half of each sclerotome with the cranial half of the next, with the cranial half of the first sclerotome fusing with the occipital bone. The intervertebral discs then form from the sclerotomes, with their nucleus pulposus core derived from notochord cells. In reptiles, birds, and mammals, the notochord almost disappears during development (e.g., remains as a pulpy nucleus in the vertebrae of mammals).
5. Define diplospondyly.
Diplospondyly – the condition in which vertebrae have two centra, the central portion of the vertebra, which in early vertebrates, was taken up by the notochord
6. Describe the vertebral column of advanced fish. Include regional differentiation and the craniovertebral joint.
In advanced fishes, the notochord is replaced by centra, and the vertebrae exhibit diplospondyly, having two or more complete vertebrae per muscle segment. In bony fishes, the centra ossify directly from mesenchyme surrounding the notochord. Regional differentiation of the vertebrae exists as dorsal vertebrae form the trunk and caudal vertebrae form the tail. The craniovertebral joint usually resembles an intervertebral joint.
7. Explain the changes in the vertebrae necessary for adaptation to terrestrial life.
The pectoral and pelvic girdles were modified and strengthened to allow the legs to support the weight of the body. The vertebral column was strengthened by the development of interlocking processes (called zygapophyses) between adjacent neural arches, and by the eventual replacement of the notochord by one or two pairs of bony rings and/or nodules (centrum elements) seen around the notochord in the crossopterygian fishes. In addition to problems of locomotion and support, the transition to land involved a large number of other challenges. Once on land, gills would prove useless. They would be incapable of providing effective absorption of oxygen from air (and release of carbon dioxide) and would release too much water from the bloodstream to the outside air, resulting in drying of animal (dessication). Internal lungs solved both of these problems by providing a specialized structure with a lot of small, branched air passageways which were effective in transfer of gases to and from the air and conservation of water. The first amphibians used the scaly exterior of the fish to prevent drying out of the body surface. Another interesting adaptation is that of hearing. The ear of terrestrial vertebrates evolved by two modifications: first, a fluid-filled channel in the inner ear improved efficiency of sound transmission to the hair cells. And, upper part of the hyoid arch, a bony jaw brace in early fishes, became the stapes, located in a new part of the ear, the middle ear. These structures improve the efficiency of transmission of sound from air to the hair cells of the inner ear.
8. Discuss the phylogeny of tetrapod vertebrae based on intercentrum (hypocentrum) and pleurocentrum.
Centrum are the principal part of a vertebra, located below spinal cord, and may consist of one or two elements. If two, the more anterior called an intercentrum, the more posterior (sometimes paired) called a pleurocentrum. If one, it may be the intercentrum (some extinct amphibians) or the pleurocentrum (amniotes). Rhipidistian crossopterygians has vertebrae similar to those of earliest amphibians (Ichthyostegalia) with the notochord persistent, a neural arch, a large intercentrum, and small, paired pleurocentra. Two principal lineages are derived from this common origin. The first lineage includes Rhachitomi, group of labyrinthodonts that replaced the notochord by a large intercentrum and a pair of small pleurocentra, and the Stereospondyli that retained only the intercentrum. The second lineage includes Embolomeri that increased the size of pleurocentrum to the size of intercentrum, and amniotes that enlarged pleurocentrum to near or complete exclusion of intercentrum.
9. Describe articulations of amphibian vertebrae.
An important anatomical feature is the presence of zygapophyses, processes of the neural arch of a vertebra that articulate with corresponding parts of adjacent vertebrae,which allow the vertebrae to interlock, forming a stiff vertebral column. In amphibians, the presence of zygapophyses limits dorsal/ventral bending.
10. Identify the regions of vertebrae in amphibians. What is a urostyle?
Cervical region – first vertebra modified
Trunk vertebrae – bear ribs, except in most anurans
Sacral vertebra – single, enlarged vertebra to articulate with pelvic girdle
Caudal vertebrae – lack zygapophyses, have hemal arches
Urostyle – a styliform process forming the posterior extremity of the vertebral column in some fishes and amphibians
11. Characterize reptilian vertebrae. Include regional differentiation. Explain how their cervical and sacral vertebrae differ from those of amphibians.
Cervical region – more distinct than amphibians; eight vertebrae, including atlas and axis which provides greater flexibility
Dorsal region – thoracic and lumbar vertebrae
Sacral region – two vertebrae; provides a stronger support and a bigger anchor for pelvis
Caudal region – lack zygapophyses; hemal arches present as chevron bones; autonomous
12. Define autotomy.
Autotomy – the ability to regenerate a new tail
13. Describe bird vertebrae. Include regional differentiation, heterocelous, synsacrum and pygostyle.
Centrum – heterocelous with saddle-shaped articular facets at both ends, concave and convex for extra mobility
Cervical region – numerous cervical vertebrae, with single occipital condyle
Trunk region – four to six vertebrae tend to fuse
Synsacrum – fusion of last thoracic, all lumbar, two sacral, and a few caudal vertebrae; allows for streamline shape and a rigid brace
Pygostyle – fusion of last few caudal vertebrae
14. Characterize mammalian vertebrae. What are intervertebral discs? What is their composition?
Centrum –unique in having centra with epiphyses; acelous, without a cavity; lay flat against each other
Cervical region – seven vertebrae with “smooth spot” for rib articulation
Trunk region – usually about twenty vertebrae divided into thoracic, anterior region which bear ribs, and lumbar, posterior region with no ribs
Sacral region – three or more sacral vertebrae fuse to form a sacrum
Caudal region – chevron bones usually restricted to base of tail; coccyx
Intervertebral discs- discs between acelous vertebrae; compose of fibrocartilage and a pulpy nucleus
15. Compare the craniovertebral joint in regards to structure and physiology amongst the vertebrate classes.
Fishes – Cranio-vertebral joint usually resembles an intervertebral joint; allows for no movement
Amphibians – the first vertebrates to have a neck; more flexible cranio-vertebral joint; first vertebra modified; second slightly modified; 1-2 occipital condyles
Reptiles and birds – ball-and-socket joint between first vertebra (atlas) and the single occipital condyle; specialized joint between atlas and axis allows for rotation
Mammals – joint between atlas and axis permits side-to-side motion and rotation around the spine; two occipital condyles
16. Discuss evolutionary trends of vertebrae.
Unlike most tetrapods today, vertebral column of earliest tetrapods did not consist of 1 bone/body segment. Crossopterygian vertebrae consisted of an hypocentrum (a large, wedge-shaped piece) plus 2 pleurocentra (smaller, intersegmental pieces). This type of vertebra is called a rachitomous vertebra. The ‘trend’ in vertebra evolution has been for pleurocentra to increase in size (and, of course, for the hypocentrum to decrease in size).
17. Discuss the functions of ribs.
The function of ribs is to protect internal organs in the thoracic cavity. Also, some animals rely on costal breathing, that is the use of the rib cage to expand the rib cage to inhale and to compress the lungs to exhale.
18. Theorize the phylogenetic origin of ribs.
Ribs are considered to be relatively new structures. The ribs were thought to be ventral processes of the axial skeleton and therefore to be derived from the sclerotomes; however, recently a dermomyotomal origin of the distal rib (the costal shaft) was suggested, with only the proximal parts (head and neck of the rib) being of sclerotomal origin. Due to the transition of water to land, early tetrapod ribs were enlarged, giving rigidity to the trunk. Modified sacral ribs provided bony connection of hind limbs to vertebral column to enhance weight-bearing. Ichthyostega, the first Devonian tetrapod, was probably “more terrestrial” having heavy ribs that formed a “barrel” inside the trunk. It could not flex trunk for walking or swimming.
19. Compare dorsal and ventral ribs.
Dorsal ribs – where septa intersect with the partition between dorsal and ventral muscle masses
Ventral ribs – form between hypaxial muscles and the lining of the coelom
20. Contrast monocipital and bicipital ribs. Define tuberculum and capitulum.
As opposed to “monocipital” ribs, or “one-headed” ribs, a bicipital rib occurs with the fusion of the first thoracic rib with cervical vertebra. The tuberculum is the dorsal head of the vertebrae that articulates with the transverse process. The capitulum is the ventral head of a vertebrae that articulates with the centrum.
21. Contrast costal and sternal ribs and costal cartilage.
Thoracic ribs usually have two parts, a bony part (vertebral/costal rib) articulating with vertebrae, and a cartilage part called a costal cartilage (or sternal rib) which articulates with the sternum.
22. Describe the ribs in fish, amphibians, reptiles, birds, and mammals.
Jawless fishes and placoderms – have no ribs
Advanced fishes – most have ribs; most are ventral ribs that enclose body of ray-finned fishes; sharks have dorsal ribs
Tetrapods – have ribs in position of ventral ribs, but they are derived from dorsal set; cervical ribs may articulate with vertebrae, fuse with vertebrae, or be lost; caudal ribsmay be free, fused, or lost
Amphibians – ribs are short in anurans and urodeles, long in apodans
Reptiles – ribs are short in neck, long in trunk
Turtles have no cervical ribs, trunk ribs fuse to form carapace, sacral ribs fuse to pelvis.
Snake ribs are long and curved.
Crocodiles have “abdominal ribs” or gastrula which are integumentary structures.
Birds – have costal and sternal ribs, with uncinate processes for muscle attachment
Mammals – have thoracic ribs, some are true, some false, and some floating
23. Define uncinate process, true rib, false rib, and floating rib.
Uncinate process – hooked projections on a bone for muscle attachment
True rib – attaches directly to the sternum with its own costal cartilage
False rib – do not directly connect to the sternum; no costal cartilage
Floating rib – false rib with no ventral attachment; no costal cartilage
24. Name the vertebrates that possess a sternum.
Sternums are features of tetrapods with the exceptions of snakes and turtles.
25. Define carina, keel, and xiphoid process.
Carina – keel-shaped structures or ridges such as that on the breastbone of a bird used for flight muscle attachment
Keel – the median ridge on the breastbone of birds that fly
Xiphoid process – the cartilage at the lower end of the sternum
26. Compare and contrast the sternum amongst the vertebrate classes.
Some amphibians have a sternum (not present in fish), and some have ribs; but in no case do the ribs contact the sternum. With the exception of snakes and turtle that do not possess a sternum, the reptile sternum is large, and may be cartilaginous or bony. In birds, the sternum is completely ossified. In ratites, the sternum lacks a distinctive keel for attachment of breast muscles. Carinates have a massively enlarged sternum to support flight muscles.
In mammals, the sternum is divided into three segments (anterior to posterior), the manubrium, the sternebrae (ossified bony elements), and the xiphisternum and xiphoid cartilage.