Bio 205, Lecture 12
Some links
To understand the function of the skeleton, it is important to know some terms.
a load is a force, for example, when standing, the
force being applied to the heel and ball of your feet is your weight. This
force is literally pushing you up by the contact points on the bottom of your
feet.
a tensile load is a force that stretches a structure.
a compressive load is a force that compresses
a structure.
a bending load is a set of forces that bends
a structure. For example, when I hold a barbell of weights with my forearms
stretched out, the load of the weights wants to rotate my forearm at the
elbow. If I resist this rotation with a force applied by my biceps muscles,
the weights apply a bending load to the forearm bones and the bones will
actually bend a little, if the load is large enough. Importantly, a bending
load puts one side of the structure in compression and the opposite side
in tension.
a torsional load is a force couple that twist
a structure.
tension, compression, bending, and torsion are different ways of deforming a structure.
A stiff structure takes a large load to deform
it only a little bit. Its opposite is compliant
A strong structure takes a large force before
it fails (permanently bends or breaks). Its opposite is weak
An elastic structure returns to its original shape when the load is released.
Its opposite is plastic.
A tough structure requires huge energy (force*deformation) before it fails.
An axial skeleton needs to resist compression in animals swimming in a dense fluid and a solid bony bar would obviously do well for this purpose. But a solid bony bar is not flexible and would not allow the undualtory movements for swimming. A series of bony bars is not a real solution because bending on one side would need to compress the inside of the bar, which is not going to happen with bone. Or, the bony bars could hinge, but this would stretch the nerve cord. So the bony bars need to be separated. One partial solution, then, is a series of separated bony bars that are taped together by a meterial that is strong in tension (so they don't snap when stretched as the column is bent) but allows bending. But separated bony bars (even ones that are taped together) is not a final solution because the interbar region can easily be compressed. So the column of bars needs to have a structure between the bony bars that allows some stretching and compression during column bending.
Terrestrial animals have a bit of a different problem, that is, a flexible vertebral column would collapse under its own weight. So terrestrial animals have added features to make the column less flexible. But flexibility is still important because it allows an animal to turn its head or to stretch its back or to curl its tail - all behaviors important in terrestrial vertebrates.
Left figure: lateral view of teleost vertebral column
middle figure: cross-section of teleost trunk anterior to anus
right figure: cross-section of post-anal trunk
This cross-section of the post-anal body illustrates the organization of the typical caudal axial skeleton of a teleost fish. At the center, there is a vertebral centrum (body), which is endochondral bone. Dorsal to the centrum is a neural arch, through which the neural tube passes. Paired spinal nerves exit the spinal cord between nueral spines through a window called the intervertebral foramen. Dorsal to the arch is a neural spine. Ventral to the centrum in the tail section is a hemal arch, through which the caudal artery and vein run and ventral to this is a hemal spine. In the pre-anal trunk, there is no hemal arch, probably because there is no muscle that can squeeze the dorsal aorta. Intervertebral pads or disks, consisting of the notochord, cartilage and connective tissue lies between the centra. The vertebral bodies are connected along the axis by intervertebral ligaments and there may be additional ligaments connecting the adjacent arches. Together, the centrum and arches make a vertebra. The array of vertebrae down the axis is called the vertebral column. Telosts generally have one or two sets of ribs. Dorsal ribs occur in the horizontal septum (see below) while ventral ribs occur in the connective tissue surrounding the coelom (the peritoneum).
The muscles surrounding the vertebra and are compartmentalized by skelatogenic septa made of dense connective tissue, including the median skelatogenous septum, the horizontal skelatogenous septum, and variable number (sometimes zero) of other horizontal septae dorsal and ventral to the main horizontal septum. The muscles themselves are cone-shaped and surrounded by myosepta, which histologically, is called the epimysium. A muscle itself is didvided into a bunch of bundles called fasciculi and each fascicle surrounded by connective tissue called the perimysium. Finally, within each fascicle is a bundle of individual muscle cells or fibers and each fiber is surrounded by connective tissue called the endomysium.
Finally, most externally, we have the skin, which includes connective tissue made of collagen and elastin fibers in the dermis and possibly dermal bones.
The trunk muscles are segmental, as they develop from distinct somites, and this segmentation is easy to see in the myomeres of amphioxus, or a skinned teleost fish. The vertebrae are intersegmental, that is, they develop at the junction of two segments. Indeed, the sclerotome splits into a cranial and caudal half and the caudal half of a more anterior somite joins with the cranial half of the adjacent, posterior somite. The intersegmental vertebrae allows the muscle in a segment to cross the joint between the two intersegmental vertebrae. For derived vertebrates, this is not a problem since muscles tend to fuse during development and cross many segments but for the origin of vertebrates with a muscle-actuated, segmented vertebral column, this design was necessary.
Teleosts typically have amphicoelous centra (concave at both anterior and posterior ends) with intervertebral pad between the centra. The pads resist small compressive loads because of the thick central section and allow bending by having more compliance than bone and a smaller peripheral section.
Occasionally, the pad of the vertebral column may slip under bending loads, which are particularly large for terrestrial vertebrates. Consequently, terrestrial vertebrates tend to have a ball-and-socket type intervertebral joint where the intervertebral pad fuses to one vertebral centrum and forms a ball that then fits into the socket of the adjacent centrum. If the centrum with the concave surface is caudal to the joint, then this is opisthocoelous. If it is cranial to the join, it is procoelous. The vertebrate allow some torsion and excessive bending does not stretch the nerve cord because the center of bending is in the center of the centrum and not at its edge.
The vertebrae of birds and mammals tend to be acoelous, that is, the cranial and caudal surfaces are flat, so this shape is more stable in bending.
Finally, the neck of birds have heterocoelous vertbrae. The articular surfaces are saddle shaped, which allows extensive bending but limits rotation.
Fish
The basic structure of the axial skeleton in teleosts was described above.
Chondrichthyes
- no ossification although calcium salts are deposited in centra in some sharks
- very short ribs
- very amphicoelus vertebrae with notochord in intervertebral space
- alternating neural and intercalary archesHagfish
- Hagfish have no vertebrae
Lampreys
- Lampreys have no centra and small cartilage elements associated with spinal nerves
primitive actinopterygians, primitive sarcopterygians
- generally consists of neural and hemal arches that rest on large notochord
- Centra small in primitive actinop and sarcopsRegional differentiation
- trunk and caudal vertebrae recognized by presence of hemal arches on caudal
- highly modified tailhetero, vs.homocercal
symmetric vs. asymmetric function
Amphibians and primitive tetrapods
- Immediate fish ancestors of tetrapods had two, dorsal pleurocentra and single, ventral, intercentrum
- general trend in tetrapods is to lose either inter- or pleurocentra and expand the other
- modern amphibians - don't know if centrum is homologous to inter or pleuro
- procoelous and opistocoelous
- zygaphophyses present
- regionalization into atlas, trunk, sacral (1), and caudal
- frogs have only 4-8 (typically 7) vertebrae plus a urostyle and have lost the ribs
- some caecilians have 285 vertebrae
- sternum - a ventral endochondral element. In many tetrapods, ribs will wrap completely around the body wall and articulate with the sternum.
amniotes
- pleurocentra expanded into centrum
- intercentra retained as small bones in reptiles or fused with chevron bones (modified hemal arch)
- atlas and axis. The atlas has no body or centrum. It is generally believed that the odontoid process of the axis, which sticks anteriorly into the atlas, is the centrum (derived from pleurocentrum) of the axis. The intercentrum and hemal arch make up most of the atlas ring.
reptiles
- variable number of cervical vertebrae with short ribs
- ribs on all vertebrae except posterior caudal
- gastralia, or abdominal ribs. Some are dermal, some endochondral. Endochondral ones may be serially homologous with the sternum.
- autotomize - tail breaks within vertebra in some lizards
- vertebrae and ribs and dorsal bones form carapace of turtle shell
- plastron composed of dermal bones, gastralia (abdominal ribs) - note that shoulder and pelvic girdle are inside of ribs
- snakes have up to 400 vertebrae with little regional differentiation
birds
- 11-25 cervical vertebrae with heterocoelus joints allowing great mobility and lacking ribs (or ribs do not articulate with the sternum)
- several thoraccic vertebrae may fuse
- synsacrum - 10 - 23 fused vertebrae and ribs that also fuses with pelvis
- pygostyle - 4 - 7 fused caudal vertebrae
- the sternum is greatly enlarged and its surface area is expanded by a large keel
Mammals
- nearly all with 7 cervical vertebrae. Related to cancer?
- cervical vertebrae without articulating ribs (ribs incorporated into vertebrae as part of transverse process)
- thorracic vertebrae with ribs
- lumbar vertebra without ribs
- sacral vertebrae and coccygeus. Sacral ribs enlarged and fused together.Many specializations of mammalian vertebral column which we will discuss in the locomotion lecture.