Lesson 10. References: Chapter 8: Reading for Next Lesson: Chapter 8:

Transcription

1 Lesson 10 Lesson Outline: General Features of the Skeletal System Axial Skeleton (Vertebral Column) Ontogenetic Origins (Morphogenesis) of the Skeletal System Functions of the Axial Skeleton Phylogenetic Trends o General o Fish o Tetrapods o Regionalization Objectives: At the end of this lesson you should be able to: Describe the embryonic origins of the different parts of the skeletal system Describe the phylogenetic trends seen in the development of the different layers and discuss the selection forces that gave rise to these differences and the functional significance of them. Describe the biological significance of this trend References: Chapter 8: Reading for Next Lesson: Chapter 8:

2 Skeletal System General All vertebrates have an endoskeleton (or internal skeleton) that usually consists of three parts: the skull; the axial skeleton (vertebral column, ribs and sterna); and the appendicular skeleton (limb girdles and appendages), which form from 3 different groups of cells of embryonic origin. Neural crest cells form the chondocranium (protects brain) and the splanchocranium which supports gill arches and forms the jaws the skull. 1) Mesodermal cells from the sclerotome of the epimere and form the vertebral column (gives the body support and protects the nerve cord) the axial skeleton. 2) Mesodermal cells from the somatic hypomere form the pectoral and pelvic girdles the appendicular skeleton. Postcranial Skeleton: Axial Skeleton The original function of the vertebral column was 1) protect the spinal cord and dorsal aorta. The structural support for the longitudinal axis of the body came from the notochord. Later, vertebrae 2) take over the role of axial support 3) become important attachment sites for body musculature. As such, the axial skeleton becomes critically important for transferring muscle contraction into body movement for locomotion. In tetrapods, the role expands to include suspension of the body. The form and function of the vertebral column are related directly to the static and dynamic demands placed on it. Static demands are associated with suspension or weight bearing. Dynamic demands are associated with locomotion. These are related to the environment the animal is in - aquatic or terrestrial - and the type of locomotion in which the vertebral column is involved.

3 Phylogenetic Trends Agnathans have a large and prominent notochord. Vertebral elements are absent in hagfish and are small cartilaginous elements resting dorsally upon the notochord in lampreys In sharks there is a cartilaginous vertebral column surrounding the notochord. The vertebral elements are the main structural elements. The first components of the vertebra to appear were the dorsal and ventral arches that rested on the notochord. The ventral, hemal and interhemal arches, protected the blood vessels. The notochord provides support. The next step in the evolution of the basic elements of the vertebra was the formation of the two centra, the intercentrum and the pleurocentrum. These formed from the expansion of the ventral arches where they met the notochord. This was the situation in primitive chondrichthyans but in extant sharks, these vertebral elements enlarge to become the predominant structural element of the body axis. The centrum fuses with the pleurocentrum and the two grow to meet the neural and interneural arches. The notochord remains but is constricted and enclosed within the vertebral centra. In advanced fishes, the vertebral column is ossified and the centra become more prominent and replace the notochord as the major structural support for the body. Neural spines and ribs become more prominent. The dorsal arches, neural and interneural (intercalary) protected the neural tube.

4 In aquatic animals, support is provided by the water. The vertebral column serves primarily as a "compression girder resisting telescoping of the body during locomotion and translating axial muscle forces into lateral swimming undulations". This means that the vertebral column is flexible enough to bend a little, becoming compressed on the bent side. If it couldn't compress, it couldn't bend. It is also stiff enough to resist collapse or buckling or folding. It doesn't compress too much. Thus alternating contractions of muscles on each side of the body produce lateral undulations that provide the propulsive force to push fish forward through the water as we described previously for the dogfish. The same lateral undulations of fishes are important for the locomotion of the early tetrapods. Now, however, besides bending from side to side due to locomotion, the column tends to bend dorso-ventrally due to the weight of the animal hanging from it suspended between the limbs, as well as the weight of the head at the front end. Thus the tetrapod vertebral column incorporates design features to deal with this.

5 Centra Remember that the vertebral column must be both strong for support yet have some flexibility for movement. The trend is to reduce the contribution from the notochord while leaving intervertebral bodies (cartilage) or intervertebral disks (mammals) between the vertebrae. Thus, the centrum gradually replaces the notochord within the vertebrae while the notochord is confined to the concavities at the articular ends of the centra and are usually capped with cartilaginous pads. These take up the compressive forces acting on the cord, and the different shapes of the centra allow different degrees of flexibility and restrict the direction in which the cord will flex. flex. Intervertebral ligaments control the stiffness of the vertebral column when it does Torque also becomes a problem in tetrapods and antitorque features appear in the design of the column in the form of the zygapophyses. The height and direction of neural spines reflect their role as levers, delivering forces to the vertebral centra and moving or stabilizing the vertebral column. The column becomes regionalized reflecting functional demands. In fishes the column is relatively undifferentiated with trunk and caudal regions (absence/presence of basapophysis vs hemal arches). This reflects the fact that the column has a common function along its entire length - muscular attachment for swimming. In tetrapods, now limbs are used for locomotion and the vertebral column is used for support. Locomotor forces are, however, still transmitted to the body through the vertebral column. A cranial region differentiates first for cranial mobility. A sacral region differentiates for attachment of the hips. The lumbar region becomes differentiated from the thoracic region due to the demands of locomotion and the caudal region retains much of its original role.