Vertebrate Structure and Development
Lecture Notes

Body Skeleton

The body skeleton which includes everything but the head has a number of functions: 1) protect internal organs; 2) help in respiration; 3) store minerals - especially calcium and phosphorus; 4) provide a rigid internal frame; and 5) provide muscle attachment and with movable segments allow movement.

We can divide the skeleton in another way. The axial skeleton includes the head and backbone. The appendicular skeleton includes the pectoral girdle and front limbs and the pelvic girdle and hind limbs.

We've started our discussion of the axial skeleton so we'll finish that first. The main structures left are the ribs, vertebrae and sternum.

The vertebrae come from mesenchyme that aggregates on each side of the notochord and neural tube. The somites are divided into various parts: sclerotome, myotome, and dermotome. Dermatome forms skin, myotome forms muscle, and sclerotome forms the vertebrae. Sclerotome breaks up into mesenchyme and surrounds the spinal cord, and notochord, and becomes cartilage. The sclerotomes break up into mesenchyme and surround the spinal cord and notochord and also become cartilage. Initially, the vertebrae are layed down as cartilage and later replacement bone comes in.

The myotomes and the precursors to the vertebrae develop between each other in an alternating fashion. In the middle of each myotome (muscle segment) a spinal ganglion is formed. The spinal ganglion innervates its muscle segment. The vertebrae form between the spinal ganglia and therefore sit underneath 2 muscle segments.

The vertebrae can be subdivided into various parts. The main part or body is called the centrum. This sits below the dorsal nerve cord (or spinal cord). The centrum may be associated with the notochord or actually replace it. In some vertebrate groups the centrum has 2 parts. The more anterior part is the intercentrum and the hind part is the pleurocentrum. Dorsal to the centrum is an arch - the neural arch which surrounds the spinal cord. In some species a sharp projection - the neural spine sticks up dorsally. Less frequently there is a hemal arch and hemal spine ventral to the centrum.

When 2 vertebrae meet (articulate) it is by the centra or in tetrapods by the centra and projections on the neural arch called zygapophyses. The prezygapophysis sits anteriorly and the postzygopophysis sits posteriorly. Some projections stick straight out at the sides of the vertebrae. These are the transverse processes.

Anatomists can use the articular surfaces of a vertebra to tell about functions, evolution and what group it belong to. Amphicoelous means both ends of the centrum are concave (fish). This allows limited movement. Procoelous means concave in front and convex in back (anurans and reptiles). Opisthocoelous means convex in front and concave in back (terrestrial salamanders). These allow more movement. Acoelous refers to vertebrae than are flat on both ends (mammals).

The notochord is present in the 3 classes of fishes throughout life. In the advanced osteichthyes the notochord is replaced by the centrum. In the fishes the vertebral column is uniform along its entire length. This takes a drastic change when vertebrates moved onto land. The spinal column became involved in limb support and we started having differentiation.

In tetrapods the cervical vertebrae are in the neck. The trunk vertebrae have the ribs (except anurans). The exception to this is in amniotes (except snakes and turtles) where there are 2 types of trunk vertebrae: the thoracic vertebrae have ribs and the lumbar vertebrae don't. Sacral vertebrae are associated with the pelvic girdle and caudal vertebrae with the tail.

Some tetrapod exceptions or additions are the urostyle of anurans. The synsacrum of birds which is made of fused trunk, sacral and caudal vertebrae. The pygostyle of birds which supports tail feathers. The sacrum of mammals which is fused sacral vertebrae.

Ribs articulate with the vertebrae. Only the Agnatha lack them. Fishes have both dorsal and ventral ribs. Tetrapods have retained the dorsal set, but they have moved ventrally.

Ribs start as mesenchyme derived from sclerotomes and lateral mesoderm. Initially cartilage is layed down and then it may be replaced by bone.

The sternum is a ventral bone unique to tetrapods, but a number of groups such as some amphibians, snakes and turtles lack it. Birds have a ridge on the sternum called a keel (carina) to attach flight muscles. We aren't really sure where the sternum originates evolutionarily.

Appendicular Skeleton

The appendicular skeleton includes the fins of fishes which are used for movement and staying upright during swimming. In fishes the pectoral girdle is larger and more complex than the pelvic girdle. The pectoral girdle articulates with the skull. In amphibians this connection is lost. The bones of the pectoral girdle are the clavicle, scapula, cleithrum, supracleithrum, suprascapula, and coracoid. The furcula of birds is the wishbone. Tetrapods have larger pelvic girdles than fishes. The 3 bones are the ilium, ischium and pubis.

All vertebrates have paired appendages except Agnatha. Fish used their fins for swimming and probably evolved fins to walk along the bottom of the water or get on the edge to catch bugs. The limbs of tetrapods are very similar in all 4 classes. There is 1 proximal limb bone, then 2 bones in the next distal segment, followed by numerous small bones - tarsals or carpals. Hand or foot bones - metacarpals or metatarsals and phalanges. Some mammalian distinct features include the calcaneus or heel bone and the olecranon process. We also have tiny - usually nameless bones embedded in tendons near joints called sesamoid bones. These can show great individual variation. The largest, most consistent is the patella. Some mammals have a bone within the penis called the baculum.

Last updated on 1 Mar 2007
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