ABSTRACT
This chapter has explored the enormously diverse nature of the notochord in adult vertebrates and the cellular mechanisms that regulate retention or modification of the notochord.
In all vertebrates without complete vertebral bodies the notochord maintains its embryonic function, which is axial support as a non-constricted hydroskeleton. In those vertebrates in which vertebral bodies develop, the notochord (or notochord-derived cartilage) becomes a functional component of the intervertebral joints. Only a minority of vertebrate species maintains a notochord composed of the vacuolated cells described in the previous chapter.
A non-constricted notochord is present in those vertebrates that lack vertebral centra, have only partly developed centra, or have simple, ring-shaped centra. The non-constricted notochords of extant jawless vertebrates (hagfishes and lampreys) and extant sarcopterygians (lungfish, Latimeria) are examples.
Constricted notochords develop in those vertebrates in which centra develop around the notochord in a characteristic hourglass shape (Figure 7.1). There is an initial pattern of notochord expansion in the intervertebral spaces. In amphibians and amniotes, the notochord becomes restricted intervertebrally but can remain expanded in intravertebral spaces; in extant mammals, intervertebral notochord forms the intervertebral disks, while in non-mammalian tetrapods (amphibians and reptiles, including birds) a variety of mechanisms result in notochord constriction and the development of cartilage derived from the notochord or from sclerotomal cells. An intervertebrally expanded notochord forms the intervertebral disks in bony fish (actinopterygians), using a considerable diversity of different tissues discussed in this chapter. In mammals, the notochord is discontinuous and contributes the cells that form both the nucleus pulposus and the inner annulus fibrosus, which are, the gelatinous core and the outer portion of the intervertebral disks, respectively. As part of the complexity of notochord evolution, discontinuous notochords are found not only in mammals but also in several other groups of vertebrates.
