ABSTRACT
The differentiated notochord, one of four synapomorphies that separate chordates from all other animals, consists of large vacuolated cells (chordocytes) connected by desmosomes and surrounded by an expanded and multilayered basement membrane known as the notochord sheath. The history of the development of the sheath has been studied since the 1860s and is examined in some detail in this chapter. The inner layer of the sheath contains, in common with cartilage, collagen type II, proteoglycans and in some vertebrates, cells. Morphogenesis of the notochord is based on the “stack of coins” arrangement, a mechanism that the notochord shares with developing rods of cartilage in vertebrates and in invertebrates.
Early in development and continuing to the pharyngula or phylotypic stage, the vertebrate notochord is the signaling center that triggers the differentiation of neural ectoderm, neural plate and neural tube with antagonists of Bmp and production of SHH as the key signaling factors. In tetrapods the notochord is the signaling center that triggers the differentiation and segmentation of sclerotomal mesoderm into somites and then into vertebrae. In teleost fish, however, mineralization of the notochord sheath provides the anlagen of the vertebral centra. The fate of the notochord varies in different vertebrates, either (i) being retained as the primary axial skeleton, (ii) replaced, (iii) forming intervertebral disks between the vertebrae or forming (iv) intravertebral cartilage within the vertebrae. The chapter concludes with an introduction of the topic of notochord as cartilage and of the challenges such a concept raises.
