Meeting Abstract
Vertebral anatomy and development vary across jawed vertebrates (gnathostomes). Centra, in particular, have originated independently in numerous gnathostome clades, with distinct morphologies and tissues arising in different taxa. There is evidence that the embryonic origins of the vertebral skeleton may be distinct in different gnathostome groups. All vertebral elements in tetrapods derive from sclerotome (skeletogenic cells of the ventral portions of the somites), while teleost vertebrae are more variable, with layers of bone in the centra apparently deposited by the notochord. To test whether a notochordal contribution to centra is unique to teleosts, or whether it is a more general gnathostome characteristic, data from the sister group of bony fishes, the chondrichthyans, are needed. Here we determine the embryonic origin of the vertebral skeleton in the little skate, Leucoraja erinacea, using somite and notochord fate mapping experiments. CM-DiI injected into ventral somites was traced to the neural and hemal arches, as well as the inner and outer layers of the centrum. Also, a layer of spindle-shaped cells surrounding the notochord, which has been posited as a derivative of the notochord epithelium in elasmobranchs, was discovered to be somite-derived. Complementary long-term notochord fate mapping experiments show that notochord cells do not contribute to any vertebral elements in the skate. These experiments demonstrate that a sclerotomal origin of the vertebral skeleton is a general feature of gnathostomes, and that the teleost condition, in which the notochord contributes to the centrum, is probably a specialized exception. Significantly, tetrapods and elasmobranchs employ the same tissues to build centra, despite having evolved these structures independently.
