Abstract

The vertebral column is a defining feature of vertebrates, but it varies widely in its components and tissue types. Ancestral character estimations indicate that there is substantial convergence in the evolution of the axial column complex. Specifically, centra have originated independently numerous times in different gnathostome groups, and the developmental processes contributing to centrum formation vary accordingly. There is evidence that teleost centra can form through secretions of bone matrix from the notochord, while amniote centra develop exclusively from the sclerotome. To test which mode of development is general for jawed vertebrates, I studied embryonic development in a cartilaginous fish, the little skate (Leucoraja erinacea). MicroCT scans of a series of skate embryos show a continuous condensation of tissue around the notochord and neural tube before the cells differentiate into cartilage and subdivide. Centra consist of three layers of tissue: an inner hyaline cartilage, a middle areolar calcification, and an outer hyaline cartilage. To investigate the embryonic origin of skate centra, I performed complementary somite and notochord fate mapping experiments. I labeled the ventral somite and early notochord progenitors with CM-DiI. DiI-labeled somite cells were subsequently identified both in all layers of the centrum, as well as the arches. Notochord progenitor cells gave rise to only the notochord and its epithelium. These results suggest that sclerotome ancestrally contributed to the development of all components of gnathostome vertebrae.