Fig. 1. Models of resegmentation during formation of vertebrae from somites. (A) Primary elements of the axial skeleton include the centrum (c) or vertebral body, which develops around the notochord (n); neural arches (na), paired dorsal elements that together with the dorsal margin of the centrum enclose regions of the neural tube (nt); hemal arches (ha), paired ventral elements that extend from the centrum adjacent to midline blood vessels (bv); and ribs (r), which extend ventrolaterally from the centrum or hemal arch. Viewed from anterolateral with anterior towards the left and dorsal towards the top. Resegmentation models (B,C) describe development of vertebrae from somites. Views of a generic vertebrate from the dorsal aspect. (B) Compartmental resegmentation model proposes that somites (S) are comprised of anterior (A) and posterior (P) halves. Each vertebra (V) develops by recombination of posterior half-sclerotomes from one somite pair with anterior half-sclerotomes from the next posterior somite pair (yellow arrows). In this strict model, half-sclerotome derivatives are restricted to one-half of the vertebra as well. This model resembles the contribution of parasegmental anterior and posterior compartments to segments in fly integument, in which patterning is maintained by lineage restriction (reviewed by Dahmann and Basler, 1999). (C) Leaky resegmentation model in which sclerotome cells from one somite (S) contribute to (yellow arrows) two adjacent vertebrae (V) in a manner that is not strictly dependent upon the A or P domain of origin in the somite.