Schematic representation of segmentation, re-segmentation, and vertebrae development in amniotes. (A) Embryo of a chick (Gallus gallus domesticus), around 40 h post-fertilization. Note the rounded somites developing bilaterally to the neural tube. We also represent a scheme of a transversal section, showing the paired somites (red), surrounding the neural tube (green) and notochord (grey), under the dorsal ectoderm (blue). X, Y, Z: lateral, dorsal, and anterior axes, respectively. (B) Somites are regionalized and polarized soon after their formation. The dorsolateral area (light pink) constitutes the dermomyotome, differentiating eventually into axial muscles and dermis. The ventromedial area forms the sclerotome (orange), cellular precursor of vertebral skeletal elements. Sclerotomal cells delaminate from somites and migrate ventrally towards the notochord, and dorsally towards the neural tube. (C) Scheme of a longitudinal section (anterior to the left), showing three somites (pink) and their boundaries. Polarization of somites results in a rostral-caudal differentiation of the cellular domains (light and dark pink). Schematized is the initial migration of sclerotomal cells (orange), forming an unsegmented layer around the notochord, and perichordal rings at the level of the middle of each somite. Sclerotomal cells also aggregate dorsally, developing eventually into the neural arches. (D) Diagram of two chondrified vertebrae, with centra enveloping the notochord, and neural arches surrounding the neural tube. Intervertebral discs develop from the perichordal rings in (C), and centra and neural arches from the cells that migrate from the caudal part of the sclerotome of one somite (shaded) and the rostral part of the sclerotome of the adjacent somite (not shaded). Thus, vertebrae form at the intersomitic boundaries from cells from two adjacent somites. Vertebrae do not reflect the embryonic primary segmentation (somite position), but are displaced half a segment, a phenomenon known as re-segmentation. Chick picture is a courtesy of Sophie Miller.