Segmentation of the vertebrate body.

The segmental character of the vertebrate body wall is reflected by metamerically arranged tissues that are patterned during embryonic life as a consequence of somite formation, compartmentalization and differentiation. The somites bud off the paraxial mesoderm in a cranio-caudal sequence and are compartmentalized by local signals from adjacent structures. These signals may be mediated by diffusible substances such as Sonic hedgehog (Shh), Wnts and Bone morphogenetic protein (BMPs) or by cell-cell interactions via membrane-bound receptors and ligands such as Delta and Notch. Compartmentalization of the somites and their derivatives is reflected by the differential expression of developmental regulatory genes such as Pax-1, 3, 7 and 9, MyoD, paraxis, twist and others. Secondary segmentation is imposed upon other tissues, such as blood vessels and nerves, by the rearrangement and regionalization of the somitic derivatives, especially the sclerotome. Early cranio-caudal identity is determined by the expression of different Hox genes. Finally, fusion of segmental anlagen occurs to form segment-overbridging skeletal elements and muscles. The expression of homologous genes indicates that the process of segmentation in vertebrates and invertebrates is homologous, derived by descent from a common ancestor.