Vertebrate somitogenesis: a novel paradigm for animal segmentation?

In vertebrates, the primary segmented tissue of the body axis is the paraxial mesoderm, which lies bilaterally to the axial organs, neural tube and notochord. The segmental pattern of the paraxial mesoderm is established during embryogenesis through the production of the somites which are transient embryonic segments giving rise to the vertebrae, the skeletal muscles and the dorsal dermis. Somitogenesis can be subdivided into three major phases (see Fig. 1). First a growth phase during which new paraxial mesoderm cells are produced by a growth zone (epiblast and blastopore margin or primitive streak and later on tail bud) and become organized as two rods of mesenchymal tissue,forming the presomitic mesoderm. Second a patterning phase occuring in the PSM, during which the segmental pattern is established at the molecular level. Third, the somitic boundaries are formed during the morphological segmentation phase. In all vertebrates, all cells of the paraxial mesoderm, during their maturation in the PSM, go successively through these three phases, which are tightly regulated at the spatio-temporal level. The first phase of paraxial mesoderm production falls out of the scope of this review, as it essentially pertains to the gastrulation process. Here, I essentially discuss the segmental patterning phase in vertebrates. Recent data suggest that establishment of the segmental pattern relies on a clock and wavefront mechanism which has been conserved in vertebrates. Furthermore, conservation of this system could extend to invertebrates, suggesting that the clock and wavefront is an ancestral mechanism.