FIGURE 1.

Main characteristics of somite compartmentalization in vertebrates. (A) Comparison of somite organization between amniotes and anamniotes. Schematic view of somites slightly after the phylotypic stage. In anamniotes, the somite organization is adapted to the ondulatory swimming of the larvae and harbors a chevron shape with the myotome occupying the majority of the somite. The thin layer of dermomyotome cells is in dorso-lateral position and the sclerotome layer is ventro-medially stranded between the myotome and the midline structures (neural tube and notochord). The syndetome at the origin of the dorsal tendons arises from the sclerotome. The tenocytes project cytoplasmic extensions between muscle cells of adjacent somites. In amniotes, the spatial organization is the same, but the myotome compartment is reduced and the sclerotome is larger. (B) Somite compartmentalization in amphioxus. The anterior and intermediate somites are formed by enterocoely from the endoderm at the early neurula stage. The somites are subdivided into a medial myotome and a lateral domain at late neurula stage. The sclerotome-like cells seem to migrate from the lateral domain to position themselves medially between the myotome and the axial structures. The lateral domain also gives rise to the dorsal external cells, the medial fin box mesothelium (FBM) and the latero-ventral perivisceral mesothelium (PVM). Modified from Mansfield et al. (2015) and Yong et al. (2021). ES, epithelial somite; NC, notochord; NP, neural plate; END, endoderm; ECT, ectoderm. (C) Somite compartmentalization in Xenopus. The somite is initially medio-laterally organized with the myotome in medial position and multipotent somitic cells (MSCs) in lateral one. The myotome forms first and is initially made up of a medial- and a lateral-population of muscle cells. The MSCs appear at lateral somitic Frontier (LSF) at the beginning of neurulation and envelop next dorsally and ventrally the myotome to give rise to both dermomyotome and sclerotome.