Comparison of compartmentalization modes between amphioxus, zebrafish, Xenopus, chick and mouse. (A) The lateral domain of amphioxus somite is already compartmentalized at mid neurula stage and possesses progenitors that give rise to the external cells layer, the sclerotome-like compartment but also the lateral plate mesoderm and the fin box mesothelium at stage G9. The lateral domain can be subdivided into three subdomains which express different set of genes. For example, Pax3/7 is expressed in the central domain (CD), Pax1/9 in the dorsal domain (DD) and Hand in the ventral domain (VD). Adapted from Yong et al. (2021). (B) The first phases of compartmentalization are both medio-lateral and antero-posterior in zebrafish, mainly medio-lateral in Xenopus and mainly dorso-ventral in chick and mouse. In zebrafish, an apparent movement of somite rotation relocated the different cell populations during the segmentation period. The posterior cells elongate toward the anterior region of the somites (straight black arrows) and give rise to fast fibers, the anterior cells were relocated in the surface outside of the somites (curved black arrows) and give rise to the dermomyotome. In addition, the medially adaxial cells migrate laterally toward the myotome periphery (grey arrows) and differentiate both into pioneer cells and superficial slow fibers. The endotome cells migrate toward the midline aorta (light blue hollow arrows). The appearance and location of MSCs are unknown and difficult to infer in zebrafish. Somites at 12 and 24 hpf (hour post fertilization). In Xenopus, lateral MSCs envelop the myotome ventrally and dorsally to give rise to the dermomyotome and the sclerotome (bended black arrows). Both in Xenopus and zebrafish, the lateral fast fibers are in dorsal and ventral position around the medial ones which are located close to the notochord. Somites at mid-neurulation (stage 18) and at tailbud stage (stage 28). In amniotes, chick, and mouse, the newly formed somites are naïve structures, made up of MSCs which subdivide into a dorso-lateral dermomyotome and a ventro-medial sclerotome. The dermomyotome cells remain epithelial whereas the sclerotome cells undergo EMT (epithelial mesenchymal transition). In chick, the pioneer cells begin to express Myf5 and Myod1 medially at epithelial somite stage, and become the first myocytes used as a scaffold for the construction of primary myotome. For zebrafish modified from Buckingham and Vincent, (2009) and Keenan and Currie, (2019). For chick and mouse, modified from Buckingham, (2001). (C) Comparison of muscle cell movements during somitogenesis between zebrafish, Xenopus, and axolotl. Zebrafish: Cell movements during apparent somite rotation. Lineage tracing of cells inside a somite makes it possible to follow their movements. Myogenic cells (curved arrow), Dermomyotome precursors (straight arrow). Explained in (A). Xenopus: Myogenic cells are first oriented perpendicular to the antero-posterior axis, before becoming parallel to it during apparent somite rotation (black arrows). Axolotl: Differentiation of myogenic cells inside somites is characterized by cell elongation in antero-posterior direction progressing medio‐laterally (hollow arrows). Adaxial cells have been described in axolotl but are not represented here (Banfi et al., 2012). For zebrafish, modified from Stellabotte et al. (2007). For Xenopus, modified from Keller, (2000). For axolotl summarized from Neff et al. (1989), Radice et al. (1989), and Keller, (2000).