Xenbase Image ID: 176433

Fig. 4. Twist1 marks lateral migrating sclerotome cells in axolotl. Scheme of experimental procedure: postero-anterior transverse sections (a to g) of axolotl embryos at stage 31 after whole mount in situ hybridization with Twist1 (A) or immunohistochemistry with 12/101 antibody (B). Arrows design migrating sclerotome progenitors. Rectangle dotted line designs the area magnification at the bottom of each panel (A and B, a’ to g’). (C) Schematic representation of somite compartmentalization in Xenopus. MSC domain is located at LSF at early neurulation (stage 13). Lateral Myod1 expression domain could overlap the MSC domain. At mid-neurulation (stage 17/18), differentiation of lateral myotome is initiated and early markers of dermomyotome and sclerotome like Pax3 (della Gaspera et al., 2012b) and Twist1 begin to be expressed. At stage 22 lateral cells migrate dorsally to give rise the dermomyotome. Next, sclerotome progenitors expressing Twist1 migrate ventrally to give rise to the sclerotome located medially at stage 28. (D) Evolution of somite compartmentalization based on axochord hypothesis. The axochord hypothesis (Brunet et al., 2015) proposes that the notochord evolves from a ventromedian muscle (the axochord in annelids) present in Urbilateria, the last common ancestor of bilaterian animals, and suggests that transverse muscles attached to it, could give rise to the ancestral myotome in cephalochordates. The origin of MSCs in Urbilateria is unknown. MSCs probably already exist in cephalochordates. The transition from anamniotes to amniotes is characterized by extension of MSCs domain at the expense of ancestral myotome. The chordate dorso-ventral axis is inverted compared with Urbilateria. VM, ventromedian muscle; TM, transverse muscle; M, medial somite; L, lateral somite. Image published in: Della Gaspera B et al. (2019) Copyright © 2019. Image reproduced with permission of the Publisher, Elsevier B. V. Larger Image Printer Friendly View

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