Fig. 4. Sequential dissociation of the epithelial DM and formation of its derivatives as revealed by morphology and Pax7 mRNA expression. Transverse sections in-situ hybridized with PAX7 (blue) and immunolabeled with desmin (brown). (A) The epithelial DM at E3 expresses PAX7, but the underlying myotome is devoid of PAX7+ cells. (B) At E3.5 the central DM partially dissociates into a compact cell aggregate (ID) that retains PAX7 expression. Few PAX7+ cells become apparent in the myotome but no dermis is yet apparent. (C) By E4 three distinct domains are clearly visible, the desmin+ myotome containing PAX7+ cells, the compact ID, which expresses PAX7, and the beginning of a PAX7-negative dermis composed of scattered mesenchymal cells. See also a similar expression pattern of PAX3 between the myotome and dermis in Fig. 3A. (D) At E5 the ID domain has almost disappeared and most PAX7+ cells are in the myotome intermingled among desmin+ fibers and also located medial to them. A well developed dermis is present. D, dermis; DM, dermomyotome; ID, intermediate domain; M, myotome; NT, neural tube. Scale bar: 40 µm in A; 55 µm in B; 60 µm in C; 120 µm in D.