Fig. 7. The formation of the primary myotome by pioneer myoblasts. Successive stages in the formation of the early myotome (first wave, red color) represented as a function of somitic level in a 28-somite embryo. (A) Pioneer myoblasts originate along the entire R-C extent of the dorsomedial wall of epithelial somites. (B) During somite dissociation, pioneer myoblasts bend underneath the forming DML, delaminate and begin migrating to rostral portions of the somite (curved arrow). White mesenchymal cells represent sclerotome (Scl). (C) Migrating mesenchymal myoblasts give rise to a transient triangular pattern. (D) The first growing myofibers differentiate apposed to the DML. The direction of myofiber formation proceeds from R to C (red arrows) and from M to L. Mesenchymal cells are located lateral to the differentiating fibers. The overall pattern of the forming myotome is still triangular. (E) All fibers have reached the caudal and lateral edges, giving rise to the early myotome (rectangular pattern), which is attached to the R and C lips of the dermomyotome. These are mononucleated fibers whose nuclei concentrate at the center (black dots). The second wave of myotome colonization (not shown) begins at segmental levels D/E. The drawings represented in the figure depict characteristic phases, yet the processes of myoblast delamination, migration and fiber generation are continuous and overlapping over discrete segmental levels. Hence, segments are found that reveal simultaneously delaminating and migrating cells, or migrating and differentiating cells. In C-E, only the four dermomyotome (DM) lips (DML, VLL, R and C) were left. The dorsal layer of the DM has been removed to appreciate the underlying myoblasts. Intersecting arrows represent coordinates for orientation: R, rostral; C, caudal; M, medial; L, lateral; D, dorsal; V, ventral.