Fig. 2. Pioneer myoblasts migrate from caudal regions of the medial epithelial somite to the rostral edge prior to generating myofibers. The medial wall of epithelial somites at interlimb levels of the axis was focally labeled with CM-DiI approaching the caudal tip of the segment (A,A' time 0). Six (B,B') and 12 (C,C') hours after injection, mesenchymal myoblasts (open arrowheads) have relocated towards the rostral half of the somite. (D,D') Eighteen hours after labeling, differentiating fibers (between arrows and white arrowheads) are detected that are attached to the rostral pole and elongate caudally. Mesenchymal myoblasts located laterally always begun differentiating later than medial ones (see also Fig. 1). (A-D) Dye labeling on top of the phase contrast image. (A'-D') Dye-labeled fibers only. C, caudal; L, lateral; M, medial; R, rostral; NT, neural tube.
