Fig. 2. Mitotically active cells enter the myotome from the extreme rostral and caudal lips of the dermomyotome. Iontophoretic labeling with CM-DiI (red) of (A) the center of the rostral lip, (B) the medial aspect of the caudal lip, (C) the DML of dermomyotomes. Embryos were incubated for additional 20 hours and then pulsed with BrdU (green) for one hour prior to fixation. (A,B) Note the presence within the myotomes of double-labeled cells containing BrdU-immunoreactive nuclei surrounded by CM-DiI-positive material (arrows). The double labeling represents cells that entered from the respective lips and continued proliferating within the myotome. Myofibers spanning part of or the entire rostrocaudal length of the myotome are also apparent (arrowheads in A and C). One day after CM-DiI labeling, dye distribution adopts a dotted appearance in the cell membrane. BrdU-positive/CM-DiI-negative cells were also apparent as only a small number of lip cells were dye labeled. The presence of CM-DiI-positive cells that lack BrdU in their nuclei was evident as well; this finding was expected as a 1 hour pulse enables the detection of only a subset of the mitotic population given a cell cycle length of about 8 hours (Langman and Nelson, 1968; Summerbell et al., 1986). D, dermis; EC, ectoderm; Myo, myotome; Scl, sclerotome. Scale bar: 11 µm.