Fig. 2. DM-sheet-derived cells that enter the myotome are mitotically competent progenitors that retain PAX7 expression. (A-D) Sequential expression of PAX7 immunoreactive protein to developing somites. (A) E2; (B) E3; (C) E3.5; (D) E5. See text for details. (E) DM-derived cells traced by labeling with DiI co-express PAX7 in the myotome. Time zero (t=0) image, showing a dorsal view of a somite at E2.5 immediately following labeling with DiI to the central DM sheet. The four panels below illustrate sections through the myotome of E4 embryos expressing nuclear PAX7 (green) surrounded by membrane-associated DiI (red). (F-I) PAX7-positive cells in the myotome are mitotically active. E4 embryos were pulsed for 1 hour with BrdU, fixed and processed for PAX7 and BrdU immunolabeling. (F) PAX7. (G) BrdU. (H,I) Overlay between the images in F and G. Yellow cells co-express both PAX7 and BrdU. See text for details. D, dermis; DM, dermomyotome; M, myotome; NT, neural tube. Scale bar: 25 µm in A,I; 44 µm in B,C; 165 µm in D; 5 µm in E (sections); 37 µm in F-H.