Short exposure to Notch signaling permits myotomal colonization but not myogenic differentiation. (A,B) Dorsal view of whole-mount segments. (A,A’) Presumptive lateral DM’s were electroporated with an inducible GFP control plasmid. Eight hours post-electroporation doxycyclin was administered to silence plasmid expression. A further 32 hours of incubation proceeded. Cells have initiated myogenesis and begun elongating to form terminally differentiated myofibers (N = 4). (B,B’) aNotch2 was expressed for 8 hours and an additional 32 hours of incubation without expression followed. Cells entered the desmin + myotome but failed to differentiate into myofibers (N = 4). (C-D) Notch maintains Pax7 expression after delamination. (C) Transverse sections of control-GFP-treated embryos 16 hours post-electroporation. Cells that entered the myotome mostly silenced Pax7 expression (N = 4). (D) Transverse sections of aNotch2-treated embryos 16 hours post-electroporation. Cells maintained low Pax7 immunoreactivity even after exiting the DM (arrow). A non-cell autonomous maintenance of Pax7 expression was also observed (arrowhead) (N = 8). (E-F) Notch enhances the incorporation of BrdU into nuclei at the lateral DM. Control-GFP (E,E’) or aNotch2 (F,F’) was electroporated into the lateral DM and proliferation was monitored eight hours post-treatment. Short exposure to Notch enhances BrdU incorporation in the DM (F,F’ yellow arrows compared with BrdU-negative nuclei in E,E’, arrows). (G) Quantification of Pax7-expressing cells (mean ± SEM). (H) Quantification of BrdU + labeled cells within the lateral DM (mean ± SEM). *P ≤0.05, **P ≤0.01. Bar: (C-F) 50 μm. BrdU, bromodeoxyuridine; CV, cardinal vein; Des, desmin; DM, dermomyotome; M, myotome; Scl, sclerotome; SEM, standard error of the mean.