Time course of Notch activity in the lateral DM. (A-C) Electroporation of the lateral DM at E2. Control cells are epithelial and located in lateral dermomyotome (DM) both at four hours (A) and eight hours (B) post-electroporation (N = 4 and 4, respectively). (C) By 20 hours, although most cells reside in the DM, some have delaminated into an intermediate zone between the lateral DM and myotome (N = 6). Inset shows lateral DM by Hoechst staining. (D-F) Electroporation of aNotch2. By four hours and eight hours the epithelial morphology of labeled cells is abrogated and delamination from the lateral DM begun (N = 4 and 7, respectively). (F) At 20 hours cells have migrated towards, and partly arrived at, the cardinal vein (CV) (N = 5). Inset shows lateral DM by Hoechst staining. (G-J) Transient Notch activation. (G) Control GFP expressed for 40 hours reveals labeled progeny in myotome (M) and CV (arrows and insets i and ii, N = 8), with residual cells still populating the lateral DM. Qh1 staining of endothelial cells is in blue. (H) Eight hours of aNotch2 missexpression + 32 hours of Tet-off. Cells are clustered in the ventro-medial portion of the myotome, do not express desmin/SMA or display myofiber morphology (see Figure 2A,B) (N = 5). (I) Sixteen hours of aNotch2 + 24 hours Tet-off. Most labeled cells are outside the myotome and many of them express ectopic desmin/SMA while others have integrated into the CV wall (arrows and insets i and ii, N = 5). (J) aNotch2 missexpression for 40 hours. Many Notch + cells co-expressing desmin/SMA are located in the wall of the CV (arrows and insets i and ii), while others are still on their way through the sclerotome (N = 3). Lateral DM in C,F and G-I is marked by a dashed white line. Myotome is outlined with a yellow solid line (G’,H’,I’,J’) Hoechst staining of lateral DM. Bar: (A-F) 65 μm; (G-J) 100 μm. Des, desmin; DM, dermomyotome; SMA, smooth muscle actin.