Fig. 3

Mesp-ba^Myc inhibits fast fiber differentiation and promotes dermomyotome development. Tg(hsp70l:mesp-ba^myc) embryos and wild-type siblings 1h (A-D), 6h (24-somite stage) (E-H) and 9h (24-h stage) (I,J) after the end of heat shock. (A-D) Transgenic embryos show downregulation of myoD (A) and myf5 mRNA (B) in the lateral paraxial mesoderm, and upregulation of meox1 (C), but no difference in pax3 (D). Most recently formed somite SI is outlined. (E,F) myoD expression shows smaller myotomes (E, brackets) in somites formed after heat shock (orange arrowhead); meox1 mRNA is upregulated throughout those somites (F, brackets). (G,H) In transgenic embryos (Myc⁺, gray), the number of slow muscle nuclei (Mef2, bright red) is similar to wild type, but fast fiber nuclei (Mef2, faint red) and myosin heavy chain protein (MF20, blue) are reduced (arrowheads). (I,J) Immunolabeling shows increase in Pax7⁺ dermomyotome cells (green) in Tg(hsp70l:mesp-ba^myc) embryos (number indicated); boxed areas show underlying myotomes (MF20, blue) on the right. Orange arrowheads mark last somite formed before heat shock. Scale bars: 100µm in D,F; 50µm in H,J.