Fig. 3. Fz7 signaling is required for MyoD, but not for Myf5 initiation. (A) MyoD expression in an embryo injected with control alkaline phosphatase-transfected cells. Eighty-eight percent of the embryos presented normal MyoD expression (n=23/26). (B) MyoD expression in an embryo injected with S-Fz7-transfected cells. S-Fz7-secreting cells were injected at the caudal, middle and rostral presomitic level, as well as in the third or fourth newly formed somite. Seventy-seven percent of the embryos presented an inhibition of MyoD expression in somites formed in the presence of S-Fz7-transfected cells (n=23/30). Inhibition was more pronounced around S-Fz7-expressing cells injected in the caudal and middle presomitic mesoderm (shown here). MyoD inhibition was sometimes observed when cells were injected in the rostral presomitic mesoderm, although this effect was seldom observed when they were injected in somites which already expressed MyoD. (C) In sharp contrast, embryos (87%) injected with S-Fz7-transfected cells presented normal Myf5 expression in injected somites (n=20/23). (D) Transverse section of B. (E) Transverse section of C. Section levels are indicated by broken lines. Injected cells were stained with the fluorescent dye DiI and observed under UV illumination. (D,E) Composites of a bright-field image of transverse sections and a fluorescent image showing the DiI-labeled S-Fz7-expressing cells. Arrowheads indicate the position of the injected cells.