Figure 5.

Adaxial precursors transfate to produce Shh-independent fast muscle derivatives in embryos with diminished RyR function. (A) Schematic representation of experimental setup and summary of results, with embryos shown in transverse views. At the 5-somite stage Kaede was photoconverted to Kaede* in small groups of adaxial (Ad) slow muscle precursors or lateral (Lat) fast muscle precursors. At 26hpf (14 hours later), embryos were fixed and stained for F59 myosin expression. (B,D,F,H) Labeling of muscle precursor cells for lineage analysis. Clusters of slow muscle precursor adaxial (B–B′, F–F′, H–H′) or fast muscle precursor lateral (D–D′) cells were labeled by photoconversion (magenta) at the 5-somite stage. At 26hpf, the positions of Kaede*-labeled (magenta) cells in the somites were determined relative to the superficial slow muscle, detected with the F59 antibody (myosin, green). (C–C″) Kaede*-labeled adaxial cells always gave rise to F59⁺ descendants in the parallel array of superficial slow muscle cells (white arrowheads) in control embryos. (E–E″) Kaede*-labeled lateral somite precursors always became deep fast muscle regardless of ryr gene expression (yellow arrowheads). (G–G″ and I–I″) In ryr1a,3 morphant or azumolene-treated embryos, adaxial cells gave rise to both slow (white arrowheads) and fast (yellow arrowheads) muscle fibers.