Figure 5.

 Slow muscle forms beneath the floorplate in flh embryos. (A–C) Differentiated muscle visualized by MyHC-staining in whole-mount 15-somite wild-type (A) or flh (B,C) embryos observed after flat-mounting. Dual labeling for shh mRNA (red; C) and MyHC (green; C) confirmed that the most posterior differentiated muscle was at the midline (arrow, C). (D–I) Transverse cryosections show that differentiated muscle in flh midline mesoderm expresses slow but not fast MyHC at 15 somites (D–F). By 24 hr, flh embryos have a single fused myotome (G), with lateral slow and medial fast muscle (H,I). Despite the ectopic location of formation of slow muscle cells in the midline the slow muscle cells appear to migrate normally so that the only detectable residual slow muscle defect is lack of muscle pioneers. (J,K) Dual labeling for MyHC (green) and ptc1 mRNA (red) in the two most posterior MyHC-containing serial sections of a 15-somite flh embryo. Newly differentiating muscle cells close to the midline do not always express detectable ptc1 (arrows), even though ptc1 is expressed abundantly in the mesoderm underlying residual floor plate (K).