Fig. 9. Localization of MyHC message within the developing myotome. Embryos hybridized with isoform-specific MyHC probes were subsequently stained with a monoclonal anti-desmin antibody (green). (A) In somites 26 and 27 of an HH19 embryo, the first efast MyHC expression is detected by in situ hybridization in the ventrolateral regions of the myotome (arrowhead), and desmin-positive, MyHC-negative fibers are positioned primarily in the dorsal region. (B) In more-mature (rostral) somites of the same embryo, the expression of efast MyHC extends more dorsomedially. (C) In somites 31 and 32 of an HH21 embryo, the first fibers to express sMyHC3 are detected near the middle of the myotome (arrowhead), and desmin-positive, slow-MyHC-negative fibers are present both dorsal and ventral to this region. (D) After an initial extension to the ventrolateral margin of the myotome, sMyHC3 expression expands in the dorsomedial direction in more-mature (rostral) somites. The broken line outlines the extent of the desmin-positive fibers in the myotome. Not all fibers are within the focal plane of the photo.

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