Fig. 3. Intracellular expression patterns of slow and fast MyHC mRNA within myotomal fibers. HH 15 (A,B) and HH 19 (C,D) embryos were hybridized with probes for efast MyHC and sMyHC3. (A) At HH 15, efast MyHC mRNA is found throughout the cytoplasm of myotomal fibers. (B) sMyHC3 mRNA is localized to the myotomal nuclear domain. Notice also that fewer somites express slow MyHC then efast MyHC at this stage. (C) At HH 19, efast MyHC expression has expanded along the dorsoventral axis of each somite, but retains its cytoplasmic localization. (D) sMyHC3 expression has expanded in the dorsoventral direction and some signal is now apparent in the cytoplasm. (E) In sagittal sections through immature somites, in situ hybridization indicates a restriction of the sMyHC3 mRNA transcripts to the central domain of myotomal fibers where the nuclei are located. (F) In more mature somites, sagittal sections show sMyHC3 mRNA transcripts distributed beyond the nuclear domain.

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