Fig. 10. Model of fiber formation in the myotome. The myotome as defined by desmin staining is represented as closed boxes. Fast MyHC transcripts appear first in the myotome and are distributed throughout the width of myotomal fibers, whereas transcripts for slow MyHCs appear slightly later and are initially confined to the central nuclear domain. Fast MyHC expression begins near the ventrolateral edge of the myotome and expands dorsomedially as development continues. Slow MyHCs are initially expressed more centrally in the myotome and expression initially expands ventrolaterally. Subsequently, slow MyHC expression expands dorsomedially within the myotome. In the most-mature somites, slow MyHC transcripts become distribute more evenly throughout the myotomal fibers, eventually losing their nuclear localization completely to mimic the distribution of fast MyHC transcripts.