Fig. 4. Myotomal fibers express both fast and slow MyHC proteins. A cross section (A,B) through a rostral somite of a HH 20 embryo and a longitudinal section (C-E) through a rostral myotome of a HH 24 embryo, double-stained with F59 for fast and S58 for slow MyHC protein. (A,C) Fast MyHC (red) is present in fibers of the myotome. (B,E) Slow MyHC (green) appeared in essentially all the fibers as well. (D) Nearly total overlap of the two fluorescent markers (yellow) demonstrates colocalization of fast and slow MyHC proteins in all fibers. The outline of the embryo was visualized by counterstaining nuclei with DAPI (blue). A and B, and C-E depict the same field viewed in different fluorescent channels.