Histochemistry and immunocytochemistry using an antibody to adult rat slow-type myosin demonstrated that about 10% of the fibers in the mouse extensor digitorum longus and semimembranosus muscles contain slow myosin during the first month after birth. In adult animals, these muscles have only t&0.8% slow myosin-containing fibers. These results demonstrate a developmentally linked disappearance of an adult-type myosin, and show that the adult phenotype of muscle fibers is not necessarily determined before birth as previously suggested.
Histochemistry Immunocytochemistry
Muscle from dystrophic (129 ReJ dy/dy) mice, aged 1, 2, 3, 4, and 6 months, was cocultured with normal embryonic mouse spinal cord. Cultures were scored for regeneration (myotubes) and differentiation (cross-striated or contracting fibers) over a 28-day period. Although muscle from dystrophic mice of all ages was able to regenerate, the ability to differentiate decreased with the increasing age of the mouse. Muscle from litter-mate control mice of different ages regenerated and differentiated equally well.
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