Mammalian myoblasts become fast or slow myocytes within the somitic myotome.

A monoclonal antibody has been prepared to slow skeletal muscle type myosin heavy chain which in the rat distinguishes fast and slow myocytes within the somitic myotome at 11.5 days in utero. The distribution of slow myotubes identified by this antibody in developing limb buds is also restricted to presumptive slow muscle cell type regions only. No slow myoblasts in hindlimb buds, however, were detected at 14.5 day in utero when a small number of fast muscle cells were already present. The presence of slow muscle cell population detected by this specific antibody became apparent a day later. This study thus demonstrated the diversification into different muscle cell types during both early embryonic and foetal development.