Initiation of differentiation into skeletal muscle fiber types.

A monoclonal antibody directed against a slow isoform of a 100-K myofibrillar protein has been used to study the differentiation of muscle fibers in the fast and slow muscles of the rat. Immunohistochemical studies have shown that initiation of differentiation into fast and slow muscle fibers occurs very early during development. The distribution pattern of cell types in different muscles is unique and is also determined very early during fetal muscle development. It is concluded that motor innervation is probably not essential to initiate differentiation into distinct muscle fiber types.