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Postnatal growth and differentiation of muscle fibres in the mouse. I. A histochemical and morphometrical investigation of normal muscle.

A histochemical classification of muscle fibres based on three enzymes (ATPase, pre-inc. pH 4:35; succinic dehydrogenase and alpha-glycerol-phosphate-dehydrogenase) was used to describe postnatal growth and differentiation of muscle fibres. The m. soleus, m. plantaris and m. gastrocnemius were examined in normal mice from birth to the young adult stage. At birth, differentiation of the gastrocnemius muscle was in a more advanced stage than that of the plantaris and the soleus muscles, while the last of these was the least developed. During growth, as well as in the (young) adult, there was a distinct relation between fibre type and size, which, however, differed per muscle (region). The development of muscle fibres was a gradual process, rather than a succession of distinct stages, although a change in fibre type was often accompanied by a change in size. Differentiation of fibres already occurred perinatally, and the "adult fast' fibre types appeared during the second week postnatally, varying with the muscle region. During development, a percentage of fibres remained as a population that was histochemically and morphometrically intermediate between the fast-oxidative-glycolytic and fast-glycolytic adult fibres. A model is presented in which the most probable pathways of development are depicted.