An enzyme histochemical study of large muscle fibres in the neonatal mouse.

Small clusters of extra large muscle fibres were identified in hindlimb muscles of neonatal mice (strain C57BL/10ScSn). At two days of age they had a significantly greater cross-sectional area than their normal counterparts (P less than 0.01). Fibre typing methods (NADH-tetrazolium reductase, ATPase and phosphorylase) classified them as 2A fast oxidative glycolytic (FOG fibres). The activity of NADH-tetrazolium reductase and the lysosomal enzymes beta-glucuronidase, acid phosphatase and dipeptidyl peptidase II were all elevated in the large fibres. Microsomal aminopeptidase (mAPP), a membrane-bound enzyme, also showed increased activity. The fibres are probably the mouse equivalent of the Wohlfart B fibres of the human fetus, with which comparison is made.