Enzyme histochemistry on skeletal muscle of the human foetus.

The differentiation of fibre types in developing human skeletal muscle was studied. The material consisted of muscle samples from different muscles of 86 foetuses (abortions) between 12 weeks gestation and delivery and 50 children 1 day to 7 years old. The latter samples were obtained at surgery. Histochemical stains for myofibrillar ATPase were made after preincubations at pH 4.3, 4.6 and 10.3 in order to identify the subgroups A and B of type II fibres and undifferentiated fibres (type II C). Stains for glycogen and lipids were also performed as well as for NADH-diaphorase and alpha-glycerophosphate dehydrogenase. After 20 weeks gestation a few large size type I fibers could be found in some muscles, but not until after the 30th week were some type II A fibres seen. During the last 3 months of gestation a very rapid further differentiation occurred, but at delivery the differentiation process was still not completed. At birth 15-20% of the fibres were classified as undifferentiated. This picture only gradually changed with a slow increase in the number of type I, II A and II B fibres. The stains for metabolic enzymes and substrates were pale until late in foetal life when some distinction between fibre types became discernible.