[Histochemical, quantitative and ultrastructural maturation of human fetal muscle]

Histochemical studies of muscle specimens from human fetuses showed: (a) a uniform fiber type population having the properties of Type IIC fibres up to 19 weeks of development; (b) a progressive appearance of Type I fibres after that age; (c) a decrease in number of Type IIC fibres during the last 3 months of pregnancy, accompanied by the appearance of Type IIB and Type IIA fibres; (d) the presence after the myotube stage of fibers with a light peripheral halo in sections stained for mitochondrial dehydrogenases. Electron-microscopic examination of the muscle fibres confirmed the existence of a peripheral halo devoid of myofibrils and mitochondria and showed: (a) scarcity of myofibrils in comparison with mature muscle fibres and (b) irregularity in shape of the myofibrils. In addition, quantitative studies demonstrated an important variation of the fibre diameters up to 21 weeks and the increase of the mean diameter after this age. It is suggested that the persistence after birth of some features of immaturity identical to those described in this work may be considered as a pathological finding.