An ultrastructural study of the differentiation of skeletal muscle in the bovine fetus.

The differentiation of skeletal muscle was studied by electron microscopy in bovine fetuses from 47 days gestation to neonatal calves 3 days of age. Initially, the muscle was composed of clusters of myotubes with mononucleated myoblasts between them. In 2-month-old fetuses these myoblasts became apposed to the differentiating muscle cells and were enclosed within the rudimentary basal lamina of the myotubes. At this stage the clusters of myotubes consisted of central, larger diameter, more differentiated myotubes and also the mononucleated satellite cells. The differentiated myotubes separated from the clusters accompanied by satellite cells which continued proliferating and fused together to form new generations of satellite myotubes. In this manner new clusters of myotubes were formed. By 4-5 months some of the separating myotubes began to form individual myotubes and independent myofibers were prominent in fetuses of 5-8 months of age. The myofibers in the 8-month-old fetuses showed diversification into fiber types by differences in the thickness of the Z-line, the prominence of the sarcotubular system, the amount of glycogen and lipid droplets and also the number of mitochondria.