Human ontogenesis. 3. Cell death in fetal muscle.

Naturally occurring muscle cell death in normal human fetal muscle was examined to determine the timing and structural differences with respect to muscle maturity. Two types of degenerative changes in developing muscle were found: cytoplasmic and nuclear. Degeneration of the primary and mature myotubes between 10 and 16 weeks of gestation entailed cytoplasmic condensation and disruption, swelling of mitochondria and dilatation of sarcoplasmic reticulum. In contrast, the formation of immature muscle fibres was associated with disintegration of satellite myofibres characterized by nuclear degenerative changes. These findings indicate that naturally occurring muscle cell death appears as a two-successive-stage phenomenon of cell necrosis. Initially, at the myotube stage, a number of muscle cells are eliminated. In the later stage a single cell is removed from the cluster which seemingly is responsible for final shape and size of the muscle fibre.