Ultrastructural analysis of skeletal muscle development in the fetal pig.

Myogenesis was investigated at the ultrastructural level in the fetal pig at 35, 52, 65, 80, 95 and 110 days of gestation. At each stage of gestation, the predominant type II fiber type portion of the peroneus longus and sartorius muscles was examined. The sequence of events for normal myogenesis in the pig was generally similar to that reported for the skeletal muscle of other vertebrate species. In addition, the time interval for sequential development of myogenesis was similar between the two muscles. Centrioles were identified in primary fetal fibers and also in secondary fibers. No morphological evidence of mitotic activity was found in the myonuclei. The organization of myofibers into fasciculi was unrelated both temporally and spatially to capillary and neural development.