Human ontogenesis. I. Ultrastructural characteristics of developing human muscle.

Muscle cells of 25 humans fetuses 7, 9, 10, 14, 17, 20, and 24 weeks old were examined by electron microscopy to determine the structural characteristics of developing muscle. Three structurally different levels of maturation of muscle cells, the primitive myotube, the mature myotube, and the immature muscle fibers, have been observed in the process of human ontogenesis. The sequence of morphological changes and the timing of transformation of one form of fetal muscle into a more mature form are presented.