Human ontogenesis. II. Development of the human neuromuscular junction.

The ultrastructural pattern of the early stages of end-plate formation is described in the quadriceps femoris muscle of 9- to 20-week-end human fetuses. Features of the neuromuscular junction are first observed in the ninth week of fetal life. The primitive motor end-plate contains a few axons always covered by one Schwann cell and does not seem to change either in number or in structure from the tenth through the twentieth week. Continuous modification of the post-synaptic apparatus is observed from the tenth to the twentieth weeks of human fetal life.