Acetylcholinesterase activity of developing muscles in the lower limb of the rat.

A cytochemical study of acetylcholinesterase was done in the lower limb of the prenatal rat and in the gastrocnemius muscle of the postnatal rat. Between 15 and 17 days of gestation, mesenchymal cells constituting the muscle primordia are characterized by the presence of enzyme activity in their rough endoplasmic cisterns and nuclear envelopes, while those involved in the formation of the neocapillary and cartilage do not show enzyme activity. This suggests that mesenchymal cells destined to myogenic cells actively produce acetylcholinesterase in a limited period, which may play a role in cellular aggregation and fusion during the muscular morphogenesis. Cytochemical findings as to extensive networks of secondary synaptic folds of the neuromuscular junctions and invaginations of the sarcolemma in the extrasynaptic regions are also illustrated in the differentiating gastrocnemius muscles.