Ultrastructure of sites of cholinesterase activity on amphibian embryonic muscle cells cultured without nerve.

Muscle cells, derived from 1-day-old embryos of Xenopus laevis, were cultured in the absence of nerve and then examined histochemically for cholinesterase activity. Virtually all of the cells exhibited one or more discrete patches of cholinesterase activity on their surface. The patches were located not only on the lower cell surface which was apposed to the floor of the culture dish but also on the lateral and upper surfaces which were not apposed to any solid substrate. At many of the cholinesterase patches the cell surface was invaginated and the invaginations also became filled with the histochemical reaction product. Surface sites of cholinesterase activity were often associated with a thickened plasma membrane and overlying basal lamina-like material. Regions of the cell surface without cholinesterase were typically smooth and ultrastructurally unspecialized. The cholinesterase patches and their associated ultrastructural features developed in the absence of electrical and contractile activity. The similarities between the muscle specializations at cholinesterase sites on cultured, noninnervated, Xenopus muscle cells and those at the neuromuscular and myotendinous junctions of normal skeletal muscle are discussed.