The fine structure of neuromuscular junctions and contact zones between body wall muscle cells of Ascaris lumbricoides (var. suum)

Summary Neuromuscular junctions and close membrane apposition between body wall muscle cells of Ascaris lumbricoides (var. suum) have been examined with the light and electron microscopes. It was found that the body wall muscle cells send out elongate processes from their basal, myofibril containing portion to terminate on dorsal and ventral nerves. When observed with the aid of the electron microscope the neuromuscular junctions were seen to consist of several muscle cell processes in apposition to a single axon. The intersynaptic cleft was approximately 350–500 Å wide. Both the axolemma and sarcolemma were triple layered membranes which were 75–80 Å thick. Electron dense patches were observed at intervals on the apposed membranes which were due to increased thickness of the inner membrane leaflets of axolemma and sarcolemma. Muscle cell membranes, at the level of the neuromuscular junction, were in close apposition resulting in an apparently five-layered membrane complex which was 170–210 Å thick. The sarcolemmata in these regions were separated by 10–50 Å. Presynaptic axons contained mitochondria, microtubules which were 180–270 Å in diameter, and two, morphologically distinct types and sizes of synaptic vesicles. One was 200–600 Å in diameter, with a single, triple-layered membrane bounding a center of low electron density. The other was 600–1200 Å in diameter, with a single, triple-layered membrane bounding a central, electron dense granule of 500–800 Å size.The functional significances of the close membrane appositions between body wall muscle cells and of the two types of synaptic vesicles found at the neuromuscular junctions of Ascaris lumbricoides were discussed with respect to their possible role in neuromuscular physiology.Supported by U.S.P.H.S. Grant No. NB-01528 and Research Career Development Award No. 9-K3-NB-15255. — The author wishes to express his grateful appreciation for the excellent technical assistance given by Miss Gabrielle Rouiller during the course of this investigation.