The fine structure of the mouse and human intercostal muscle neuromuscular junction was studied after brief fixation in a new formol-sucrose fixative. This primary formalin fixation was followed by brief postosmication in buffered 1 per cent osmium tetroxide. Muscle blocks were embedded in methacrylate or Epon 812 epoxy resin. Marked similarities between mouse and human motor end-plates were observed. Neuromuscular junctions from both mouse and human intercostal muscle showed synaptic vesicles, primary and secondary synaptic clefts, and layered differentiation of the amorphous surface material (ASM) present on the surface of the Schwann cell plasma membrane and on the muscle surface membrane in the region of the neuromuscular junction. An attempt to stain the ASM with lead was unsuccessful. Observations on thick and thin plastic-embedded sections stained by PAS after diastase digestion showed that the ASM within the subneural apparatus is PAS positive. Alcian blue stained the endoneurium and perineurium of peripheral nerve bundles and portions of the end-plates. The similarity of the PAS-positive ASM to other basement membranes described in other sites is discussed and its possible physiologic significance within the subsynaptic apparatus is considered.

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