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The development of the neuromuscular synapse is initiated by an interaction between the motoneuron processes and the skeletal muscle. Previously we showed that basic polypeptide-coated latex beads can mimic the nerve in effecting a postsynaptic differentiation (Peng, H.B. and Cheng, P.-C.: J. Neurosci., 2:1760-1774, 1982). In this study, we examined whether these beads can also induce a presynaptic differentiation along the nerve processes. Explant cultures were prepared from the spinal cords of Xenopus larvae and polyornithine-coated latex beads (0.45-4.5 micron in diameter) were applied. After 1-2 days, the cultures were processed for light and electron microscopy. For light microscopy, the cells were permeabilized and labeled with a monoclonal antibody against a 65-KD antigen on synaptic vesicles. Indirect immunofluorescence revealed that this antigen was concentrated at 60% of the bead-neurite contacts, therefore suggesting the clustering of synaptic versicles at these sites. This phenomenon was not observed at the contacts between neurites and negatively charged (polycarboxylate) beads. However, a concentration of this antigen was also observed at the varicosities along the neurites cultured on polylysine substrate. Thin-section electron microscopy showed the following features: (1) The neurites formed terminal-like varicosities on the bead. (2) Within these varicosities, clusters of 50-60-nm clear vesicles were prominent at the bead-neurite contact. (3) Large (80-100 nm) dense-cored vesicles were also present in the varicosity, but they did not form clusters. (4) Basement membrane did not form at the bead-neurite interface, in contrast to its appearance at the bead-muscle contact from our previous study.(ABSTRACT TRUNCATED AT 250 WORDS)

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