Development of presynaptic specializations induced by basic polypeptide‐coated latex beads in spinal cord cultures

The development of the neuromuscular synapse in 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 μm 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. (5) Coated pits were occasionally observed at the contact area. Thus, our results show that the beads can induce a differentiation characteristic of the presynaptic nerve terminal along the spinal cord neurites.