The time course of synapse formation of mouse neuroblastoma cells in monolayer cultures.

Neuroblastoma cells grown on substrates in culture develop long processes and assume the morphology of normal neurons as judged light microscopically. The development of synapses in the cultured tissue is studied by periodic electron microscopic examination of the areas of contact between cells. The initial explants are free of any apparent synaptic contacts. After 48 h in culture, simple swellings or boutons are detected at the periphery of the cells or at the end of the fine processes. These initial synaptic profiles contain a few vesicles but lack mitochondria. The synaptic vesicles appear to originate from the smooth endoplasmic reticulum. Further explants remain primitive, only the number of vesicles in the cytoplasmic swellings or boutons increases. These clusters of vesicles are 40-60 nm in diameter and morphologically distinguishable from the synaptic vesicles of normal neurons. There are no postsynaptic folds or membrane thickenings. Specialized cell contacts between cells are also present.