The morphology of synaptic profiles in explants of foetal and neonatal mouse cerebral cortex maintained in a magnesium-enriched environment.

The present study examines the ultrastructure of synaptic profiles developed in explant cultures of immature mouse cerebral cortex, maintained for prolonged periods in a magnesium-rich environment. The ethanolic phosphotungstic acid method was employed in addition to conventional preparation procedures so that paramembranous densities could be clearly observed. Although presynaptic terminals were frequently packed to capacity with vesicles in cultures maintained in magnesium-rich media, there was always a proportion which contained loosely collected vesicles. Few other changes in synaptic morphology were apparent and the paramembranous densities were unaffected. The reasons for the effectiveness of the transmission block, the absence of any change in the morphology of paramembranous densities, and the excessive crowding of presynaptic vesicles are considered and discussed.