Expression of synaptophysin during synapse formation between dissociated cortical neurons.

Cerebral cortical cells from fetal rats (14 days) were cultured in vitro (7-104 days). Localization of synaptophysin was examined immunocytochemically using a monoclonal antibody (SY-38) against synaptophysin. Synaptophysin-immunoreactive (SP-IR) spots were distributed on the surface of the cells. Electron-microscopic observation showed that the SP-IR staining was localized at presynaptic terminals, indicating that the SP-IR spots observed will, fluorescence microscopy represent presynaptic terminals. The time-course of the distribution of SP-IR spots was examined. SP-IR spots could not be observed in cells at 7 days in vitro (DIV). At 14 DIV, SP-IR spots could be first observed on the surface of cell bodies and neurites. Thereafter, the density of SP-IR spots on the cell bodies and neurites increased with DIV. At 42 and 104 DIV, SP-IR spots were almost continued, surrounding the neurites and cell bodies of the cultured cells. These immunocytochemical studies suggest that the dissociated 14-day fetal cerebral cortical neurons make synapses by at least 14 DIV and that the density of synapses increases with time.