The formation and maturation of synapses in the visual cortex of the rat. I. Qualitative analysis.

Synapse formation and maturation were examined in the visual cortex of albino rats from birth to maturity. During the first few days of postnatal life, synapses were sparsely scattered in the subplate zone and in layer I. They appeared immature as judged by the irregular shapes of the presynaptic and postsynaptic profiles, the relatively poorly defined membrane specializations and the presence of only a few synaptic vesicles in the presynaptic structures. As the neuropil matured, synapses were observed throughout the cortex, showing increased thickening of the membrane specializations and more vesicles. However, it was not until the end of the fourth postnatal week that they appeared qualitatively indistinguishable from synapses identified in the adult material. A feature characteristic of the developing visual cortex was the presence of vacant membrane specializations that resembled type I postsynaptic densities. These specializations, which were located either opposite extracellular space or opposite another neuronal process, were only evident during the initial stages of synaptogenesis and their frequency decreased as the number of synapses increased. In addition, transitional forms between these densities and true type I synapses were identified during the first two postnatal weeks. Structures that resembled vacant postsynaptic densities typical of type II synapses were not observed. The earliest identified forms of type II synaptic contacts identified consisted of two profiles that exhibited symmetrical membrane specializations and cleft material. Based on these observations, a scheme has been proposed for the formation of type I and type II synapses in the visual cortex of the rat.