An electron microscope study of the early postnatal development of the visual cortex of the hooded rat.

Synaptic plasticity in response to environmental events has been clearly demonstrated in the visual cortex of the rat, but no detailed data concerning the course of early synaptogenesis in this area are available. In this study, synaptogenesis in the visual cortex of hooded rats at 1, 3, 5, 7 and 10 postnatal days of age (P1-P10) was examined with electron microscopy. The cortex was divided into the molecular layer, the superficial layers (II-IV) and deep layers (V-VI). In the visual cortex at P1, very few synapses are present in the molecular and deep layers and virtually none in the yet undifferentiated layers II-IV that compose the cortical plate at this age. The synapses that are present are axodendritic and often symmetrical with little membrane thickening and few vesicles. Axosomatic synapses were seen as early as P3 but very rarely. There are marked increases in axodendritic synaptic density and maturity with increasing age. By P7 and P10, many synapses appear mature in form and the majority can be classified a symmetrical. Axospinal synapses first appeared at P7 and were more frequent by P10. However, this classification was somewhat uncertain since no spinal apparatus was detected. Rate of synaptogenesis appeared to increase over the ages studied and showed no signs of leveling off except in the deep layers. Synaptic length was extremely variable and did not change systematically with age.