Sexual differences in the numerical density of synaptic profiles of developing rat visual cortex

Axo-dendritic synaptic profiles were quantified along the whole depth of the visual cortex of 10-day-old male and female rats. In both sexes the numerical density of synaptic profiles on spine-like structures was greater than the numerical density of synapses on dendritic shafts. Females had a significantly greater numerical density of synaptic profiles on spine-like structures, than did males at a distance of 200–400 and 500–600 μm from the pia surface, which corresponds to layers II–III and IV of the cortex, respectively. A small percentage (2%–4%) of spine-like structures received two presynaptic terminals. This type of double synapses was three times more abundant in females. No sex differences were found in the numerical density of synapses on dendritic shafts in any cortical layer. 1994 John Wiley & Sons, Inc.