Sex differences in neuron number in the binocular area of the rat visual cortex.

We have previously shown that the thickness of the binocular area of the primary visual cortex is sexually dimorphic in rats. In the present study, sex differences in the number of neurons in this cortical area were examined in nine littermate pairs of 90-day-old Long-Evans hooded rats. Cytoarchitectonic characteristics were used to define the binocular visual cortex, and its volume was estimated through three-dimensional reconstruction of serial coronal sections for each hemisphere. Neuronal and glial density as well as neuronal soma size were estimated from semithin sections through a stereological technique, the disector, in the same animals that were used to estimate volume. The volume of the binocular area was 19% greater in males than in females. While there were no sex differences in soma size or in neuronal density, the differences in the volume of the binocular area resulted in significant sex differences (male greater than female) in the number of neurons overall and in every layer, except layer IV. Glial density was not different between the sexes, but the total number of glial cells was higher in males than in females. These results demonstrate that the binocular visual cortex of the rat is sexually dimorphic in its volume and much of the difference is due to sex differences in the number of neurons and glial cells.