Sex differences in the rat forebrain.

In the first postnatal month, the thickness of the cerebral cortex has different patterns of development in the male and female Long-Evans rat. All areas of the male cortex appear to develop at similar rates, very rapidly for the first 10 days, then more gradually reaching a peak somewhere between 30 and 40 days of age before beginning to decrease in thickness. The same is not true for the female; for example, at birth her medial frontal cortex is further developed than her lateral posterior parietal cortex and the rate of growth in the first weeks varies immensely. The male right cortex is thicker than the left from birth to 904 days of age, but the differences become less significant with age. The female cortex is thicker on the left side in the majority of the cases, from 7 to 400 days of age, but the differences are not in general statistically significant. Estrogen receptors are found in both male and female rat cerebral cortices at birth and apparently disappear after one month. In the male the greatest concentration of receptors is in the left cortex, and in the female, in the right. The number of neurons and glia per unit area is greater in the right cortex (area 39) in the male; and in the female, in the left cortex. Ninety days after ovariectomy at birth, the female right cortex is thicker than her left, establishing a pattern similar to that of the male. In the male gonadectomized at birth, the left cortex is thicker 90 days later in the frontal and somatosensory regions but not in the occipital or posterior region.(ABSTRACT TRUNCATED AT 250 WORDS)