Sex and environmental influences on the size and ultrastructure of the rat corpus callosum.

A contested report of sex differences in the size of the splenium of the corpus callosum in humans prompted the present examination of the corpus callosum in the rat. We have previously found that sex differences can vary with the rearing environment. Consequently, male and female rats were raised from weaning to 55 days of age in either a complex or an isolated environment. There were no sex differences in the size of the corpus callosum in sagittal cross section in these rats; however, rats of both sexes had a larger posterior third of the corpus callosum if they were raised in the complex environment. Because the corpus callosum continues to grow in size past 55 days of age, we examined socially housed rats at 113 days and again found no sex differences. The splenium was examined with electron microscopy in complex and isolation reared rats at 55 days of age. The ultrastructural analysis revealed differences that were not apparent from gross size measures. Females had more unmyelinated axons regardless of environment, and females from the complex environment had more myelinated axons than comparably housed males. In contrast, males in the complex environment had larger myelinated axons than females. Rats of both sexes from the complex environment had larger and more unmyelinated axons than isolated rats. In addition in myelinated axons, plasticity in the females occurred through changes in axon number and in males, through axon size. Thus sex differences exist in axonal number and size and the environment influences these differences.