Sex differences in cortical thickness and the dendritic tree in the monocular and binocular subfields of the rat visual cortex at weaning age.

The visual cortex of adult rats is sexually dimorphic at both the gross size and dendritic levels [Brain Res., 295 (1984) 27-34; J. Comp. Neurol., in press]. In addition, sex differences in the dendritic tree are dynamic and can be altered by environmental conditions imposed at weaning [Brain Res., 295 (1984) 27-34]. The present study examines sex differences in cortical thickness and in the dendritic tree of the monocular (Oc1M) and binocular (Oc1B) subfields in littermate male and female pairs of Long-Evans rats at weaning age (25 days). From Nissl-stained tissue, it was found that the whole cortex and layer II-IV of Oc1B was thicker in males than females. No sex differences were found in the thickness of Oc1M. Golgi-Cox-stained pyramidal neurons in layer III from the Oc1M and Oc1B regions were quantified in 7 littermate pairs of weaning-age rats. There were no sex differences in the basilar tree, while the apical oblique branches were sexually dimorphic, especially in the monocular region. Females had greater total dendritic length and longer terminal branches in Oc1M compared to males. Females also had longer bifurcating branches in both Oc1M and Oc1B than males. The present study found that sex differences at weaning age do not completely mirror the dimorphisms found in the visual cortex of the adult rat. This study also indicates that related subfields can differ in their morphology and should be examined separately.