Different Brains, Different Realities?
Most of us go through our days not questioning why separate aisles in toy stores are designated to boy-toys such as puzzles and blocks and girl-toys such as dolls and tea sets. We do not always consciously notice that men dominate certain professional fields while women do others. And why are there a higher percentage of gay men (or more "feminized" men) in the dramatic, passionate world of the performing arts? Are these sex trends all enculturation? If we do not stop to explore the origins and implications of our observations, we make the mistake of glossing over them with the non-informative, "fact-of-life" explanation that neglects the why of ...view middle of the document...
The effects of steroid binding are realized in alterations in regional cell growth, proliferation, or death, which may then influence cell number, size, or packing density. Early migrational patterns, dendritic growth, and neuronal myelination may also be modified (2).
One place where steroid binding appears to exert its effect is in the hypothalamus. The sexually dimorphic nucleus of the medial preoptic area (SDN-POA) is a sub-nucleus in the medial preoptic area that is approximately 2.5 times larger in males than in females. In addition, the presence of two sexually dimorphic cell groups has been confirmed in the preoptic-anterior hypothalamic area. There are four interstitial nuclei of the anterior hypothalamus and the two that are larger in the male brain are the INAH3 and INAH1. These hypothalamic findings are particularly noteworthy because the preoptic area has been shown to be sexually dimorphic in several other non-human species and more important, to be sensitive to prenatal or perinatal hormonal influences.
The SDN-POA can be enlarged in female rats through the administration of a synthetic estrogen (diethylstilbestrol) which does not bind to AFP, indicating that masculinization of this structure is dependent on the intracellular conversion of testosterone to estrogen. Estrogen, therefore, may mediate this sexual variation by preventing a developmental loss of neurons within the medial preoptic nucleus (2). Perhaps these hypothalamic discrepancies are also related to the contrasting natures of the male and female hypothalamus; the male's being constant and the female's being cyclic.
Neonatal testosterone also appears to be involved in the sexual differentiation of the cerebral cortex. Certain regions of the cortex are significantly thicker in the right hemisphere than in the left in male rats, whereas females showed a non-significant trend toward asymmetry in the opposite direction. This distinction appears to be mediated at least partly by androgen exposure, since neonatally gonadectomized male rats fail to show the right-left pattern of cortical asymmetry seen in intact males (2).
Perhaps yet another steroid-induced phenomenon lies in the differing proportions of white and gray matter seen in male vs. female brains. Researchers at the University of Pennsylvania Medical Center have recently reported that women have a higher proportion of gray matter to cranial volume while men have a higher proportion of white matter (gray matter is where computation takes place, while white matter is responsible for communication between groups of cells in different areas of the brain) (3). This finding complicates the earlier observations concerning the corpus callosum, a large body of nerve fibers that connects the right and left hemispheres of the brain. Those studies showed that women have a larger corpus callosum than men and therefore show a more bilateral representation of function which decreases specialization but integrates the...