Sexual Differentiation of the Human Brain A Historical Perspective

Abstract

This chapter discusses the historical perspective of the sexual differentiation of human brain. Systematic observations concerning morphological sex differences in the human brain, on the other hand do not go back much further than a hundred years. The sex differences in macroscopic appearance of the human brain, that have been reported so far, are relatively small and, those that seem to be truly present are related to a sex difference in lateralization. The literature on macroscopic sex differences of the human brain was, and still is, a remarkable mixture of scientific observations and cultural bias. Those sex differences that seem to be really present––that is, the larger splenium of the corpus callosum in women arid the larger left temporal planum in men––involve sex differences in brain lateralization. Staining with antibodies raised to vasopressin enables morphometric studies on this hypothalamic clock. These studies reveal that the suprachiasmatic nucleus (SCN) in women tends to be larger in its cranio-caudal diameter while in men it has a larger maximal diameter in the middle of the nucleus. On the whole, however, neither the total volume nor the cell number of the SCN turns out to differ. This sexual dimorphism in the SCN shape is the first gender-linked difference in an identified neurotransmitter system that has been reported within the human brain.