Many of the sex differences seen in the brain manifest themselves in a variety of ways, such as sizes of particular regions of the brain, number of nerve cells in specific regions, distribution of neurotransmitters, and even development of behavior (Goldstein et al., 2001; Hutchinson, 1997; Segovia et al., 1999). In humans, for instance, there are region-specific dimorphisms, with some structures larger in females (caudate nucleus, hippocampus, Broca's area, anterior commissure, and right parietal lobe) and some larger in males (hypothalamus, stria terminalis, and amygdala) (Goldstein et al., 2001). In rodents, numerous sexually dimorphic nuclei have been studied, the following two perhaps most extensively. One is the sexually dimorphic nucleus of the preoptic area (SDN-POA), which is larger in males than females, and another is the anteroventral periventricular nucleus (AVPv), which is larger in females than males (Davis et al., 1995; Gorski et al., 1980; Simerly et al., 1988). Sex differences in distributions of neurotransmitters have also been documented. For instance, neurochem-ical analysis of the AVPv showed that females harbor more cells secreting TH than males, while males synthesize more enkephalin than females (Simerly and Swanson, 1987; Simerly et al., 1985). Most sex differences in the brain have been investigated in regions important for sexual function and reproduction, such as the hypothalamus. The influence of gonadal hormones on the sexual differentiation of these structures has been studied extensively, for instance in the case of the SDN-POA (Swaab and Fliers, 1985).
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