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Figure 2 Brain volume changes in male-to-female and female-to-male transsexuals. Changes in hypothalamus, total brain, third and lateral ventricle volumes in milliliters (mean change from 0 at first measurement in bars, 95% confidence interval in error bars, individual values in circles) in transsexuals following cross-sex hormone treatment as compared with age-and sex-matched healthy comparison subjects. |
structures surrounding these ventricles (including, but not limited to, the hypothalamus, which lies in close proximity to the third ventricle). Considering that the effects were not specific for gray (neurons, glia) or white (myelinated axonal fibers) matter suggests that both alterations in nerve cells as well as in axonal fibers may be implicated in the anatomical brain changes following cross-sex hormone treatment in humans. It is not surprising that the influences of sex hormones on the brain were not limited to the hypothalamus, but were also expressed as changes in total brain size. Estrogen |
and androgen receptor mRNA containing neurons are not limited to the hypothalamus, but are distributed throughout the adult human brain (18). |
Prior to cross-sex hormone treatment, no differences in brain volumes between transsexuals and comparison subjects were found. Transsexuals had brain volumes in agreement with their sex at birth. The intracranial volume (and hence overall brain size) and the hypothalamus volume of males were larger than the females, irrespective of the condition of transsexualism. This supports the notion that brain volume changes in |
Figure 3 Brain volumes in male-to-female and female-to-male transsexuals just prior to treatment. Hypothalamus, total brain, third and lateral ventricle volumes at scan 1 in milliliters (mean volumes in bars, 95% confidence interval in error bars, individual values in circles) in transsexuals as compared with age-and sex-matched healthy comparison subjects (individual value of one control male’s lateral |
ventricle (62 ml) not printed). |
transsexuals are related to cross-sex hormone treatment and do not reflect pre-existent differences in brain volume between transsexuals and comparisons originating from earlier (pre-or perinatal) brain development. Thus, our findings imply plasticity of adult human brain structure to develop towards the size of the opposite sex under the influence of cross-sex hormones. |
Our findings of plastic changes in the total brain volume and hypothalamus volume of transsexuals after |
cross-sex hormone treatment are corroborated by the observation of (in vitro) structural and functional brain changes in animals upon hormonal manipulations. Since the first study on the reversal of natural sex differences in neuronal connectivity following testosterone hormone treatment in the developing rat (19), numerous studies have documented the influence of sex hormones on brain structure. These include volumetric studies on the influence of cross-sex hormone treatment in adult animals. In the medial preoptic nucleus of the |
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