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Male-to-Female Transsexuals Have Female Neuron Numbers in a Limbic Nucleus

Title: Male-to-Female Transsexuals Have Female Neuron Numbers in a Limbic Nucleus

Link: http://jcem.endojournals.org/content/85/5/2034.full

Summary: (abstract)

Transsexuals experience themselves as being of the opposite sex, despite having the biological characteristics of one sex. A crucial question resulting from a previous brain study in male-to-female transsexuals was whether the reported difference according to gender identity in the central part of the bed nucleus of the stria terminalis (BSTc) was based on a neuronal difference in the BSTc itself or just a reflection of a difference in vasoactive intestinal polypeptide innervation from the amygdala, which was used as a marker. Therefore, we determined in 42 subjects the number of somatostatin-expressing neurons in the BSTc in relation to sex, sexual orientation, gender identity, and past or present hormonal status. Regardless of sexual orientation, men had almost twice as many somatostatin neurons as women (P < 0.006). The number of neurons in the BSTc of male-to-female transsexuals was similar to that of the females (P = 0.83). In contrast, the neuron number of a female-to-male transsexual was found to be in the male range. Hormone treatment or sex hormone level variations in adulthood did not seem to have influenced BSTc neuron numbers. The present findings of somatostatin neuronal sex differences in the BSTc and its sex reversal in the transsexual brain clearly support the paradigm that in transsexuals sexual differentiation of the brain and genitals may go into opposite directions and point to a neurobiological basis of gender identity disorder.

A sex difference in the human brain and its relation to transsexuality

Title: A sex difference in the human brain and its relation to transsexuality

Link: http://www.nature.com/nature/journal/v378/n6552/abs/378068a0.html

Summary:

TRANSSEXUALS have the strong feeling, often from childhood onwards, of having been born the wrong sex. The possible psycho-genie or biological aetiology of transsexuality has been the subject of debate for many years1,2. Here we show that the volume of the central subdivision of the bed nucleus of the stria terminalis (BSTc), a brain area that is essential for sexual behaviour3,4, is larger in men than in women. A female-sized BSTc was found in male-to-female transsexuals. The size of the BSTc was not influenced by sex hormones in adulthood and was independent of sexual orientation. Our study is the first to show a female brain structure in genetically male transsexuals and supports the hypothesis that gender identity develops as a result of an interaction between the developing brain and sex hormones5,6.

***Note: follow link to get links to the papers referenced in footnotes.

Phantom Penises in Transexuals

Title: Phantom Penises in Transexuals

Linkhttp://www.ingentaconnect.com/content/imp/jcs/2008/00000015/00000001/art00001

Summary: (abstract)

How the brain constructs one’s inner sense of gender identity is poorly understood. On the other hand, the phenomenon of phantom sensations– the feeling of still having a body-part after amputation–has been much studied. Around 60% of men experience a phantom penis post-penectomy. As transsexuals report a mismatch between their inner gender identity and that of their body, we wondered what could be learnt from this regarding innate gender-specific body image. We surveyed male-to-female transsexuals regarding the incidence of phantoms post-gender reassignment surgery. Additionally, we asked female-to-male transsexuals if they had ever had the sensation of having a penis when there was not one physically there. In post-operative male-to-female transsexuals the incidence of phantom penises was significantly reduced at 30%. Remarkably, over 60% of female-to-male transsexuals also reported phantom penises. We explain the absence/presence of phantoms here by postulating a mis-match between the brain’s hardwired gender-specific body image and the external somatic gender. Further studies along these lines may provide penetrating insights into the question of how nature and nurture interact to produce our brain-based body image.