Summary

This Amsterdam study followed up a 1995 paper in Nature on the BSTc, a tiny area deep in the brain. Instead of measuring its size, the researchers counted one type of its nerve cells in 42 brains donated after death. Men had about 71% more of these cells than women, whatever their sexual orientation. Six trans women had counts in the female range, and one trans man had a count in the male range. The authors said this points to a biological basis for gender identity. But the six trans women were the same people studied in 1995, all treated with estrogen and testosterone blockers and mostly castrated. So this is the same brains looked at another way, not a new test. The claim that hormones played no part rests on a few single cases. The same team later found the male-female difference in this area appears only in adulthood. No independent team has repeated the finding.

Key findings

• 研究計算了荷蘭腦庫（Netherlands Brain Bank）收到的 42 個死後捐贈大腦中，一個稱為 BSTc 的微小腦區裡的一種神經細胞。
• 對照用的大腦來自 9 名推定為異性戀的男性、9 名男同志、10 名推定為異性戀的女性，以及數名有性荷爾蒙異常的人。
• 異性戀男性約有 33,000 個這種細胞，比女性多約 71%；男同志在男性範圍內。
• 六名跨性別女性平均約有 20,000 個細胞，接近女性；她們與女性之間沒有可測量的差異。
• 一名跨性別男性的細胞數在男性範圍內，一名從未接受治療、有強烈想成為另一性別感覺的 84 歲男性，細胞數在女性範圍內。
• 為了主張荷爾蒙不是造成這個結果的原因，作者指出有跨性別女性在死前 3 到 15 個月已停用雌激素，以及一名患有分泌雌激素腫瘤的 31 歲男性，其細胞數在男性範圍內。
• 這些跨性別女性的大腦就是 Zhou 等人 1995 年研究的同一批六人，因此這是重新審視那些個案，而不是在新的人身上重複這項發現。

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.

Conclusion

The authors concluded that the somatostatin neuronal sex difference 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 in opposite directions, and point to a neurobiological basis of gender identity disorder. They argued that hormone treatment or variations in adult sex-hormone levels did not seem to have influenced BSTc neuron numbers, citing individual transsexuals who had stopped estrogen before death and non-transsexual patients with hormone disorders.