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

• 연구는 사후에 네덜란드 뇌 은행에 기증된 뇌 42개에서 BSTc라는 작은 뇌 영역의 한 종류의 신경세포를 세었습니다.
• 비교 뇌는 이성애자로 추정되는 남성 9명, 게이 남성 9명, 이성애자로 추정되는 여성 10명, 성호르몬 장애가 있는 여러 사람에게서 왔습니다.
• 이성애자 남성은 이 세포가 약 33,000개로 여성보다 약 71% 많았고, 게이 남성은 남성 범위였습니다.
• 트랜스 여성 6명은 평균 약 20,000개로 여성에 가까웠고, 여성과의 측정 가능한 차이는 없었습니다.
• 트랜스 남성 1명은 남성 범위의 수를 보였고, 다른 성이라는 강한 느낌이 있던 치료받지 않은 84세 남성은 여성 범위의 수를 보였습니다.
• 호르몬이 결과를 일으키지 않았다고 주장하기 위해, 저자들은 사망 3~15개월 전 에스트로겐을 중단한 트랜스 여성들과, 에스트로겐을 만드는 종양이 있었지만 수가 남성 범위였던 31세 남성을 들었습니다.
• 트랜스 여성의 뇌는 1995년 Zhou 등이 연구한 바로 그 6명이므로, 이는 새로운 사람에게서 발견을 반복한 것이 아니라 그 사례들을 다시 본 것입니다.

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.