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

• この研究は、死後にオランダ脳バンクに提供された42個の脳で、BSTcと呼ばれる脳のごく小さな領域にある1種類の神経細胞を数えました。
• 比較用の脳は、異性愛者と推定される男性9人、ゲイの男性9人、異性愛者と推定される女性10人、性ホルモンの障害のある数人から得られました。
• 異性愛の男性には約33,000個のこの細胞があり、女性より約71%多くなっていました。ゲイの男性は男性の範囲でした。
• トランス女性6人の平均は約20,000個で、女性に近く、女性との間に測定可能な違いはありませんでした。
• トランス男性1人の数は男性の範囲にあり、異性であるという強い感覚を持っていた、未治療の84歳の男性1人の数は女性の範囲にありました。
• ホルモンが結果を引き起こしたのではないと論じるために、著者らは、死亡の3～15か月前にエストロゲンをやめていたトランス女性と、エストロゲンを産生する腫瘍があるものの数が男性の範囲にあった31歳の男性を挙げました。
• トランス女性の脳は、1995年にZhouらが調べたのと同じ6人のものであったため、これは新たな人々で知見を再現したというより、それらの症例を再び調べたものです。

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.