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

• Studiet talte én type nerveceller i et lille hjerneområde kaldet BSTc i 42 hjerner doneret til Netherlands Brain Bank efter døden.
• Sammenligningshjernerne kom fra 9 mænd, formodet heteroseksuelle, 9 homoseksuelle mænd, 10 kvinder, formodet heteroseksuelle, og flere personer med forstyrrelser i kønshormoner.
• Heteroseksuelle mænd havde omkring 33.000 af disse celler, cirka 71 % flere end kvinder; homoseksuelle mænd lå i det mandlige område.
• De seks transkvinder havde i gennemsnit omkring 20.000 celler, tæt på kvinder; der var ingen målbar forskel mellem dem og kvinder.
• Én transmand havde et antal i det mandlige område, og en ubehandlet 84-årig mand med stærke følelser af at være det andet køn havde et antal i det kvindelige område.
• For at argumentere for, at hormoner ikke forårsagede resultatet, pegede forfatterne på transkvinder, der var stoppet med østrogen 3 til 15 måneder før deres død, og en 31-årig mand med en østrogenproducerende tumor, hvis antal lå i det mandlige område.
• Transkvindernes hjerner var de samme seks, som Zhou m.fl. undersøgte i 1995, så dette ser igen på de tilfælde frem for at gentage fundet hos nye personer.

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.