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

• Die Studie zählte eine Art von Nervenzellen in einem winzigen Hirnareal namens BSTc in 42 Gehirnen, die nach dem Tod der niederländischen Hirnbank gespendet wurden.
• Die Vergleichsgehirne stammten von 9 mutmaßlich heterosexuellen Männern, 9 schwulen Männern, 10 mutmaßlich heterosexuellen Frauen und mehreren Personen mit Störungen der Geschlechtshormone.
• Heterosexuelle Männer hatten etwa 33.000 dieser Zellen, etwa 71 % mehr als Frauen; schwule Männer lagen im männlichen Bereich.
• Die sechs trans Frauen hatten im Durchschnitt etwa 20.000 Zellen, nahe an den Frauen; es gab keinen messbaren Unterschied zwischen ihnen und den Frauen.
• Ein trans Mann hatte eine Zahl im männlichen Bereich, und ein unbehandelter 84-jähriger Mann mit starkem Gefühl, dem anderen Geschlecht anzugehören, hatte eine Zahl im weiblichen Bereich.
• Um zu argumentieren, dass Hormone das Ergebnis nicht verursachten, verwiesen die Autoren auf trans Frauen, die 3 bis 15 Monate vor dem Tod Östrogen abgesetzt hatten, und auf einen 31-jährigen Mann mit einem Östrogen bildenden Tumor, dessen Zahl im männlichen Bereich lag.
• Die Gehirne der trans Frauen waren dieselben sechs, die Zhou und andere 1995 untersuchten, die Studie betrachtet diese Fälle also erneut, statt den Befund an neuen Personen zu wiederholen.

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.