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

• De studie telde één type zenuwcel in een klein hersengebied genaamd de BSTc in 42 hersenen die na de dood aan de Nederlandse Hersenbank werden geschonken.
• Vergelijkingshersenen kwamen van 9 mannen die vermoedelijk hetero waren, 9 homomannen, 10 vrouwen die vermoedelijk hetero waren, en verschillende mensen met aandoeningen van de geslachtshormonen.
• Heteromannen hadden ongeveer 33.000 van deze cellen, ongeveer 71% meer dan vrouwen; homomannen zaten in het mannelijke bereik.
• De zes transvrouwen hadden gemiddeld ongeveer 20.000 cellen, dicht bij vrouwen; er was geen meetbaar verschil tussen hen en vrouwen.
• Eén transman had een aantal in het mannelijke bereik, en een onbehandelde man van 84 met sterke gevoelens het andere geslacht te zijn had een aantal in het vrouwelijke bereik.
• Om te betogen dat hormonen het resultaat niet veroorzaakten, wezen de auteurs op transvrouwen die 3 tot 15 maanden vóór hun dood met oestrogeen waren gestopt, en een man van 31 met een oestrogeenproducerende tumor wiens aantal in het mannelijke bereik lag.
• De hersenen van de transvrouwen waren dezelfde zes als bestudeerd door Zhou en anderen in 1995, dus dit kijkt opnieuw naar die gevallen in plaats van de bevinding in nieuwe mensen te herhalen.

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.