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

• Lo studio ha contato un tipo di cellula nervosa in una minuscola area cerebrale chiamata BSTc in 42 cervelli donati dopo la morte alla Netherlands Brain Bank.
• I cervelli di confronto provenivano da 9 uomini presumibilmente etero, 9 uomini gay, 10 donne presumibilmente etero e diverse persone con disturbi degli ormoni sessuali.
• Gli uomini etero avevano circa 33.000 di queste cellule, circa il 71% in più delle donne; gli uomini gay rientravano nella fascia maschile.
• Le sei donne trans avevano in media circa 20.000 cellule, vicino alle donne; non c'era alcuna differenza misurabile tra loro e le donne.
• Un uomo trans aveva un conteggio nella fascia maschile, e un uomo di 84 anni non trattato con forte sensazione di essere dell'altro sesso aveva un conteggio nella fascia femminile.
• Per sostenere che gli ormoni non avessero causato il risultato, gli autori hanno indicato donne trans che avevano smesso gli estrogeni da 3 a 15 mesi prima della morte e un uomo di 31 anni con un tumore che produce estrogeni il cui conteggio era nella fascia maschile.
• I cervelli delle donne trans erano gli stessi sei studiati da Zhou e altri nel 1995, quindi questo riesamina quei casi anziché ripetere il risultato in persone nuove.

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.