Six femmes trans sous hormones : nombre de neurones de type féminin dans un noyau cérébral — mêmes cerveaux qu'en 1995

Les transsexuels de l'homme vers la femme ont un nombre de neurones féminin dans un noyau limbique

Études de cohorte & registres Sexe, biologie & sport

Authors: Frank P. M. Kruijver, Jiang-Ning Zhou, Chris W. Pool, Michel A. Hofman, Louis J. G. Gooren, Dick F. Swaab
Journal: Journal of Clinical Endocrinology & Metabolism
Year: 2000
Source: https://doi.org/10.1210/jcem.85.5.6564

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

L'étude a compté un type de cellule nerveuse dans une minuscule zone du cerveau appelée BSTc dans 42 cerveaux donnés à la Banque de cerveaux des Pays-Bas après le décès.
Les cerveaux de comparaison provenaient de 9 hommes présumés hétérosexuels, 9 hommes gays, 10 femmes présumées hétérosexuelles et plusieurs personnes ayant des troubles des hormones sexuelles.
Les hommes hétérosexuels avaient environ 33 000 de ces cellules, environ 71 % de plus que les femmes ; les hommes gays se situaient dans la fourchette masculine.
Les six femmes trans avaient en moyenne environ 20 000 cellules, proche des femmes ; il n'y avait aucune différence mesurable entre elles et les femmes.
Un homme trans avait un comptage dans la fourchette masculine, et un homme de 84 ans non traité ayant un fort sentiment d'être de l'autre sexe avait un comptage dans la fourchette féminine.
Pour soutenir que les hormones n'avaient pas causé le résultat, les auteurs ont cité des femmes trans qui avaient arrêté les œstrogènes 3 à 15 mois avant leur décès, et un homme de 31 ans porteur d'une tumeur produisant des œstrogènes dont le comptage se situait dans la fourchette masculine.
Les cerveaux des femmes trans étaient les mêmes six que ceux étudiés par Zhou et autres en 1995 ; l'étude réexamine donc ces cas plutôt que de répéter le résultat chez de nouvelles personnes.

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.

Methodological limitations

Not a fresh test: The six trans women's brains were the same people studied in 1995 (Zhou and others), by the same team and brain bank. Using a new measure on the same tissue cannot show the original finding holds for other people.
Very few trans people: The conclusions rest on six trans women, one trans man and one untreated person. With so few, averages can be swung by individual differences.
Hormone treatment could explain it: All the trans women had taken estrogen (with testosterone blockers) for years, and most had been castrated. Stopping estrogen 3 to 15 months before death, as two did, does not undo years of exposure, and cross-sex hormones in adults are known to change brain structure (Hulshoff Pol and others, European Journal of Endocrinology 2006).
Hormone checks based on single cases: The claim that adult hormones do not change cell numbers rests on a handful of single cases with brief or different hormone problems (for example one man with a feminising tumour for about a year), not on a proper comparison.
Timing problem: The same team later reported that the male-female difference in this brain area only appears in adulthood (Chung, De Vries and Swaab, Journal of Neuroscience 2002). That makes it an unlikely cause of gender feelings that usually start in childhood.
'Similar to women' is not proof of being the same: Finding no measurable difference from women in such a tiny group does not show the counts really are the same.
Sexual orientation guessed: 'Presumed straight' comparison people were classed from medical records, not asked. The trans women's orientations are also uncertain, and the researcher Lawrence argues most were probably not attracted to men, which limits claims about trans women in general.
Key claims rest on single brains: The trans man and the untreated person, who carry much of the argument that the difference is inborn, are each just one brain.
Authors also provide the treatment: Co-author Louis Gooren led the Amsterdam gender clinic that treated these patients and was a leading supporter of the idea that trans people have a 'brain sex' of the other sex. A study by an independent team would carry more weight.

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Summary

Key findings

Abstract

Conclusion