Male-to-Female Transsexuals Have Female Neuron Numbers in a Limbic Nucleus

FRANK P. M. KRUIJVER, JIANG-NING ZHOU, CHRIS W. POOL,
MICHEL A. HOFMAN, LOUIS J. G. GOOREN, AND DICK F. SWAAB

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 inner-
vation 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
         
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 differenti-
ation of the brain and genitals may go into opposite directions and
point to a neurobiological basis of gender identity disorder. (J C
 

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