Transsexual brains – 1995: The Year in Science – Brief Article
Transsexuals define themselves as male or female in spite of what their anatomy tells them. Freud ascribed this sexual disharmony to “childhood conflicts within the oedipal triangle”; others have attributed it to parents who give children the message that they would be more valued if they were the opposite sex. And indeed, transsexuals themselves often trace their desire to belong to the opposite sex to their very early childhood, This past year, though, a group of Dutch researchers reported that they’d traced the origins of transsexuality even farther back–to the womb.
In November neurobiologist Dick Swaab and his colleagues at the Netherlands Institute for Brain Research in Amsterdam announced that their post-mortem study of a tiny brain region known as the BSTc–the central subdivision of the bed nucleus of the stria terminalis–had shown that it was, on average, 44 percent larger in heterosexual men than in heterosexual women. More remarkably, the BSTc’s of six male-to-female transsexuals–whose brains the researchers had painstakingly collected over the course of 11 years–were 52 percent smaller than those of the average man in the study. In fact, they were about the same size as those of the females-from-birth. The researchers chose to study the BSTc because previous research in rodents had shown that it plays a pivotal role in sexual behavior: remove it from a rat and the animal will show no interest whatsoever in sex.
Although five of the six transsexuals had been castrated and all had undergone estrogen treatments to feminize their bodies, the researchers don’t think these procedures affected the size of the brain region. “We know from animal experiments that in adulthood you cannot change the size of the nucleus using sex hormones,” says Swaab. “You can do that only in development.” A comparison with the brains of two men who had had their testes removed as a treatment for prostate cancer showed that these non-transsexuals had a BSTc in the normal male size range; a study comparing pre-and postmenopausal women’s brains, meanwhile, showed that the drop in estrogen levels following menopause also did not change the size of the structure.
These findings led Swaab to believe that in humans also, BSTc size is programmed during fetal and neonatal development–perhaps as a result of an interaction between sex hormones and the developing brain–and is probably not the result of parental or social pressures after birth. His research, he says, “shows that transsexuals are right. Their sex was judged in the wrong way at the moment of birth because people look only to the sex organs and not to the brain.”
RELATED ARTICLE: CALMENT’S CURE
Baby boomers nervously pondering taking hormones as they look forward to old age may take heart from Jeanne Calment, the French-woman who has been a senior citizen for nearly half her life. On October 17 Calment turned 120 years old and 238 days, surpassing the world record for longevity set by a Japanese man in 1986. In her long life, Calment met Vincent van Gogh in Arles when she was 14 (said he was ugly and smelled of drink) and saw the Eiffel Tower being built. Her only grandson died in 1963. Although Calment’s health has been good, her habits have not. Until ten years ago, she smoked regularly, and she has only recently given up her daily glass of port. Though blind and almost completely deaf, Calment is still sharp. Her vision of the future, she says, is “very brief.” The Calment secret? “Always keep your smile. That’s how I explain my long life. I think I will die laughing.”
RELATED ARTICLE: BISEXUAL FLIES
Neurogeneticist Ward Odenwald was studying mutant drosophilas when he noticed that the males were acting out: instead of just courting females with their alluring dances and love songs, they were also courting one another. Last summer, after a year of research, Odenwald and his colleagues at the National Institutes of Health identified the mutant gene responsible for their flies’ bisexuality–a red-eye-color gene, similar to a human gene with an unknown function. “The popular press has anthropomorphized what we’ve done, trying to draw a straight line between our research and human sexuality,” says Odenwald. “Although you can’t do that, many basic mechanisms in life have been highly conserved from day one right on up to man. By understanding molecular mechanisms in the fly, scientists are gaining new insights into similar mechanisms that occur in man.”
All fruit flies have the red-eye-color gene, but generally it’s turned on only in pigment-making cells and has no bisexual side effects. By injecting his fly embryos with both the gene and a piece of DNA that would turn on the gene in response to heat, Odenwald activated it all over his flies. Not only did they turn out red-eyed, but when heated for about an hour, the males of the group formed single-sex courtship chains and circles, each fly vamping the fly ahead.
The red-eye-color gene codes for a protein that is involved in the production of both the pigment and the neurotransmitter serotonin. In rabbits and rats a low serotonin level causes homosexual mounting behavior. “I think it’s more than a coincidence,” says Odenwald. “The next thing to ask is, are serotonin levels being changed in the fly due to misexpression of the gene? It’s a highly speculative hypothesis–but a testable one.”
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