Never let accuracy get in the way of a good story
When I first saw this item, I thought, "What rubbish! We think with our neurons and their nerve fibres. Can this be the same Breedlove that wrote such a measured response to the paper by Zhou et al(1) about the BST-C, in the same issue of Nature?
But no - it was by a reporter trying to put a spin on the story to attract the attention of the otherwise totally disinterested reader.
Most people equate thinking with the cognitive processing that goes on in the cortex. Clearly this does not happen in isolation, as when we say that we let anger cloud our judgment. Equally the anger and the body chemistry changes it brings about may well have come about from the cognitive appraisal of a social situation. The interplay between the different parts of the brain is a complex process, particularly when the aphorism "Don't get mad - get even" is difficult to carry out in practice.
The news report manages to cloud the basic issue that, at the time of Zhou's study, which was based on an earlier one by Allen and Gorski(2), the prevailing dogma was that the structure of the brain is complete at birth and remains unchanged through life.
Thus the study was seized on, by the less critical in the transsexual community, as being a cause of gender identity problems. The fundamental weakness, of course, was that there was no more than a correlation - unacceptable, for instance in a court of law.
That no neurons are produced, or that they are never replaced is still largely an assumption, but what is clear is that the nerve fibres, axons and dendrites with their synapses, (neurites) do develop and decay during life. At the time of Zhou's study, in another branch of neuroscience, its occurrence had been demonstrated during estrus in female rats. Presumably this was done by post-mortem studies of a succession of specimens at various stages of their cycle. Otherwise one has to assume that larger neurons mean more neurites and vice versa, and changes in size reflect changes in activity.
More recently other studies have supported this idea. For example, it seems the brain of a squirrel becomes larger towards its hibernation time in Winter as it memorises the location of its stores of food. One assumes that it is not simply the case that more fat is deposited for protection against extremes of temperature.(3)
Even with latest techniques it is still impossible to 'look at' individual neurites, or even a cluster of neurons as small as the BST-C, in a living human being. It should be possible to see an organ as large as the amygdala, although whether it is possible to measure subtle changes in size is debatable. It is therefore not possible to follow the dynamic changes that may occur in daily life.
As Breedlove points out, it is thought that smell is much less important to humans than it is to rats. The organs responsible are the olfactory bulbs, connected via the septum to the amygdala and in humans they have virtually disappeared. One could speculate that the main sexual organ in humans is the cortex, through the medium of imagination, and the neural and biochemical interactions are likely to be complex, to say the least.
It was with the foregoing in mind, that I offered an speculation at the Gateshead Conference(4) whereby size of the BST-C could be a during-life effect of being transsexual, rather than a cause.
It is still something of a mystery why human males produce so much testosterone, when most of it is bound to the blood plasma and is, it is thought, inert. It is also becoming clear that the amount of active testosterone varies in subtle ways on a situational basis for both women and men.
The interaction of brain structure and steroid hormones in humans is still a matter for future study. What Breedlove is demonstrating is that the removal of gonadal hormones in males, and administration of testosterone to females, has an immediate physiological effect, at least in rats.
As to the last paragraph in the news item: the actual study quoted does not say women "think and remember better as they age" - their mental powers simply deteriorate less quickly. Nor is it a matter of sex hormones as such, since, as reported in issue 2 of the Gendys journal, the neurons, in the part of the brain concerned, do not have hormone receptors. Rather it is thought that estrogens act as anti-oxidants removing certain toxins implicated in the ageing process.
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