SEX HORMONES
From the sixteenth day after fertilisation of an egg a female embryo is detectably female as determined by the presence of Barr bodies (the repressed X chromosomes mentioned earlier). By the seventh week ovaries or testes are beginning to develop, and in the male testosterone, the main male hormone, is produced. Hormones are complex chemical messengers produced by one part of the body to be active somewhere else. The release of such chemicals is under the control of an area of the brain called the hypothalamus.
In the foetus the hypothalamus is cycled. That is, its activity switches on and off spontaneously for some as yet unknown reason. In females this cycling continues and controls many body functions, including ovulation and the menstrual periods during their fertile life. The male hypothalamus becomes uncycled at some stage, probably under the influence of testosterone.
So it is the absence of testosterone that makes a foetus female. In a condition known as testicular feminisation the genital tracts and external genitals are female and breasts appear at puberty. The person is genetically a male, with XY chromosomes, but looks like a female. The same thing can occur in reverse with ‘women’ who look like men.
All of this begins to raise some very fascinating questions. If, as we have seen, the brains of male and female foetuses are behaving differently (one cycled, and the other not) might there not also be other differences in brain function? It is already known that certain brain centres are influenced by sex hormones as well as influencing them and micro-anatomical research has shown that some areas of the hypothalamus are larger in women than in men, and that the connections between the cells of this area of the brain and the brain itself are different in the sexes.
It is interesting, before leaving the subject of hormones, to look at a rare condition called the adrenogenital syndrome. Girls with this condition are exposed to abnormally high levels of male hormones right from foetal life, because their adrenal glands over-produce these particular hormones secondary to an inborn error of metabolism. These girls have an enlarged (penis-like) clitoris and, compared with their sisters, spend more time in rough-and-tumble play, are
athletically orientated and less interested in playing with dolls. In other words, they are tomboyish in every respect. This (and parallel animal work) seems to prove that levels of male hormones from birth affect the developing brain in a way that changes a child’s behaviour.
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