Conception and Development.
The Embryo - Introduction
The Germ Cells
Germ Line and Somatic Mutations
Conception and Development.
The SRY Gene.
Genetic Errors of Metabolism.
The Fetal Environment.
At eight weeks most of the features of the adult are visible, when it is referred to as a fetus. During the first few weeks, it is neither male nor female. However, a small group of cells, called the "indifferent gonads" begin to form, that are capable of becoming ovaries or testicles. At the same time, other internal features of both sexes develop, the Mullerian (female) ducts and the Wolffian (male) ducts.
The gonad contains supporting cells and others that will, in time, produce hormones, the so-called gonadotrophins, androgens or estrogens, depending on the direction of development.
The gamete producing cells, which will in the future, produce the ova or sperm, originate in another part of the embryo and migrate to it.
For a male embryo, at around the sixth week, the SRY gene, on the Y chromosome promotes a protein called the H-Y antigen. The effect of this is to bind to the DNA molecule itself, in a number of specific places, causing it to bend, in turn affecting the action of a number of genes.
About nineteen different genes are probably involved, on either the X chromosome or the autosomes.
The actual sequence of events is still largely unknown, but the hormone producing cells of the indifferent gonad become the Leydig cells, the primitive testes, which have rudimentary sperm producing tubules, while the supporting cells become the sertoli cells which will, in time, produce the sperm.
They also have the ability to produce testosterone and other androgens, along with a hormone called Mullerian Inhibiting Factor. The latter, as its name implies inhibits the further development of the female sexual features, which degenerate.
This in turn sets off a complex sequence of hormonal changes which encourage the action of the genes in developing some characteristics and inhibiting others.
In a female embryo, from about the sixth week, the Wolffian ducts degenerate, and the Mullerian ducts develop towards the Fallopian tubes, uterus and vagina. Meanwhile, by the twelfth week, the indifferent gonad begins to develop into an ovary.
The supporting cells form the cells which will surround the ova, the granulosa cells, and the hormone producing cells form the thecal cells which remain relatively quiescent. The chromosomes in the gamete cells begin to separate, but then cease their activity until puberty.
As early as 1983, the idea of femaleness being merely the absence of maleness was out of date. As Rose, Lewontin and Kamin(1) point out, there is a specific 'feminisation' process. Clearly there must be two X chromosomes, otherwise the gamete cells die, as in X0 (Turner's Syndrome), and the ovary atrophies. Moreover there is a rise in estrogens in the female embryo at six weeks, paralleling the rise in androgens in the male.
Nevertheless, the process of female development does not appear to have been studied in depth, being taken as a sort of default state, and Fausto-Sterling(2) criticises "the willingness of researchers to accept at face value the idea of passive female development."
Until the eighth week, the external genitalia grow identically for either sex, but by twelve weeks, for a female, the genital tubule develops into a clitoris, while the urogenital membrane develops towards the labia.
In the male fetus, an enzyme, 5 alpha reductase, converts testosterone to dihydrotestosterone. The genital tubule instead becomes a penis, while the urogenital membrane fuses to become the scrotum.
Bibliography and good reading.
Bland, J., (1998) About Gender: Conception and Development.
Book graphics courtesy of Amazon.co.uk
Web page copyright 1998-2006 Derby TV/TS Group. Text copyright Jed Bland.
24.05.98 Last amended 03.03.04, 01.05.14