This is the 2007 version. Click here for the 2017 chapter 16 table of contents.

Sexual Development

A tiny fetus growing in its mother's womb starts out sexually undifferentiated, neither male nor female in outward appearance. Male and female sexual anatomies develop from the same starting point. Up until the age of about 5 or 6 weeks, male and female embryos cannot be distinguished. Then, around the 8th week, the sex glands begin to differentiate (become different).

Normally, sexual differentiation is triggered by the sex chromosomes. If the fetus has two X chromosomes, it is genetically female. Normally the sex glands of a genetic female will develop into ovaries, full of tiny egg cells that will start to mature during puberty.

What are details of the sexual differentiation process?

If the fetus has an X and a Y sex chromosome, making it genetically male, then normally a chemical called H-Y antigen will cause the sex glands to develop into testes. As the testes develop in the womb, they begin to secrete two substances, testosterone and Muellerian-inhibiting substance (MIS) that trigger development of male reproductive structure.

What if the fetus is genetically male but there are no male hormones?

If anything goes wrong with the rather complex process of hormone regulation, a person's sexual identity can be altered, either biologically or psychologically. The presence of XX or XY genes, in themselves, do not make a person male or female. If something abnormal happens to the hormonal system, a genetic female can develop as a male or vice versa. If there are no male sex hormones present, the fetus naturally develops as a female, even if the embryo has an XY (male) chromosome pattern.

This first phase of sexual differentiation is called the gonadal phase. During this time the gonads (hormone-releasing sex organs) first develop, and they contribute to the rest of sexual differentiation. The next phase is development of other internal sex structures. Finally, in a third phase, the external sexual structures develop. Hormonal disturbances at any point can disrupt development so that (for example) a genetic female can have ovaries, a uterus, and Fallopian tubes, but masculine external genitals (a rare condition known as pseudohermaphroditism ).


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