A human embryo normally undergoes about 9 months of gestation. At about 4 to 6 weeks, the first tissues that will become the embryo’s gonads develop. Sexual differentiation begins a week or two later and is initiated by the sex chromosomes, which control at least four important aspects of sexual development: (1) the internal sexual organs (for example, whether the fetus develops ovaries or testicles); (2) the external sex organs (such as the penis or clitoris); (3) the hormonal environment of the embryo; and (4) the sexual differentiation of the brain (Money & Norman, 1987).
Internal Sex Organs
In the first few weeks of development, XX (female) and XY (male) embryos are identical. Around the 5th to 6th week, the primitive gonads form, and at this point they can potentially develop into either testes or ovaries. Traditional developmental models claim that the “default” development is female; without the specific masculinizing signals sent by the Y chromosome and the SRY gene, the gonads will develop as female. The SRY is a Y chromosome-specific gene that plays a central role in sexual differentiation and development in males (Knower et al., 2003). However, it may not be only testosterone or the SRY gene that differentiates males from females—it may also be the presence of ovarian hormones (Fitch & Bimonte, 2002).
In most males, the testes begin to differentiate from the primitive gonad by the 7th to 8th week following conception. In most females, the development of the primitive gonad begins to differentiate into ovaries by the 10th or 11th week. The primitive duct system, the Mullerian (myul-EAR-ee-an) duct (female) or the Wolffian (WOOL-fee-an) duct (male), also appear at this time (Warne & Kanumakala, 2002). Once the gonads have developed, they then hormonally control the development of the ducts into either the female or male reproductive system.
In female embryos, the lack of male hormones results in the regression and disappearance of the Wolffian ducts, and the Mullerian duct fuses to form the uterus and inner third of the vagina. The unfused portion of the duct remains and develops into the two oviducts or Fallopian tubes (see Figure 3.1).
In the presence of a Y chromosome, the gonads develop into testes, which soon begin producing Mullerian inhibiting factor (MIF) and testosterone. MIF causes the Mullerian ducts to disappear during the 3rd month, and testosterone stimulates the Wolffian duct to develop into the structures surrounding the testicles. The body converts some testosterone into another androgen, called dihydrotestosterone (DHT), to stimulate the development of the male external sex organs.