Sex Determination and Differentiation

In humans, sex is determined by a specific set of chromosomes. Females have two X chromosomes (XX), whereas males have an X and a Y chromosome (XY). A mature female will produce eggs, each with one X chromosome, a mature male will produce sperm with either an X chromosome or a Y chromosome. When an egg and a sperm fuse during reproduction, the chromosome that the sperm carries determines the sex of the child.

Sometimes an individual may receive an abnormal amount of chromosomes, such as in the case of Turner's Syndrome where women have only one (functional) X chromosome (XO).

Human Sex Differentiation

Sex differentiation, which is when a fetus gains characteristics of either males or females during development, depends on gonadal steroid hormones. These hormones perform 'organizing functions' to permanently differentiate sex organs during development. This process starts before the developing child is even old enough to be considered a fetus, and is instead still an embryo.

By the sixth week of development, all embryos have both Wolffian ducts and Müllerian ducts. In this stage, the internal organs are bipotential, meaning they have the potential to develop into both male and female sex organs. The way they develop is influenced by hormones, and each fetus will only have one of pair of these ducts by the end of differentiation.

The differentiation process is started for males by a gene on the Y chromosome known as the SRY-gene (Sex determining Region Y). When expressed, this gene initiates the correct biochemistry inside of a male fetus for him to develop male sex organs. The Wolffian ducts develop into the vas deferens and the seminal vesicle, and the Müllerian ducts degenerate. The undifferentiated gonads develop into testes, and other structures such as the prostate gland and scrotum develop. The illustration shows the male fetal sex organ development at four months.

Females have two X chromosomes, so they do not have the initiation of sex differentiation signaled by the SRY-gene; instead, their sex organs develop in the absence of these cues. The Müllerian ducts. The undifferentiated gonads develop into ovaries, and other structures such as the labia and vagina develop. The illustration shows the female fetal sex organ development at five months.

This is the sex differentiation process for mammals as a whole, but not all animals undergo this process. For example, sex differentiation in birds is seen as the reverse as in mammals. It depends on the presence of hormones to begin differentiation in the female, and a lack of those hormones to begin differentiation in the male.

Development of the External Sex Organs

In this next illustration, you can see that the external genitalia of males and females are not as radically different as one might expect. The external genitalia are undifferentiated at four and six weeks, and then start to differentiate at ten weeks for males and twenty weeks for females. The final illustrations are the external genitalia at birth. It is interesting to realize that the same initial tissue makes up different structures in males and females, such as how the genital tubercle develops into the glans in males and the clitoris in females.

Special Circumstances

Some people have different circumstances that prevent them from developing along the same path that others do. The results of these circumstances may be intersex individuals who have sex characteristics of both males and females. The following are three examples of hormonal circumstances that change the way that the sex organs develop in males and females.

Androgen Insensitivity Syndrome

Androgen Insensitivity Syndrome (AIS) can occur in both males and females, but really only has a large impact on males. In this syndrome, males have tissues that lack sensitivity to androgens, meaning they do not respond normally to the hormones that cause masculinization. There are different extents to which a male is insensitive, so development may vary from person to person with AIS. In the most extreme case, males will develop external genitalia that resembles the genitalia of a female. Because the testes secrete anti-Müllerian hormone and the Wolffian ducts do not receive the proper signals to develop, both duct systems degenerate.

Congenital Adrenal Hyperplasia

Congenital adrenal hyperplasia (CAH) is an affliction in females who have overactive adrenal glands. This produces extra cortisol, a hormone that is structurally and functionally similar to testosterone. These recessive genes can also be found in males, but they do not have a large impact. Females will develop masculinized genitalia to different degrees, such as enlargement of the clitoris or possibly labial fusion. If it is found early and treated, females are said to grow up to be completely normal in her 'female behaviors' and have no conflict with her sexual identity.

5 α–Reductase Deficiency

Males with 5 α–reductase deficiency (5-ARD) do not undergo the same prenatal sexual differentiation as other males. The enzyme 5 α–reductase helps to control proportions of male sex hormones in the body. When there is a deficiency (not enough) of this enzyme, normal sex differentiation does not occur. Males with this condition may look like females until puberty, and then they have elevated enough levels of testosterone to experience the same changes as males without this condition. Even after puberty their genitalia do not look fully masculinized, though many are reported to live perfectly normal male lives.

Last Updated 10 December 2014.

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