Complete androgen insensitivity syndrome
In complete androgen insensitivity syndrome, an XY person has female external genitalia because tissues cannot respond to testosterone, and typically no uterus because testes make the hormone that removes it.
Remember
In complete androgen insensitivity the uterus is typically absent because the testes still make the hormone that removes it, while the body cannot respond to testosterone, so the external genitalia are female.
Memory aid
XY, female outside, no uterus inside, testes in the wrong place
Run through it in order to recall the chromosomes, the outer appearance, the uterus and the testes.
In complete androgen insensitivity syndrome (CAIS), a person has a 46,XY chromosome pattern (the usual male pattern of sex chromosomes, one X and one Y). The SRY gene (the Y-chromosome gene that switches on testis development) works, so normal testes form. The testes make testosterone (an androgen, or male sex hormone), but the androgen receptor, the protein that cells need to respond to testosterone, does not work.
The testes send two messages, and only one is heard. Sertoli cells (the supporting cells of the testis) make anti-Müllerian hormone, which makes the paramesonephric (Müllerian) ducts, the embryonic tissue that would form the uterus, regress (shrink away). So the uterus is typically absent. Testosterone tells the mesonephric (Wolffian) ducts, which would have become the male internal tubes and glands, to develop. With no response to it, those ducts do not develop and the external genitalia follow the default female pathway.
The result is an XY person who looks female but has testes and typically no uterus. The testes are normally formed but often undescended, meaning they remain inside the abdomen or the groin. Because the baby looks female, CAIS is often not noticed until puberty, when periods do not start.
Partial androgen insensitivity differs, because testosterone still has some effect and the genitalia often look atypical.
Key facts
- Chromosomes and testes
- 46,XY with normal testes, but an androgen receptor that does not work.
- External genitalia
- female, because the tissues do not respond to testosterone.
- Uterus
- typically absent, because the testes make anti-Müllerian hormone and the paramesonephric ducts regress.
- Male internal tubes
- the mesonephric ducts do not develop, because the androgen receptor does not respond to testosterone.
- Testes
- present but often undescended, lying in the abdomen or the groin.
- Presentation
- often not noticed until puberty, when periods do not start.
Exam traps
- Loss of the mesonephric ducts explains the missing male internal tubes, not a missing uterus, because the uterus depends on the paramesonephric ducts.
- A mutation in the gene for anti-Müllerian hormone or its receptor would leave the paramesonephric ducts in place and so keep a uterus; in androgen insensitivity the hormone works normally, so this does not explain the missing uterus.
- In androgen insensitivity, an XY person with no uterus and ambiguous external genitalia points to the partial form, where the receptor responds a little to testosterone.
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