Location: Outside U.S.
Date: Spring 2012
I heard that the male fetus has to go through stages to definitely become a male - that all fetuses are initially female, and that only after 8 weeks does the male fetus develop testicles, and that after that, I understand there are 3 more stages of development for the fetus to go through before definitely becoming male. What are these stages?
Embryos are male or female from the moment of conception because they have either XX or XY chromosomes. But they don't start to develop their sexual phenotypes for about 6-8 weeks. Embryos have two nonspecific gonads and two sets of nonspecific tubes.
If there is a Y chromosome, a gene called SRY turns on around 6-8 weeks and starts male development. The nonspecific gonads become testes and the tubes become the vas deferens. It causes the second set of tubes to degenerate. It also causes the testes to begin secreting testosterone which also adds to the masculine phenotype. There is a step by step process that happens. If there is a block in the process everything stops there. So some parts could develop and others not.For instance, internal structures might develop but external not. This would result in an ambiguous phenotype which may be mistaken for a female.
If there is no Y, the nonspecific gonads become the ovaries and the tubes become the fallopian tubes.
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Update: June 2012