Female Mouse Embryos Remove Male Reproductive Systems

A protein called COUP-TFII determines whether a mouse embryo develops a male reproductive tract, according to researchers at NIH and their colleagues at Baylor College of Medicine. The discovery, which appeared online Aug. 18 in the journal Science, changes the long-standing belief that an embryo will automatically become female unless androgens, or male hormones, in the embryo make it male.

Dr. Humphrey Hung-Chang Yao, head of the NIEHS reproductive developmental biology group, studies how male and female mouse embryos acquire their sex-specific reproductive systems. He said all early-stage mammalian embryos, regardless of their sex, contain structures for both male and female reproductive tracts. For a mouse or human to end up with the reproductive tract of one sex after birth, the other tract has to disintegrate.

“I learned in graduate school that androgens are needed to maintain the male reproductive tract, but our work finds that maintenance of the male reproductive tract can be achieved without androgen,” Yao said.

Since the 1950s, scientists have believed that androgens produced by embryonic testes, promote the survival of the male reproductive tract. The scientific consensus favored a female by default scenario, in which the absence of androgens in female embryos resulted in the breakdown of the male reproductive tract. However, Yao’s work demonstrated that female embryos actively promote the elimination of the male tract through the action of COUP-TFII, challenging the female-by-default theory.