The embryo in the left side of the picture is normal, while the other one
Girls are made of sugar, spice, and everything nice, or so the nursery rhyme goes. However, we may have to add one more ingredient to that list: a protein that kills off male reproductive tissue in embryos.
Scientists have long thought that female is the default sex for mammals. Embryos are automatically female unless androgens—male hormones—make the embryos male. However, recent findings show that this may not be the case.
It turns out that the development of the female reproductive organ doesn't just happen, so to speak. The development of male reproductive organs doesn't need a catalyst like androgen. However, the development of the female reproductive organ is an active process that entails getting rid of the Wolffian duct, a primitive male tissue. A protein called COUP-TFII is what facilitates the removal of the Wolffian duct, thereby allowing the development of the female reproductive organ. If the Wolffian duct remains, it eventually turns into parts of the male reproductive system involved in the ejaculation of sperm.
The stages of development of mouse emryos [photo by Azusa Inoue Shinpei Yamaguchi]
Researchers found evidence to challenge the female by default theory in a mouse model. They wanted to test how reproductive tissues interacted with the tract linings in female mouse embryos. At first, the researchers merely suspected that the protein COUP-TFII was involved in the process. To confirm their suspicions, the researchers genetically modified female mouse embryos by removing the gene that produced the protein. As a result, male reproductive tracts remained in the embryos, resulting in intersex babies with both male and female reproductive tracts.
This indicated that COUP-TFII was responsible for removing the Wolffian duct, and subsequently the male reproductive tracts. The researchers also found that androgen wasn't responsible for the presence of the male reproductive tract alongside the female one. Thus, it may be true that the default sex for mammals—mice, at least—isn't in fact female.
Perhaps it may help to think of COUP-TFII as the foreman of a particularly lazy group of construction workers on a demolition site. Without the foreman, the workers sit around and no demolition takes place. The structure—the Wolffian duct in this analogy—remains. One thing's that missing, however, is an explanation of what produces COUP-TFII and how. The study, while already informative, opens new questions as it answers old ones.
One thing you may have noticed by now is that the study only used mice. However, the researchers theorize that their findings may also be true for other mammals. After all, it's rare for female mammals to carry remnants of a Wolffian duct.
"This work is just the beginning and many interesting questions remain unanswered," says Humphrey Hung-Chang Yao, lead author of the study. "We will continue to study how the embryo develops a functional reproductive system."
This research can also contribute to existing knowledge about birth defects in the reproductive system come to be. These birth defects can lead to disorders like Klinefelter Syndrome, Turner Syndrome, undescended testicles, and cryptorchidism. Perhaps if researchers delve more into why female isn't the default sex for mammals, we'll find out more about the development of reproduction in mammals.
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