By Paul Recer
Associated Press
Washington
Researchers have isolated a genetic switch that separates the boys from the girls. It turns off the female in the human embryo and starts biological changes that eventually put hair on the chest. Dr. Michael Weiss of the University of Chicago said the new study advances the understanding of the complex cascade that determines whether people are male or female and sheds new light on how this process can sometimes go awry.
In research to be published today in the journal Science, Weiss and his team use sophisticated imaging techniques to explore on an atomic level the biological pathway womanhood.
Weiss said science has long known that everybody at conception is female, but the precise biological mechanism that changes an embryo t() male is still incompletely understood. Solving the puzzle on a molecular level may answer questions about other basic cellular changes, such as the development of cancer. For the first weeks after. conception, all mammal embryos start forming the basic female structures - uterus, fallopian tubes and vagina.
The embryo destined to become a boy begins as a female," Weiss said. "It lays down first female structures, and not male structures, at a phase when the embryo looks like a recognizable mammal, with toes, fingers and eyes and a heart. This is 35 to 4() days into human gestation."
After that, he said, a gene called SRY switches on to start the embryo on its way toward manhood. The new study shows that SRY also triggers the work of another gene called MIS. that dissolves the female parts of the original embryo. "SRY is the master switch," said Weiss. ìFor the first time we have shown that SRY can activate a male specific pattern of gene expression leading to activation of MIS, which is the key signaling molecule for half of the male pathway." Sex determination in mammals originates with the chromosome of the sperm that fertilizes the egg. Male sperm can carry one of two chromosomes, X or Y. The Y-chromosome is the male element that carries the SRY gene. The female egg has an X-chromosome.
If the egg is fertilized with the father's X-chromosome, the embryo continues its development as a female. But if the egg is fertilized with Y, the SRY gene sets off a series of changes that eventually creates a male.
In experiments on embryonic mice, Weiss and his team used nuclear magnetic resonance images to show that the SRY gene sends chemical signals to the MIS gene. The MIS gene then causes the nascent female organs in the embryo to disappear. SRY also starts the growth of testicles, which, in turn, start making testosterone, the hormone that causes the development of male characteristic the penis, masculine-like muscles and, eventually, facial hair.