Female development appears to be ‘default’
San Francisco, CA -- (SBWIRE) -- 12/31/2012 -- There have been surprising developments from Johns Hopkins, where mechanisms that promote male sex hormones such as testosterone have been isolated. Such hormones can control the mammary gland nerves that help handle the amount of milk available to the milk ducts, as well as other gender-specific actions.
The report was released in December in Science and says that the hormones are used to alter the availability of nerve growth. These nerves are noted under the acronym BDNF.
The macro-differences between male and female genders are not a mystery to humans, but these difference are portrayed at a more fundamental level. The most obvious difference between the genders is, of course, the physical structure. Yet, the nerves are also formatted in gender-specific ways.
"We now think we have a broader understanding of how sex-specific nerves reach their proper target in a given sex, say in mammary milk ducts in females, but disappear in the other sex," says David Ginty, Ph.D., a Howard Hughes investigator and professor of neuroscience in the Institute for Basic Biomedical Sciences at the Johns Hopkins School of Medicine.
Yin Liu, a student in Ginty's laboratory, studied the nerves in mice for the experiments regarding sex-specific neural wiring. The study was conducted around the monitoring of fullness of milk within the female milk ducts.
If the supply is low, nerves are believed to report the information to the brain in order to stimulate production, according to Ginty. Like so many differences in the sexes, in early embryonic development, there is a lack of true difference between the glands for both genders. As development continues, the nerves are more or less lost in the males, while the females' persists.
In a particular experiment, the researchers looked to how the nerves work through the immature mammary glands in development. Researchers found that the gland cells relied on four proteins that encouraged nerve growth. Only BDNF was found to be in significant quantities.
Ginty stated that BDNF is a known protein binder of TrkB, which is found in the surface of nerve cells. That binding is what triggers messages within nerve tells, enacting growth. "The cells of the early mammary glands released the signaling molecule BDNF, which was detected by TrkB on the nerve cells, which made them grow toward the mammary glands. What remained a mystery was why these nerve cells are lost a few hours later in males,” said Ginty.
Researchers noted that males development was devoid of a “suicide signal” for the glands, which would have been an obvious cause of the lack of development. Thus, the cells were considered to be reacting to something, rather than dying off.
Researchers then found through subsequent testing that female hormones were the “default” while male hormones caused withdrawal of nerves and glands of either sex.
"At this point, we knew that BDNF is found at comparable levels in the glands of both sexes, that TrkB is found at comparable levels on the nerve cells of both sexes, and that male sex hormones were still somehow creating a difference in the system," says Ginty.
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