SCARCE AS HEN'S TEETH
Research involving chicken DNA may eventually lead to chemically coaxing stem cells to produce missing tissues or organs.
Which came first: the chicken tooth or the bio-engineered liver?
In a study in the August 29, 2000 issue of the Proceedings of the National Academy of Sciences (PNAS), Cheng-Ming Chuong, M.D., Ph.D., professor of pathology at the Keck School of Medicine, and his colleagues, showed that although birds stopped producing teeth some 60 million years ago, chick embryos still retain the genetic programming to perform the feat. In fact, they showed that certain embryonic tissues, given the right cocktail of growth hormones and other proteins, can be prompted to take several key strides down that developmental pathway.
This is the first step, says Chuong, toward the "long-term goal of tissue engineering for epithelial organs such as hair, teeth, glands, liver, etc."
His colleagues on the study are from Harvard Medical School, Tulane University in New Orleans and Guy's Hospital in London.
The key to such goals, Chuong explains, are the epithelial stem cells-cells whose ultimate fate has yet to be decided. If these cells can be grabbed before they have made any irrevocable moves towards becoming one type of organ or another, researchers might then learn how to chemically coax them into producing or regenerating missing tissues or organs-not only in chicks and other birds, but in humans.
The study published in PNAS, says Chuong, is a promising move in the right direction. Although the cells examined were not stem cells per se, they were cells from the chick's oral epithelium at a relatively early stage of development-early enough, in fact, that they could still be persuaded to develop at least the rudiments of teeth.
The researchers found that birds are missing-or, to be more specific, no longer producing-a protein called BMP4 that, in mice, turns dental epithelium into teeth. When they exposed tissue from chick embryos to BMP4, the tissues organized themselves into primitive tooth buds. The chicks do carry the gene to respond to BMP4, they found, but it has been disabled over evolutionary time.
"Basically, birds used to have teeth, which were lost in evolution," Chuong explains. "In the lab, we are now able to wake up that potential-at least partially."
Chuong points out that, in a different study published earlier this year, his laboratory was able to take chicken foot scales and turn them into feathers. That paper was published in the journal Developmental Biology.
"These reports demonstrate the potential of epidermal stem cells," Chuong notes, "and show that our basic research in understanding the development of these appendages can now be translated to alter or guide the fate of epithelial stem cells."
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