Why You Grow Hair in Some Place

Why You Grow Hair in Some Place

Why You Grow Hair in Some Places and Not in Others
Researchers are focusing on an inhibitor that blocks signals. Their work could lead to new treatments for baldness.
Share on PinterestScientists are zeroing in on the mysteries of hair growth on the human body. Getty Images
One of the mysteries of human evolution has been why hair grows in some places on our bodies and not elsewhere.
Scientists at the University of Pennsylvania’s Perelman School of Medicine have found clues they say may help unlock that puzzle.
Using mouse skin, the researchers found the existence of a naturally occurring inhibitor that’s secreted in developing hairless skin.
The inhibitor blocks a signaling pathway, known as the WNT, that controls hair growth.
The scientists published the results of their study in the open-access online journal Cell Reports.
This breakthrough may open the door to developing products that could help restore hair where women and men want it — on their heads — and to prevent it from growing where women don’t want it, such as on their faces.
The American Academy of Dermatology reports that more than 80 million people in the United States have androgenetic alopecia, also called male pattern or female pattern baldness.
Sarah E. Millar, PhD, a professor in dermatology and director of the Penn Skin Biology and Diseases Resource-based Center, is a co-senior author of the study.
“WNT signaling is critical for the development of hair follicles,” Millar told Healthline. “Blocking it causes hairless skin and switching it on causes formation of more hair. In our study, we’ve shown the skin in hairless regions naturally produces an inhibitor that stops WNT from doing its job.”
Millar said that the pathway is switched on initially in fetal skin, where it activates genes that promote hair follicle development.
“In adult life, hair follicles go through cycles of growth, regression, rest, and regrowth,” she said. “WNT signaling is important in controlling adult hair growth as well as in the initial development of hair follicles. High levels of WNT signaling promote hair growth and when WNT signaling is blocked, the hair follicles stop growing.”
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Removing the blockade
The natural inhibitor that stops WNT from doing its job is called DKK2.
This protein, found in specific embryonic and adult tissues, plays a variety of roles.
When Millar and her colleagues tested plantar (sole of the foot) skin from mice — which is like the bare underside of the human wrist — they discovered a high expression of DKK2.
When they removed DKK2 genetically, Millar said they observed hair growth in this normally hairless skin region.
“This is significant because it tells us WNT is still present in hairless regions,” Millar said. “It’s just being blocked.”
While hair follicles develop in the human fetus, the body stops producing them after birth.
That’s why hair follicles fail to regrow after severe burns or extensive, deep wounds in the skin.
“Severe wounds and burns destroy hair follicles and their associated stem cells,” Millar said. “The mechanisms that generate new hair follicles in fetal skin no longer function in adult human skin. We are investigating whether this may be due to expression of naturally occurring WNT inhibitors.”

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