Why we have hair here, but not there

Why do humans have hair on their arms and legs but not on the palms of their hands and the soles of their feet? It’s a fundamental question in human evolution that researchers at the Perelman School of Medicine say they’ve found clues to in a new study. Their findings reveal the existence of a naturally occurring inhibitor secreted in developing hairless skin that blocks a signaling pathway, known as the WNT pathway, which controls hair growth. Cell Reports published the study.

one-half of a child's face with a fake mustache

“We know that WNT signaling is critical for the development of hair follicles; blocking it causes hairless skin, and switching it on causes formation of more hair,” says the study’s co-senior author Sarah E. Millar, the Albert M. Kligman Professor in Dermatology and director of the Penn Skin Biology and Diseases Resource-based Center. “In this study, we’ve shown the skin in hairless regions naturally produces an inhibitor that stops WNT from doing its job.”

That natural inhibitor is Dickkopf 2 (DKK2)—a protein that is found in specific embryonic and adult tissues where it plays a variety of roles. Researchers tested plantar skin from mice—roughly the equivalent of the underside of the human wrist—and found that DKK2 was highly expressed. Furthermore, when they genetically removed DKK2, hair began to grow in this normally hairless skin region.

“This is significant because it tells us WNT is still present in hairless regions, it’s just being blocked,” Millar says.

Read more at Penn Medicine News.