Why do people have hair on their arms and legs, but not on the palms and soles of the
feet? It's a fundamental question in human evolution that researchers say have found clues 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.
“We know that WNT signaling is critical for hair follicle development; blocking it causes hairless skin and turning it on causes more hair to form," said the study's co-senior author. “In this study, we showed that the skin in hairless areas naturally forms an inhibitor that prevents WNT from doing its job.”
That natural inhibitor is Dickkopf 2 (DKK2) – a protein found in specific embryonic and adult tissues where it plays different roles. Researchers tested mouse plantar skin — roughly the equivalent of the underside of the human wrist — and found that DKK2 was highly expressed. In addition, when they genetically removed DKK2, hair started to grow in this normally hairless skin area.
"This is important because it tells us that WNT is still present in hairless areas, it's just blocked," said the researcher.
Some mammals, such as rabbits and polar bears, naturally develop hair in their plantar skin. This research group found that DKK2, unlike mice, is not expressed at high levels in the plantar skin of rabbits, explaining why hair can develop there. These findings suggest that the production of DKK2 in specific skin regions is altered during evolution to form different patterns of hairless or hairy skin according to the needs of the animal.
Hair follicles develop during fetal life, but their production stops after birth. As a result, hair follicles fail to regrow after severe burns or extensive, deep wounds in the skin.
