Ever wondered why humans sprout hair on arms and legs yet keep palms and soles smooth? A pivotal study in human evolution uncovers a key clue: a natural inhibitor in developing hairless skin that blocks the WNT signaling pathway, crucial for hair growth.
"We know WNT signaling is critical for hair follicle development—blocking it results in hairless skin, while activating it promotes more hair," explained the study's co-senior author. "Here, we demonstrate that hairless skin naturally produces an inhibitor that neutralizes WNT's effects."
This inhibitor, Dickkopf 2 (DKK2), appears in select embryonic and adult tissues with diverse functions. Testing mouse plantar skin—the equivalent of the human palm or sole—researchers found elevated DKK2 levels. Genetically eliminating DKK2 triggered hair growth in these typically bare areas.
"Crucially, this shows WNT signaling persists in hairless regions; it's simply inhibited," the researcher added.
Some mammals, including rabbits and polar bears, grow hair on plantar skin. Unlike mice, rabbits express low DKK2 there, enabling growth. These insights suggest evolutionary tweaks in DKK2 production shape species-specific hair patterns to meet environmental demands.
Hair follicles form in fetal life but halt production post-birth, preventing regrowth after severe burns or deep wounds.
