We may finally have a clue to why humans don’t have hair on the undersides of hands and feet.

In a paper published in the journal Cell Reports, researchers from the University of Pennsylvania in the US say they’ve found a naturally-occurring inhibitor secreted in developing hairless skin that blocks what geneticists call the Wnt pathway, which controls hair growth.

(“Wnt” is an abbreviation of “wingless/integrated”.)

“We know that Wnt signalling is critical for the development of hair follicles; blocking it causes hairless skin, and switching it on causes formation of more hair,” says dermatologist and one of the study authors, Sarah Millar.

“In this study, we’ve shown the skin in hairless regions naturally produces an inhibitor that stops WNT from doing its job.”

That inhibitor is Dickkopf 2 (DKK2), a protein found in specific embryonic and adult tissues, where it plays a variety of roles.

Millar and colleagues tested plantar skin, located on the sole of the foot, from mice, and found DKK2 was highly expressed. When they genetically removed it, hair began to grow in a normally hairless region.

DKK2 is not expressed at high levels in rabbit plantar skin, however, explaining why hair can develop there. Millar says the findings suggest production of DKK2 in specific skin regions has been altered during evolution to allow different patterns of hairless or hairy skin to form according to the needs of the animal.

The significant point is that it appears Wnt is still present in hairless regions, but it’s being blocked.

Hair follicles develop during foetal life, but their production stops after birth. As a result, hair follicles don’t re-grow after severe burns or deep wounds. Millar and her team are now investigating whether secreted Wnt inhibitors suppress hair follicle development in these scenarios, as well as other episodes of hair loss.