A recent study investigates why some parts of the human body remain hairless while others become hirsute. The findings may offer clues to male- and female-pattern baldness.

Humans have hair in some places but not in others. The tops of our heads are generally replete with hair, while the soles of our feet are not.

This is certainly not news, but exactly why we have hair on our legs and arms but not on the palms of our hands is still a puzzle.

Hair loss affects a considerable proportion of society, particularly men, and it can cause significant distress for some people.

Male- and female-pattern baldness, which are common hereditary forms of hair loss, affect around 80 million people in the US.

Due to the prevalence of hair loss and its potential psychological impact, there is a great deal of interest in the mechanisms that regulate hair growth.

The mystery of hair distribution

Although we now have a good understanding of how hair grows, the reason why it grows in some places but not others is still up for debate.

A team of researchers from the Perelman School of Medicine at the University of Pennsylvania in Philadelphia recently took a new approach to this issue. The journal Cell Reports has published their findings today.

In particular, the researchers focused on WNT pathways. These signaling pathways are pivotal during embryonic development and continue to play a part in regenerating certain bodily tissues into adulthood.

Co-senior author Prof. Sarah E. Millar, PhD explains why they focused on this pathway:

"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."

The team was also interested in a protein called Dickkopf 2 (DKK2), which belongs to a family of natural inhibitors that play a significant role in embryonic development. DKK2 is known to inhibit WNT pathways.

To investigate the potential role of WNT pathways and DKK2 in hair distribution, the team studied plantar skin in mice, which is analogous to the underside of the human wrist.

In the mouse plantar skin, there were high levels of DKK2 expression. The researchers also found that if they removed genes responsible for the production of DKK2, hair grew on the skin sample.

Prof. Millar explains that "[t]his is significant because it tells us WNT is still present in hairless regions, it's just being blocked."

Mice compared with rabbits

In another experiment, the team decided to look at the plantar skin of rabbits because hair does grow there in these animals. As expected, there were much lower levels of DKK2 in rabbit plantar tissue compared with mouse tissue.

As a result of the lower levels of DKK2, there is no inhibition of WNT, and this allows hair to grow. The team plans to continue investigating this mechanism in other scenarios.

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

—Prof. Sarah E. Millar

As we develop in the womb, our hair follicles grow. However, once we are born, we no longer produce them. We have around 5 million hair follicles at birth, and they must last us for life.

This explains why skin remains hairless following a burn or injury. The team wants to understand if suppression of the WNT pathways plays a part here too.

Previous research has revealed an association between the DKK2 gene and male- and female-pattern baldness. Understanding more about the molecular basis of this relationship might help shape treatments for hair loss in the future.

Prof. Millar notes that this is certainly not the end of the line.

"We hope that these lines of investigation will reveal new ways to improve wound healing and hair growth, and we plan to continue to pursue these goals moving forward," she says.