Desperate bald people shouldn't try this at home, but researchers have found that mice will regrow hair, not just skin, after a flesh wound. The discovery dashes the dogma that adult mammals cannot produce new hair follicles, and it suggests ways of improving skin grafts and reversing hair loss.
Anchored in the skin's dermis, a hair follicle packs dead cells together at its base and sends them up to sprout as hair. Researchers have long believed that a mammal is born with a full set of follicles, leading to an irreversible decline in fuzziness with every one lost or damaged. Fifty years ago, researchers noticed hair regrowing on the injured skin of mice, rabbits, and humans, but the work "was kind of ignored," says investigative dermatologist George Cotsarelis of the University of Pennsylvania School of Medicine in Philadelphia.
Recently, while studying how skin stem cells help heal wounds, Cotsarelis and his colleagues noticed the same phenomenon. To comb for an answer, the team removed skin patches from mice and let the wounds heal naturally. Hair sprouted in the center of the healed patches of skin. The team then tagged hair bulges--the pocket of a follicle where stem cells reside--in the healthy skin near the wound. Although the bulges made cells that moved in and helped close the injury, they didn't make new follicles.
Further experiments showed that rather than developing from follicular stem cells, the new follicles came from stem cells of the epidermis. "They grew from cells that don't [normally] make hair follicles," says Cotsarelis. "They had to be reprogrammed." A key player in this reprogramming appears to be the protein Wnt, which plays many roles during development and wound-healing; the more Wnt the mice naturally produced, the more hair grew. And the injured area had to be sizable to stimulate hair growth, the team reports tomorrow in Nature. Follica, a company Cotsarelis co-founded, is now exploring how to apply the finding to reverse hair loss.
"Regenerating a new organ from scratch is quite striking," says Bruce Morgan, a molecular geneticist at Harvard Medical School in Boston. He notes that the findings may lead to better skin grafts; current grafting techniques don't enable growth of the follicles and glands needed for the skin to maintain itself. He is not surprised that there appears to be a crossover, with skin stem cells giving rise to follicles, while follicles send in cells to help close the wound: "When there's a trauma, everybody pitches in."