A popular idiom says that if you pluck one gray hair, many more will grow to take its place. Now it seems there may be a tiny grain of scientific truth behind the old saying—and perhaps some hope in the eternal battle against baldness. Pulling out hairs actually can induce them to grow back, along with up to five times as many new hairs in the surrounding area, according to tests done on rats.
The experiments suggest that hair regeneration is driven by a type of quorum sensing, the same phenomenon that bacteria and other microbial systems use to communicate and guide behavior. Finding hints that this same process happens in mammalian skin could lead to better understanding of the ways organ tissue systems decide to respond to damage that impacts only a small part of the whole. In addition to treating alopecia, or sudden hair loss, the work holds possible implications for regenerating tissue or even battling cancer.
“Of course we'd like to help alopecia patients,” explains Cheng-Ming Chuong of the Keck School of Medicine at the University of Southern California. “But it would be even more exciting if we could help bring out information on a fundamental mechanism that could help people's understanding of disease.”
Individual hair follicles follow an almost seasonal cycle, with phases of growth interspersed with rest, Chuong explains. Humans can alter this timeline—by plucking a hair from its follicle, they can activate it and cause the growth period to begin earlier. Most people studying hair regrowth have thought of each follicle as a single organ, Chuong says. But work from his lab suggested that when hairs were plucked, the follicles secrete a distress signal to their neighbors. The proteins they release in turn recruit immune cells to the injury site, where they produce signaling molecules that eventually cause the follicles to regrow hair.
“We started to wonder if we could position and pluck follicles in a specific way,” he says. “So that if a number of neighboring follicles got hurt and sent distress signals, a follicle in a resting stage but not plucked directly may also get activated without itself being plucked. And that is indeed the case.” When the team plucked 200 mouse hair follicles in high-density patterns, they actually caused between 450 and 1,300 new hairs to grow. Those hairs sprouted in the area that had actually been plucked as well as in nearby unplucked zones, according to results published this week in the journal Cell. Chuong calls the regrowth “pretty amazing to us.”
Pulling smaller numbers of hairs in a low-density pattern failed to generate the same type of hair growth, suggesting that there must be a tipping point that guides the quorum sensing phenomenon. Only when enough neighboring follicles signaled their distress did others begin the all-or-none process of new hair growth.
“We also speculate that this threshold might shift up and down with different body conditions,” Chuong explains. “So that the body might be over-sensitive or might not respond to a danger signal, depending on a person's general condition.” That might be tough news for people suffering from some hair-loss conditions, because it could limit the process's ability to get their hair growing again. In any case, Chuong cautions, no one should start plucking out their remaining hair out just yet.
“With people who only have 500 hairs left, we're probably not going to pluck all of them out to give them more hair,” he says. “That would take a lot of faith. I think more practically we'll try to study more about the molecular basis [of the process] in our animal system and then try to apply this to human patients in the future. But I do think that the results of this study certainly help us to move a big step towards that direction.”
Studying quorum sensing communication among bacteria is an emerging frontier of disease research, so there's also hope that the type of quorum sensing process detailed in the study may prove useful in battling an array of serious problems. “We can speculate that this kind of phenomenon probably exists in other organs,” Chuong says. “People haven't looked a lot before at quorum sensing at the organ level. I think that with this mechanism there are a lot of possibilities.”
When asked to speculate on possible areas of future research, he notes that some skin diseases manifest with lesions that appear in a pattern, which may be a suggestion that the same sort of mechanism is in play. Even cancer research, he notes, may someday make use of quorum sensing data. “Some people may have cancer cells in their body, but they are under control until defense mechanisms fall apart, and then the cancer manifests itself,” he says. “The finding we observe here might also be applicable there, and quorum sensing might actually work either way. It could be a help to the body's defenses in fighting cancer, or it could work on the cancer's part by sensing when the defense has crumbled, so this kind of threshold response could also be in different pathogenic mechanisms.”