Plucking a patch of hair provokes nearby unplucked, resting hairs to regenerate, new research has shown.
A long-standing question in biology and medicine is how organs, including the skin, self-repair. Are they responding to central "top-down" regenerative signals, or can tissues self-organise their regenerative efforts?
To find out, scientists at the University of Southern California, Los Angeles, painstakingly plucked hairs from the backs of mice.
By removing exactly 200 hairs each time but changing the diameter of the area they were taken from, they were able to see whether the plucked hair follicles affected those nearby that were untouched.
When 200 hairs were removed from a wide area, none of the hairs re-grew, even after a month.
But the researchers, writing in Cell, were surprised to find that if they removed 200 hairs from a smaller area, not only did the plucked hairs regrow, but they were also able to cause up to 1,000 nearby undamaged follicles to ‘wake up’ and produce hair.
This means that the hair follicles are not just able to repair themselves, but they are also able to communicate with nearby cells, up to 1 mm away, to repair damage efficiently.
The researchers called this effect ‘quorum sensing’. “It’s a democratic behaviour that describes how a population determines what to do,” explained Cheng-Ming Chuong, who led the project.
Quorum sensing has previously been used to describe how colonies of bacteria can communicate, but this is the first time it has ever been described in organs.
He likens the quorum sensing seen in hair follicles to a herd of wilderbeast grazing. Whilst the presence of a single lion may make a few of them nervous, it takes a certain number of predators to cause the whole herd to panic and stampede.
With the hair follicles, the level of injury nearby must exceed a certain threshold before they will respond.
The researchers believe communication between the cells happens in two stages, since chemical signals spread over only very short distances, suggesting that some other effect was at work.
Samples collected from the plucked areas showed an increase in the levels of an immune signal called CCL2, which was produced by injured hair follicles.
This, the team found, recruits large, mobile immune cells called macrophages, which migrate into the area and secrete a further immune signal called TNF-alpha. This turns on both the plucked and resting hair follicles, causing the re-growth of hairs locally.
This finding could be significant in explaining how organs in the body independently make decisions when repairing injuries. “It allows the body to preserve its energy to respond to a really dangerous signal,” explains Chuong.
Chuong suspects that this finding could one day be used to explain why some people suffer from auto-immune diseases, or even cancer. “When this threshold is not set right, we think the patient may not respond to a signal they should respond to, or over-react to a signal they should have ignored.
Although quorum sensing has yet to be investigated in humans, Chuong is upbeat. “I think this will help inspire people to look at the diseases they study with a new perspective”