At first, follicles release inflammatory proteins, or cytokines, which alert the immune system to a wound, researchers say.
The immune system responds by sending macrophages to the problem area. Macrophages are white blood cells that engulf and devour pathogens, but they also release cytokines that can trigger a variety of responses in cells, such as causing them to proliferate.
In this particular situation, macrophages will secrete signaling molecules called tumor necrosis factor alpha, which at certain levels, will prompt plucked and unplucked follicles to grow hair.
A robust regenerative response seemed to be dependent on the density of signaling behaviors, researchers said.
For example, when researchers plucked mouse hairs in a diffuse pattern, in an area with a diameter larger than 6 millimeters, no hairs regenerated.
But when a dense concentration of hairs were pulled in an area 3 to 5 millimeters across, the plucked hairs grew back and new hairs sprouted nearby.
“The quorum sensing circuit we describe here provides a way for injured hair follicles to collectively assess the magnitude and extent of injury that the skin has sustained and make an all-or-none decision whether or not to regenerate.”
Researchers discover trick to regrowing lost hair – LA Times.