A Hair Follicle Can Only Regenerate in Concert with Other Follicles, but Not by Itself
(A) When the density of plucked hairs from the mouse skin is lower than a certain density (i.e., “threshold density”), no follicles regenerate. If the density of plucked hairs is higher than the threshold, all the follicles of plucked hairs (“distressed follicles”) and the follicles of surrounding intact hairs (“healthy follicles”) regenerate by entering growth (anagen) phase from a dormant (telogen) phase.
(B) A distressed hair follicle secretes the cytokine Ccl2. M1 macrophages sense Ccl2 and swim toward the distressed follicle.
(C) Macrophages secrete Tnf-α. Tnf-α activates regeneration of distressed and healthy follicles. Higher density of distressed follicles leads to a higher density of M1 macrophages recruited to the follicles.
(D) Main factors that a group of cells may use to collectively make a binary decision.