FIGURE 4. Functional phases of the hair cycle.

a, Illustration of the bulge niche microenvironment and interfollicular dermal macroenvironment, including dermis, subcutaneous fat and adjacent follicles. Anagen-stimulating (black and green) or -inhibiting (red) activities are depicted with coloured arrows. Follicles are in different stages: A, refractory telogen; B, competent telogen; C, propagating anagen; and D, autonomous anagen follicles. Blue circle in A, intrafollicular microenvironment; colour-coded similar to panel b. b, New functional phases (coloured outer circle) mapped against classical hair-cycle stages (black and white inner circle). On the basis of the growth-inducing ability of the follicles, anagen is divided into propagating (inducing blue) and autonomous (non-inducing, yellow) phases. On the basis of the ability to respond to regenerative signals, telogen is divided into refractory telogen (red) and competent (green) phases.