FIGURE 3.

Diagram of selected stages of the HF cycle. The HF cycle can be divided into three phases, i.e., anagen, catagen, telogen. Once it matures, the HF enters into catagen to begin the cycle, following the activation of numerous signals. In catagen, the lower 2/3 of the HF regresses due to apoptosis in the IRS, ORS and hair matrix, and eventually forms the epithelial column. The DP moves up and is in close proximity to the bulge. In the telogen, the primary HS transforms into the club hair, and proximity between the DP and bulge is maintained. The interaction between the bulge and DP activates the transition to anagen. The stem cells in the upper 1/3 of the HF are activated. The HF grows downward to construct the bulb and other structures. New hair is generated, and the old club hair is shed. The signalling pathways involved in the transition are indicated in the bubbles (Paus and Cotsarelis, 1999; Al-Nuaimi et al., 2010; Shimomura and Christiano, 2010; Houschyar et al., 2020). HF, hair follicle; IRS, inner root sheath; ORS, outer root sheath; DP, dermal papilla; HS, hair shaft; HFSC, hair follicle stem cell; SG, sebaceous gland; APM, arrector pili muscle.