Figure 1.

Follicle structure during the hair cycle. The structure of the hair follicle at different stages of the hair cycle is shown. The hair shaft is produced during the anagen phase. At this stage, the follicle projects deep beneath the epidermis. The hair bulb at the base of the follicle is shown magnified above. Matrix progenitor cells (rose) abutting the DP (bright green) generate progeny that proliferate a few more times in the hair matrix before differentiating into the cell types of the inner root sheath (light green) and hair shaft cuticle, cortex (yellow), and medulla (gray). The hair shaft and IRS are extruded through the outer root sheath (blue) until the hair shaft projects from the surface of the skin and the IRS is shed. Blue cells abutting the DP are the progenitors of the proximal outer root sheath. The keratinocyte compartment is encased by dermal sheath cells (orange). At the end of the growth phase, most keratinocytes of the lower follicle either apoptose or differentiate into the base of the hair club. The DP is drawn upward by the receding epithelial strand and comes to rest at the base of the permanent follicle during the quiescent telogen stage. Keratinocytes of the secondary germ (pink) lie between the DP and the “bulge.” These proliferate in early anagen to regenerate the hair bulb and engulf the DP.