Figure 1. Schematic of the human hair follicle.

The hair follicle contains both mesenchymal and epithelial components, separated by a basement membrane. The mesenchymal dermal papilla and connective tissue sheath (dermal sheath) direct growth and differentiation of subjacent epithelial cells. In the epithelial compartment, stem cells exit the bulge, proliferate, and contribute to the outer root sheath. Cells in the matrix, which forms from the hair germ in the telogen to anagen transition, proliferate, differentiate, and migrate upwards towards the skin surface giving rise to the three concentric layers of the hair fiber (medulla, cortex and cuticle), the three layers of the inner root sheath (cuticle, Huxleys, and Henles), and the companion layer.