OF ALL structures in or on the mammalian body, the hair follicle has one of the most complex functions. It must produce, on the skin surface during a long period, a multicellular product, the hair shaft, and yet preserve in the deep dermis an "epithelial finger" that produces the shaft. The cells making up the shaft contain the machinery to produce strong cytoskeletal and cellular adhesions and to be molded by the inner root sheath. The combined hair shaft–inner root sheath structure moves outward as a unit, sliding along a slippage plane provided by the innermost layer of the outer root sheath. The outer root sheath remains behind and intact. The shaft is liberated from the sheath at a level just below the sebaceous duct, and it exits the pilary canal as a sheath-free hair fiber.
Stenn KS, Sundberg JP, Sperling LC. Hair Follicle Biology, the Sebaceous Gland, and Scarring Alopecias. Arch Dermatol. 1999;135(8):973-974. doi:10.1001/archderm.135.8.973