Hair loss physiology and transplantation principles

In the healthy scalp, hair is replenished commensurate with the rate at which hair is lost, resulting in a cosmetically stable hair pattern. In male and female patterned hair loss, two major disruptions to this equilibrium take place: hair cycle perturbation and follicular miniaturization. Androgen-driven alteration of the hair cycle truncates the anagen phase, which can effectively shorten total hair length until hair shafts no longer reach the surface of the skin. During the catagen/telogen phase, which is protracted in patterned hair loss, the follicular units miniaturize and form into thin vellus hairs from thick pigmented terminal hairs. The ratio of terminal to vellus hair also decreases, further contributing to the appearance of balding [1].