Alopecias. Practical Evaluation and Management

Abstract

The human hair growth is cyclical so that its follicle undergoes intermittent periods of growth when a new hair is produced, followed by periods of quiescence, when resting hair is retained. All human hairs show various stages of hair growth: the so called anagen or growth phase that lasts about 6 years; the regression catagen phase of about 3 weeks, and the resting telogenic phase of about 3 months. While on one hand anagen duration varies depending on age, season, anatomic region, sex, hormonal levels, and genetic predisposition, on the other hand the physiological trigger mechanism for the initiation of each hair cycle remains the main problem concerning the control of hair growth. Thus, it is estimated that the healthy individual loses 100 hairs daily and each follicle completes its cycle 10-20 times over a life-time. Unfortunately, the mechanism of cycle activation is unknown, but could involve a diffusible direct cell-cell contact. The biologic clock determining the end of the anagen phase and the beginning of the cartagen-telogen phase is, in fact, a complex phenomenon in which multiple proteins partecipate.Various metabolic and physiologic changes are capable of adjusting this biologic clock within hair follicles, and it is possible to enter the telogen phase simultaneously for abnormally large number of hairs. When this happen, approximately 300 hairs are shed daily. However, hair growth on the scalp can last for several years, and the duration is dependent on the continued activity of the matrix cells. But hair cycle is also affected during the aging process, especially with respect to the duration of the growth and resting phases. As an individual ages, the duration of telogenic become longer, probably as a consequence of the declining capability of hair follicle stem cells to initiate the anagen cycle.