We’re born with approximately 100,000 hair follicles and do not develop new ones after birth. Hair follicles change in size over time as they pass through various phases of the hair cycle.
Each hair follicle completes 10 to 20 cycles in a lifetime. There are three classic stages to the hair cycle: anagen, catagen, and telogen. Anagen, or growth phase, lasts 3 to 10 years. Catagen, the trasitional, phase is only a matter of 2 to 3 weeks, while telogen, or resting phase, lasts 3 to 4 months. The majority of hairs remain in growth phase, with an average of 13% and 1% of scalp hairs in telogen and catagen phases, respectively. Hair follicle cycles throughout the scalp are not synchronous.
Since the 1940s male pattern baldness (MPB) has been recognized as an androgen-dependent condition in which (a) the anagen phase is shortened, and (b) the hair follicles become slightly finer and shorter with each cycle. The role of androgens in female pattern hair loss (FPHL), however, remains uncertain. Both MPB and FPHL are often familial and have an inheritance pattern that has both maternal and paternal contributions.