Individual hair follicles are in a constant cycle of growth, transition and resting. Each individual hair is formed deep inside the hair follicle in a root bulb that has it's own blood supply. A hair follicle is a powerful unit, which produces many inches of hair and pauses only a few months before shedding the hair and starting the hair growth cycle again. Individual hairs grow for many years and are subjected to tortuous treatment that includes shampooing, cutting, being blown dry, chemically treated, colored and exposed to sun and wind. None of this torture affects the growth of the hair follicle. Some treatments may seriously damage the hair shaft, none will cause the hair to be shed. However at the end of the growth cycle the hair falls out on it's own. The follicle rests for a short period, and then starts to produce yet another new hair. It is very important to understand the hair cycle to understand male pattern baldness.
Adult scalp hair typically grows about 6 inches per year. Growth of any individual hair follicle is random and not influenced by seasonal changes but it will be in one of three distinct phases. Either growing, resting or shedding.
The distinct hair growth stages are anagen, the growth phase where the hair is activly growing and increasing in length. Catagen the transitional phase and telogen the resting phase.
The Anagen phase is the active phase of the hair cycle where cells in the root of the hair are dividing rapidly. As new hair is formed it pushes the hair up the follicle shaft. During this phase scalp hair grows about 1/2" per month and can remain in this stage for two years to as long as seven years. The hair for an average male will stay in this phase about two years and will be able to obtain a maximum hair length of around 12 to 18". Females will average longer and can achieve hair lengths twice as long as males due to the effects of estrogen on the Anagen cycle. At the end of this phase blood supply to the hair bulb slows and will stop completely as a result the hair stops growing and moves to the next phase.
Body hair typically have a very short anagen phase and thus the short length.
The Catagen phase is a transitional stage where up to 5% of all scalp hairs will be at one time. This phase only lasts a few weeks while the root shrinks. The hair may be shed during this phase.
The Telogen phase is a resting phase and may include up to 18% of all scalp hair at a given time. The phase lasts for about three months and hairs in this phase are easily shed if they have not already shed during the Catagen phase. This shedding is normal and makes way for a new hair to form as the cycle begins anew. The shed root of a hair that is in this phase will have a solid hard white bulb. Typically 50 to 100 hairs in the telogen phase are shed each day.
Once the telogen cycle is complete the hair follicle quickly returns to the anagen phase and a new hair is seen after a few days.