The hair follicle is a small tubular sac just under the surface layer of the skin. Each of these sacs contains a bulb at its base where rapidly dividing cells help produce hair. Old cells are packed one under the next, pushing the hair forward and extending its length. As the hair grows, its follicle goes through what is often known as a growth cycle, which consists of three separate phases.
Anagen Phase
The anagen phase is the stage in the growth cycle responsible for producing hair, explains the American Academy of Dermatology. Anywhere between 85 and 90 percent of hair is in this phase at one time. With more than 100,000 hairs making up the scalp, that's upwards of 90,000 hairs growing on your head. This phase lasts about two to six years, producing an average of 1/2 inch of hair growth each month.
Catagen Phase
After the anagen phase, the follicle moves into the catagen phase, which is a period of transition for the hair. During this time, the hair ceases to grow and the follicle begins to shrink in size. Roughly 5 percent of the follicles are in this phase at one time, working out to about 5,000 hairs. This phase is by far the shortest of all phases, ranging from one to two weeks.
Telogen Phase
As soon as the follicles complete their transition, they move into what is known as the telogen phase. This phase is often considered the resting phase of the follicles. Much like the catagen phase, no hair growth is seen during this time. Less than 10 percent of follicles are in this phase at one time, notes the International Society of Hair Restoration Surgery, which is under 10,000 hairs on your head. This phase lasts around five to six weeks, and ends with a shedding of the hair.
Shedding
The shedding of hair is essentially triggered by the beginning of the growth cycle of the follicle. After any one follicle goes through a full growth cycle, the process starts over again with another anagen phase. The follicle returns to its original size and begins to grow a new hair. It's the start of this phase that forces the resting hair out of the scalp with the tip of the new hair. Between 50 and 100 hairs are shed each day.
Shrinkage
There's another form of shrinkage that occurs with the hair follicles. This one is a result of dihydrotestosterone, or DHT, an androgen derived from the hormone testosterone. Follicles sensitive to DHT also shrink in size. But this shrinkage is permanent, and the follicle no longer goes through the various phases of growth. This results in thinning of the hair and eventually baldness.
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