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The Hair Follicle Life Cycle

Each hair follicle rotates through three stages:

1. Anagen - the growth phase
2. Catagen - the involution phase
3. Telogen - the resting phase

The anagen phase is the growth phase where hairs remain in the follicle an average 2 to 8 years. During this phase hair grows between a quarter inch (5mm) to a half inch (10mm) a month. Around 90% of the scalp hairs are in the anagen phase at any one time.

The catagen phase, a period of time that lasts 2 - 4 weeks follows the anagen phase. During this period the follicle begins to breakdown, reflecting a programmed cell death. Toward the end of the catagen phase, the follicle papilla condenses and moves upward, coming to rest under the hair follicle bulge. In the case of healthy hair follicles, stem cells from the bulge interact with the follicle papilla and the cycle of hair growth restarts with anagen phase growth.

The telogen, or resting phase, is the final stage that lasts between 2-3 months. Hairs in the telogen phase shed, although researchers are not sure whether the shedding is an active, regulated process or a passive event that occurs when the new hair in the anagen phase grows in behind it. About 10% of the hair on the scalp is in the telogen phase and will regrow about 6 months after they have been shed.

An increase of the percentage of follicles in the telogen stage leads to excessive shedding. Therefore, reducing the percentage of follicles in the telogen phase would be valuable in reducing hair loss.

Androgenetic Alopecia - Testosterone and Miniaturization:

We know that:

1. Androgenetic baldness affects the life cycle of the hair.

2. Androgens (male sex hormones) are produced by both males and females,
although females produce it in much smaller amounts.

However, the amount of these hormones does not have to be abnormally high for hair loss to occur. Males and females who have normal level of androgens and a gene for baldness will develop male pattern hair loss (MPHL) and female pattern hair loss (FPHL) respectively.

3. Two androgens are responsible for MPHL and FPHL; testosterone and

4. Testosterone is converted to dihydrotestosterone by the enzyme known
as 5-alpha-reductase.

5. In men and women with genes for baldness, the hair follicles in the scalp
remove testosterone from circulation and convert it to DHT.

6. The action of DHT over time shortens the duration of the anagen phase
of the hair growth cycle and decreases the proportion of hairs in the
anagen phase.

Note: As the anagen phase decreases, the new hairs produced are shorter in length and in diameter.

As a larger percentage of the hairs are in the resting phase, more are lost during normal grooming. This process of the shortening and thinning of each individual hair shaft is called miniaturization.

Miniaturization is often accompanied by the loss of hair pigment production, so that the miniaturized hairs are also lighter in color. The light colored fine hairs left at the end of the miniaturization process are called vellus hairs.