Anagen - the growth phase
2. Catagen - the involution phase
3. Telogen - the resting phase
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.
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
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.
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.
Alopecia - Testosterone and Miniaturization:
Androgenetic baldness affects the life cycle of the
Androgens (male sex hormones) are produced by both males
females produce it in much smaller amounts.
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 is converted to dihydrotestosterone by
the enzyme known
In men and women with genes for baldness, the hair follicles
in the scalp
testosterone from circulation and convert it to DHT.
The action of DHT over time shortens the duration of
the anagen phase
the hair growth cycle and decreases the proportion of
hairs in the
As the anagen phase decreases, the new hairs produced
are shorter in length and in diameter.
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.
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.