HAIR
LIFE CYCLE
The
life cycle of a hair is divided into three phases. The
actively growing (Anagen) phase, the transitional (Catagen)
phase, and the resting (Telogen) phase.
During
the anagen phase, protein and keratin are continuously made.
It is during this phase that the hair shaft is manufactured
and pushed upward to its natural length. Not the large
healthy bulb at the hair base. A hair's anagen, or growth
phase, lasts from 3 to 5 years, and represents what is occurring
to about 90% of the hair on your head at any given time.
In
the catagen, or transitional phase, there are chemical and
structural changes in the hair follicle. The hair stops
growing, and remains in this phase for only two to three
weeks before moving into the next phase.
Finally,
hair enters the telogen phase where it basically just sits
on your head for about 3 months. Then, it falls out only
to be replaced by the next budding hair in the anagen phase
which begins to grow from the same hair follicle. These
replacement hairs get finer and thinner as a person ages.
In most settings of baldness, the hair follicle simply shuts
down and refuses to put out more hair to replace the ones
that have fallen out.
A
person normally sheds up to 100 hairs per day. Hair growth
occurs at about an inch per month, faster when it's hot
(summer) and slower when it's
cold (winter). This rate slows down with age, and shuts
off in more and more hair follicles as time marches on.
Things that influence hair growth include not only hormones,
but nutrition, vitamins, emotional states, and many unknown
factors.
Hair
growth starts before we're even born. We don't really know
what triggers it, or what keeps it going. Perhaps it is
a link to our genetic ancestor, the primate Our first hair,
Lanugo (Latin for fine wool), begins to grow approximately
3-6 months after conception. It is usually shed before,
or soon after birth and is replaced with the coarser hair
we all know. The pattern of our hair (e.g., where our hair
parts) is believed to be related to the formation of hair
follicles as our skin is stretched over the developing fetus.
There are many different patterns and possibilities of hair
growth, though some are much more common than others (shown
below).
After
birth, different hormones affect the follicle allowing growth
or causing hair follicles to move into the resting (telogen)
phase. A
hormone called dihydrotestosterone (DHT), a product made
from testosterone, acts on the hair follicle causing growth
to slow and ultimately stop. DHT only works on certain
hair follicles that have the genetic predisposition to be
shut off.
Usually,
these are on the front and top of our heads. An interesting
fact is that castrated males (eunuchs - who do not make
testosterone (and hence can't make DHT) because they do
not have testicles, do not have male pattern balding unless
they are given injections of testosterone. If a castrated
man gets these testosterone injections, they will immediately
start losing hair in the classic horseshoe fashion (see
the male pattern baldness section). Certain drug therapies
against balding make use of this relationship of testosterone
to DHT, as we will soon discuss.
