| HOW
HAIR GROWS
The hair bulb
The hair bulb lies inside the hair follicle. It
is a structure of actively growing cells, which
eventually produce the long fine cylinder of a
hair.
New cells are continuously produced in the lower
part of the bulb. As they grow and develop they
steadily push the previously formed cells upwards.
When the cells reach the upper part of the bulb
they begin to change, and they arrange
themselves into six cylindrical layers, one inside
the other.
The hair follicle
A hair follicle is a tiny cup-shaped pit buried
deep in the fat of the scalp. The follicle is
the point from which the hair grows. It is well
supplied with minute blood vessels, and the blood
passing through them nourishes the growing region.
The temperature around the follicle is normal
body temperature, and is not affected by cold
or hot weather.
The mid-follicle region
In this part of the follicle the actively growing
cells die and harden into what we call a hair.
As the cells below continue to divide and push
upwards, the hair grows upwards too, out of the
skin. It now consists of a mixture of different
forms of the special hair protein, keratin.
The hair shaft
This is the part of the hair that can be seen
above the scalp. It consists mainly of dead cells
that have turned into keratins and binding material,
together with small amounts of water.
Terminal hairs on the head are lubricated by a
natural oil (sebum) produced by the sebaceous
glands of the follicles. How much natural oil
your glands produce is mostly determined by your
genetic inheritance.
The center part of the hair, called the cortex,
makes up most of the hair shaft.
The cortex also contains granules of the hair
pigment melanin, produced when the hair was growing
in its follicle. The granules are of two types:
smooth, dark granules which tend to be regularly
positioned within the cortex, and lighter granules
that are more irregular in shape and which are
scattered randomly through the cortex. A hair
may contain just one type of granule or a mixture.
In some of the terminal hairs, especially gray
(unpigmented) ones, the cortex has a central hollow
core, the medulla.
The outer layer of the hair is called the cuticle.
It is made up of between six and ten overlapping
layers of long cells. Each of these cells or scales
is about 0.3 micrometers thick and around 100
micrometers long, and about 10 micrometers across.
|
HAIR
GROWTH CYCLE
The average
human hair grows for about three and a half years,
usually longer in women (up to seven years) and
slightly shorter in men. Normally we lose about
eighty or more hairs a day, although the number
is variable from day to day.
The structure of the hair
bulb
Amount of natural light, which varies according
to the time of year: it grows more quickly in
winter when the days are short.
Human hair probably behaves in the same way, growing
a little faster in winter than in summer.
| |
Anagen is the
active growth phase of hair follicles. The
cells in the root of the hair are dividing
rapidly, adding to the hair shaft.
During this phase the hair grows about
1 cm every 28 days. Scalp
hair stays in this active phase of growth
for 2-6 years. The amount of time the hair
follicle stays in the anagen phase is genetically
determined. |
|
The catagen
phase is a short transition stage that occurs
at the end of the anagen phase.
It signals the end of the active growth
of a hair. This phase lasts for
about 2-3 weeks while a club hair
is formed. |
|
The telogen phase is
the resting phase of the hair follicle.
At any given time, 10%-15% of all hairs
are in the telogen phase.
This phase lasts for about 100
days for hairs on the scalp and
much longer for hairs on the eyebrow, eyelash,
arm and leg.
During this phase the hair follicle is
completely at rest and the club hair is
completely formed. Pulling out a hair in
this phase will reveal a solid, hard, dry,
white material at the root. About
25-100 telogen hairs are shed normally each
day. |
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