Structure of the hair and hair follicle
The hair is composed of a protein
called keratin. The hair itself is arranged in three layers, an outer cuticle,
middle cortex and central medulla.
If the hair is coloured it is due to
the presence of pigments- either melanin (black or brown) or pheomelanin (red or
yellow). If these pigments are lacking the hair is white. Canites is the term
given to grey hair, it is an illusion created by the mixture of white and
coloured hairs. Actual individual "grey" hairs do not really exist.
Hair grows from a follicle. The walls
of the follicle form the outer root sheath of the hair. The lower part of the
follicle widens out to form the hair bulb that contains the germinal matrix, the
source of hair growth. Dermal tissue projects into the follicle base to form the
dermal papilla, and this has a network of capillary blood vessels to supply
oxygen, energy and the amino acids needed for growth. Melanocytes are present in
the upper part of the papilla, producing pigment granules that are distributed
throughout the cortex.
In the follicle the hair is surrounded
by an inner root sheath that has three layers. Henles layer is one cell thick
and lies to the outer root sheath. Huxley's layer is two or three cells thick
and is in the middle of the sheath. The cuticle of this inner root sheath
interlocks with the cuticle of the hair. Both the hair and the inner root sheath
grow at the same rate, but the inner root sheath breaks down about two-thirds of
the way up the follicle, so only the hair emerges past the skin surface. Uncut
hairs have a pointed tip.
Hair Growth Cycle
Each hair follicle undergoes a cycle of
activity. The hair grows to a maximum length, then hair growth ceases and the
hair is shed and replaced. At any one time we only have around 85% of our hair
on our head at a time, the rest being in the resting stages. The hair growth
cycle has three distinctive phases:
Anagen: the period of active growth
Catagen: the period of breakdown and change
Telogen: the resting stage before resumption of growth
Anagan
The epidermal cells surrounding the
dermal papilla form the germinal matrix or root of the hair. These cells are
constantly dividing, and as new cells are formed they push the older ones
upwards where they begin to change shape. By the time the cells are about
one-third of the way up the follicle they are dead and fully keratinised. A
scalp hair will grow actively for between one and a half and seven years (three
years being an average growth period). The average growth rate is about half an
inch per month. On average 85% of follicles are in the anagen stage.
Catagen
This is the end of the active growth
period, and is marked by changes occurring in the follicle. The hair stops
growing and becomes detached from the base of the follicle forming a club hair.
The hair bulb begins to break down, resulting in the follicle becoming shorter.
A small section of the outer root sheath remains in contact with the group of
cells that formed the papilla. This period of breakdown or change lasts about
three weeks. As the inner root sheath breaks down, the hair remains in the
follicle due to its shape. On average, 1% of follicles are in the catagen stage.
Telogen
The section of remaining root sheath
still in contact with the papilla is known as the secondary or root germ. It is
from this germ that a new hair can grow. The shortened follicle rests for about
three months. The hair may be brushed out at this time or at the onset of anagen.
On average 14% of follicles are in the telogen stage.
After the telogen stage the cycle
returns to anagen and the root germ begins to grow downwards and forms a new
bulb around the dermal papilla. It is the lower end of the germ that forms the
new bulb, producing a new hair. The upper part of the germ forms the new cells
that lengthen the follicle below the club hair. The new hair may push the old
hair out. Sometimes therefore you may see two hairs in the same follicle.
The average daily loss of hair on the
scalp is between sixty and one hundred hairs. If the loss is over one hundred
hairs daily, hair loss will exceed replacement, and you eventually become bald.
During the year you may notice a loss of extra hair in the autumn and spring,
than in summer which is related to daylight and the weather.
There are some rare cases of
individuals (both male and females) who lose all their hair regularly every
seven years, as their hair growth cycle is not continuos. They remain virtually
bald for about four months and then the hair regrows as normal.
Hair growth is fastest from the age of
sixteen to the late twenties. New hairs grow faster and the growth rate slows
down with the increasing length (almost half the rate when the hair is over
three feet long).
The average number of hairs on the
scalp of an adult is 100,000. Blondes have more hairs with around 140,000 while
redheads have the least with about 90,000.
It is no fallacy that on occasions,
such as when you are frightened, your hair literally stands on end. Attached to
the hair shaft are tiny muscles called the arrectores pilorum, or erect pili
muscles that, as the name suggests, cause the hairs to become erect.
Under the influence of nervous
excitation, these muscles contract and cause the hairs to which they are linked
to become rigid. Similarly, when you are cold the goose-pimples you may
experience are the result of the action of the arrectores pilorum.
Thus the hair is affected by the state
of the nervous system and by the condition of the circulatory system. As such it
is very much interrelated to the functioning of the entire human organism: body,
mind and emotions. When you ponder on the opulence of the nerve and blood supply
to the head, you will have no difficulty appreciating that whatever adversely
affects them, will in the long term, be to the detriment of the hair.
So we can see that health plays a very
important part in hair growth.
