|
A
method of measuring the rate of hair growth: both cut hairs and
newly emerging hairs can be seen
lasts for some years, generally between three and seven, without
interruption. Since human hairs grow at a rate of roughly 1 centimetre
a month, hairs can grow to a length of a metre or so.
As we have seen, hair may grow more quickly in winter
than in summer. Hair growth varies with the season as a result of
a change in the difference between hair follicles in the growing
and shedding phases.
Pigment (melanin) is made in the hair bulb throughout
this phase of the hair cycle. Less pigment is made in the hair of
older people. This is why white hairs start to appear, even though
the hair itself may still be growing strongly.
In some older people the hair cycle becomes shorter,
the follicles gradually give up producing long, strong hair, and
the hairs become thinner and shorter. The result may be a general
thinning of the hair, or even a degree of baldness.
| HAIR
FACTS |
Follicle
activity
In
everyday life, nothing interrupts the activity of the
hair follicle. Nothing that is put on the scalp or hair
can interfere with the growth of the hairs. Only severe
burns or scars can affect the hair follicle.
Certain drugs that are given for cancer
treatment can prevent hairs from growing. (This is discussed
later in this chapter.) Almost always, however, the interruption
is only temporary and hairs begin to grow again when the
medication is stopped. |
|
|
|
Catagen
(the intermediate phase)
The anagen phase is followed by a short resting phase. This catagen
phase lasts for between two and four weeks in the human scalp.
No pigment is made during that time, and the follicle stops producing
hair. The base of the follicle moves upwards towards the surface
of the skin.
Telogen (the shedding phase)
The telogen phase lasts for three or four months. This
is the time at which a new hair begins to grow from the hair follicle.
As it grows upwards the old hair will be shed naturally or may
be pulled out, which happens easily and painlessly with telogen
hairs. These are the hairs that come out when you shampoo or brush
your hair.
Shedding is part of the normal process of the replacement
of old hair with new. At any one time, around one in ten of the
follicles on an individual's head are in the shedding phase.
The new hair emerges from the same opening at the
surface of the skin as the old one, and the hair cycle begins
again.
Hair length
How long anagen lasts is determined genetically, and varies between
the sexes and from one person to another. It is the length of
this time that decides how long the hair will
Differences
in hair length depend on the length of anagen, which is genetically
determined. These two people started off with hair of the same length
and went without a haircut for 18 months: the man's hair grows only
to his collar before it falls out naturally, but the woman's anagen
period is clearly much longer [reproduced from Diseases of the Hair
and Scalp, A. Rook and R. Dawber (eds), 2nd edn, Oxford: Blackwell
Scientific Publications, 1981]
|
