The Hair Growth Cycle
Each of these long hairs has been growing for at least six years: eventually it will fall out spontaneously; this growth and fall makes up the hair cycle
Each individual hair is formed inside a hair bulb deep in a hair follicle. The follicle is a tiny but powerful factory, which throughout many people's lifetime hardly ever stops working. From a baby's birth for many decades, as much as a century in some people, the follicle continues to produce hairs. Each hair grows for many years: during this time it will be shampooed, conditioned, cut, blown dry, exposed to sun and wind, colored or bleached or permed. None of these treatments affects the growth of the hair in the hair bulb, even though some may seriously damage the hair shaft. Finally the hair spontaneously falls out. The follicle rests for a little while, and then starts to produce yet another new hair. This is the hair cycle.
You need to know about the hair cycle in order to understand many of the problems people have with their hair. These can range from the sudden appearance of hairless patches to complete baldness in men, and sometimes in women too.
Stages of the hair cycle
Between starting to grow and falling out years later, each hair passes through three distinct stages. These are so important that they have been given special names: anagen (the growing phase), catagen (the intermediate phase) and telogen (the shedding phase).
We shall look at these three stages in turn.
Naturally blond hair: this lady's anagen phase lasts for about four years
A method of measuring the rate of hair growth: both cut hairs and newly emerging hairs can be seen
Anagen (the growing phase)
The anagen phase of a new hair starts at the moment it begins to grow. At that time there is very active growth in the hair bulb. This usually 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.
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.
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 grow before it falls out. Anagen lasts between three and seven years in most people.
- As we have seen, a hair grows at a rate of about 1 centimetre a month.
- After one year it will be 12 cm long. After five years it will be 60 cm long.
- Waist-length hair is 80-90 cm long, and will have taken about seven years to grow.
- Shoulder-length hair will have taken only about three years. Thus only people with long anagen times can expect to grow their hair down to the waist.
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]
As people grow older the period of anagen shortens. For example, the hair of someone with a five-year anagen can grow to a length of 60 cm before it enters the shedding phase. If their anagen period drops to three years as they age, their hair will then grow only to shoulder length before it falls out or is brushed out.
So when a scanty-haired elderly lady boasts that when she was a girl she could sit on her hair, she may very well be telling you the truth!
What controls hair growth?
No one can answer this question with certainty. What we do know is that it takes a lot to stop hair growing!
General health and nutritional factors are increasingly believed to be important for healthy hair growth. We do know that serious anaemia affects hair growth. So too does starvation: people who go on a crash diet may start to lose their hair some six to ten weeks later. Many alcoholics have poor hair growth or even hair loss because their way of life tends to lead to malnutrition.
Some minerals may be particularly important for hair growth. For example, some people who lack zinc in their diet produce only fine, sparse hairs and even lose their hair. Vitamin B, also known as panthenol, plays a part in hair growth. It also improves the physical properties (elasticity, strength and gloss) of the hair shaft.
The hair cycle for each individual hair is influenced by the levels of various hormones in the blood. Thyroid hormone speeds up growth in resting hair follicles. Steroids taken by mouth slow it down, though steroids inhaled for the treatment of asthma do not affect hair growth.
The hair of the scalp is, however, most sensitive to the effects of male hormones (androgens), which are in the blood of people of both sexes (only in different proportions in men and women). Androgens are the most important factor regulating hair growth, and also the thickness of the hair shaft. Female hormones (oestrogens), which both sexes have too, slow down hair growth during the growing period but also make that period longer. Many women notice a difference in their hair growth during pregnancy. At this time women have vast amounts of oestrogen in their blood, far outweighing their male hormones. It is the balance between the male and female hormones which decides the growth of the hair.
We have already seen how at puberty the immature vellus-like hairs on our bodies can change to terminal hairs. This change results from the dramatic rise in androgen levels at that time. It is especially noticeable in young men. Young women who suffer from the condition of anorexia nervosa, however, stop having periods and produce very little oestrogen: their natural androgens tend to thicken their fine vellus-like hairs in the same way.
The effects of androgens continue long after puberty. Some areas of the skin respond to these hormones more vigorously than others do, and at different times of life. Pubic and armpit hair begins to grow at or soon after puberty. Most men's beards do not grow strongly until the owners reach their thirties. The growth of chest hair reaches a peak even later, and hair in the nose and ears grows most in late middle age.
This boy has a zinc deficiency, and his hair is very thin and sparse; after treatment his hair is growing more strongly
A full set: growing a beard is easier at 50 than at 15, because of the response of the hair site to male hormonse (androgens)