How Hair Gets its Shape

The shape of a hair depends on several factors, including the shape of the hair follicle and its opening; these vary from one person to another and also between races. As keratin is hardening it is compressed into the shape of the hair follicle. The hair is then held in shape by the pattern of the chemical bonds within it. Of these, the disulphide bonds are the strongest. They can only be changed by chemical methods such as perming or relaxing.

But within each hair the keratin chains are also linked by bonds of a different kind, called hydrogen bonds. There are far more hydrogen bonds than disulphide linkages. The hydrogen bonds are much weaker than the disulphide linkages and more easily broken, and they give hair its flexibility. Hydrogen bonds are broken apart whenever the hair is wetted, and form again as it dries. When they break the shape of the hair changes. If the wet hair is then wound on to rollers it will form a new shape, and if it is dried on the rollers it will keep this shape.

This is the basis of the setting process. The change in shape is only temporary. It is lost when the hair is dampened, because the new hydrogen bonds are broken again.

Hair Chemistry

Hair is Surprisingly Strong: A single hair can support a load of about 100 grams without breaking. You could even spin rope out of hair!

The keratin protein of the cortex is responsible for this unusual strength. The long keratin molecules in the cortex are compressed to form a regular structure, which is not only strong but also flexible.

Proteins are made up of long chains of amino acids. Each chain takes up a helical or coiled form, rather like a long spring, or the cable of a telephone handset.

Most protein chains are made up of various mixtures of the same 20 or so amino acids. Keratin is unique in that its chains contain high concentrations of a particular amino acid called cystine. The proteins in the matrix of the hair contain the highest levels of cystine.

Every cystine unit contains two cysteine amino acids in different chains which have come to lie near to each other and are linked together by two sulphur atoms, forming a very strong chemical bond known as a disulphide linkage. Many disulphide bonds form down the length of the keratin chains, joining them together like the rungs of a ladder.
The disulphide bond is one of the strongest bonds known anywhere in nature. This cross-linking by disulphide linkages between the keratin chains accounts for much of the strength of hair.

Structure of Hair Shaft

Your smooth, glossy hairs have a more complicated structure than you might think. Each one can be compared to a tree: all its moisture lies in its centre, behind a tough outer layer of protective bark. If the 'bark' of the hair is well looked after the whole hair remains in good condition. If the 'bark' is stripped off to expose the centre the hair may break.

The centre part of the hair, called the cortex, makes up most of the hair shaft. It is the cortex that gives hair its special qualities such as elasticity and curl. The cortex is packed with strands of keratin, lying along the length of the hair. These keratin fibres are made of the low-sulphur keratins, and are compressed into bundles of larger fibres. These are held together by a mass of sulphur-rich keratins, the matrix. The fibre-matrix combination is extremely strong and resists stretching and other strains such as twisting, much as does the glass fibre-resin mixture from which many boats are built.

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 grey (unpigmented) ones, the cortex has a central hollow core, the medulla. There are medullae in the hairs of many animals, and they play a part in the regulation of body temperature. It may be that the presence of this air space in some human hairs is an evolutionary 'throw-back' to a time when our ancestors needed extra heat insulation.
The outer layer of the hair (the 'bark') 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 micrometres thick and around 100 micrometres long, and about 10 micrometres across. (1 micrometre, written 1 µm, is one-millionth part of a metre = one-thousandth part of a millimetre.) The scales lie along the surface of the hair like tiles on a roof, with their free edges directed towards the tip. They cover the hair surface all the way along its length.

If you could look at a hair under a powerful microscope you would see that the scales growing over the youngest part of the hair (the part that grows closest to the scalp) are smooth and unbroken. Further along the hair, you would be able to see that they may have been damaged by cosmetic treatments and by mis­treatment such as over-energetic brushing. Little by little they may break away, a process called weathering.

A healthy cuticle is more than just a protective layer. Much of the shine that makes healthy hair so attractive is due to the cuticle. Intact cuticle cells are smooth and glossy, and reflect light from their surfaces. This, together with the pigment within the cortex, gives hair its characteristic appearance.

Black hair reflects less light than blond hair does. Black hair appears glossier, however, Because the bright bands of reflected light contrast more sharply with the darkness of the rest of the hair.

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. But in addition boys' and girls' glands tend to produce more oil when levels of their hormones (androgens) are high. In many teenagers, a massive surge in hormone levels leads to raised grease production. This results in a tendency to greasy hair, which many young people know all too well. The good news is that most of them outgrow it

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.

Some of these keratins contain a high level of sulphur, some much less. The sulphur plays an important part in the way the hair behaves, especially when it is given cosmetic treatments. You will find more about this later in the book.

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 inner three layers of cells become the actual hair. The outer three layers become the lining of the hair follicle - the inner root sheath.

Special cells in the hair bulb produce the pigment that colors the hair. The pigment is called melanin, and these cells are known as melanocytes. As the developing hair moves upwards in the follicle the melanin is carried upwards in the inner part of the hair

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 hair of an animal like a cat or a horse grows at different rates depending on the 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.The hair follicle can be divided into two regions.

Middle Aged Hair and Old Age Hair

Many older people find that their hair continues to grow strongly, and that they have as much hair at 80 as they had at 50. Other people find that their hair gets thinner as they age, and that by the time they are in their 80s only a few wisps remain. Again, this is determined genetically, not by anything they have done to their hair.

Puberty Hair

Before puberty, the scalp carries a mixture of short vellus-like hairs and longer terminal hairs, together with various 'in-between' hairs. After puberty, in both sexes, most of the scalp hairs are terminal hairs. These hairs are thicker in diameter than the childhood hairs, especially in dark-haired people.

At puberty, terminal hairs begin to appear in the armpits, groins and legs, and also (in males) on the chin, chest and forearms. How much body hair you develop is genetically determined (that is, it is inherited from your parents).

Baby Hair

A newly born full-term baby has two types of hair. Terminal hairs grow on the scalp and eyebrows, but nowhere else. All the rest of the hair is vellus hair.

As the baby grows, the hair on the head grows too. There are two periods during which hair grows rapidly on the scalp. In both, the hair growth begins at the forehead and then extends to the back of the neck. When the baby is two or three mdnths old, the first hairs may be shed naturally over an area on the back of the head. This is often mistakenly thought to be due to head rubbing; hairs broken by rubbing may, however, be found on other parts of the head as well.

During the first year of the baby's life, all the hairs on the head grow at the same rate. At that time the head carries an even covering of hair. Then the individual hairs begin to grow independently, at different rates and in different cycles (you will read about hair growth cycles later in this chapter). Growth patterns called 'mosaics' develop.
Many children's hair shows features which are lost in adulthood. These include:

• Unruly hair which sticks straight up

• Atural curls

• Hair without pigment, which darkens as the child grow

Hair Variations

Human hair varies enormously. Different people have hair that differs in color, in length, in diameter, and in its distribution on the body. As we shall see later, some of these factors are influenced by the person's racial type, and some by his or her age.

Managing Hair

The appearance of the hair is the one feature of the body over which humans, unlike other primates, can have direct control. You can change the length, shape, color and style of your hair (including, for men, facial hair) according to the age you wish to appear, and the economic and social status you want to claim.
The style, length and condition of hair all play a part in how we perceive the people we meet.

How much Hair do we have / Hairs on a Human Head

• On average, each person's head carries about 100,000 hair follicles. Some people have as many as 150,000.

• On a baby's head, there are about 1,100 follicles per square centimetre.

• By the age of 25, this number has fallen to about 600, but the number depends on the physical type of the individual.

• Between the ages of 30 and 50, the number drops further, to 250-300. There is only a slight further fall with age.

• Each follicle grows about 20 new hairs in a lifetime. Each new hair grows for several years, and can reach over a metre in length.

• Each hair falls out eventually, and is replaced by a new one.