print this page - save to favorites print this page add to favorites

Hair Structure

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 :ontrast more sharply with the darkness of the rest of the hair, as in the photograph below.

A perfect hair seen under the electron microscope

A perfect hair seen under the electron microscope

Another normal hair - but this came from one of our nearest relatives, an orangutan!

Another normal hair - but this came from one of our nearest relatives, an orangutan!

In this section we look at what hair is made of, how it is able to grow as long as it does, and what helps it to grow.

A hair is a specialised outgrowth of part of the skin called the epidermis. It has two distinct parts, the hair follicle and the hair shaft.

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 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.

Structure of hair follicle while it is in the growing phase [Source Color Atlas of Differential Diagnosis of Hair Loss David A. Whiting and F. Lester Howsden, Fairfield, NJ: Canfield Publishing Inc.]

Structure of hair follicle while it is in the growing phase [Source Color Atlas of Differential Diagnosis of Hair Loss David A. Whiting and F. Lester Howsden, Fairfield, NJ: Canfield Publishing Inc.]

The hair follicle can be divided into two regions.

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.

The structure of the hair bulb

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.

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.

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.

The structure of the hair bulb: the living cells gradually die, and are compressed to form the hair shaft [Source Color Atlas of Differential Diagnosis of Hair Loss, David A. Whiting and F. Lester Howsden, Fairfield, NJ: Canfield Publishing Inc.]

The structure of the hair bulb: the living cells gradually die, and are compressed to form the hair shaft [Source Color Atlas of Differential Diagnosis of Hair Loss, David A. Whiting and F. Lester Howsden, Fairfield, NJ: Canfield Publishing Inc.]

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.

The structure of the hair bulb

The structure of the hair bulb

Structure of the 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 hair shaft seen with the light microscope: light is reflected from the colorless cuticle and bent as it passes through the hair - this effect gives hair the color we see

The hair shaft seen with the light microscope: light is reflected from the colorless cuticle and bent as it passes through the hair - this effect gives hair the color we see

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 (see also Hair Structure and Hair Color).

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.

Pigment granules in the hair shaft

Pigment granules in the hair shaft

 

HAIR FACTS


The chemistry of hair

As we have seen, the main constituent of hair is the protein keratin. Keratin is a remarkable protein which is resistant to wear and tear. It is the protein that makes up feathers, claws, nails and hoofs, as well as hair. Like other proteins, keratin has very large molecules made up of smaller units called amino acids, joined together in chains like beads on a string.

Hair also contains fats, pigment (melanin), small amounts of vitamins, and traces of zinc and other metals. Hair also contains water which, although it makes up only 10-13% of the hair, is extremely important for its physical and chemical properties.

 

Hair in good condition

Hair in good condition

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 see (Chapter 2).

The cuticle scale on a normal hair (electronmicrograph)

The cuticle scale on a normal hair (electronmicrograph)

Slight lifting of the edge of the hair cuticle

Slight lifting of the edge of the hair cuticle

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 :ontrast more sharply with the darkness of the rest of the hair, as in the photograph below.

P&G Beauty & Grooming

Some expert attention would be useful here!

Some expert attention would be useful here!


Previous: Variations in Human Hair | Contents | Next: Hair Strength
© 2013 Procter & Gamble | Home | Site Map | Privacy Notice | Ad ChoicesAd Choices P&G Beauty & Grooming
The Procter & Gamble Company BBB Business Review