Hair Stucture
Hair is composed of strong structural protein called keratin. This is the same kind of protein that makes up the nails and the outer layer of skin.
Each strand of hair consists of three layers.
- An innermost layer or medulla which is only present in large thick hairs.
- The middle layer known as the cortex. The cortex provides strength and both the color and the texture of hair.
- The outermost layer is known as the cuticle. The cuticle is thin and colorless and serves as a protector of the cortex.
Structure of the hair root
Below the surface of the skin is the hair root, which is enclosed within a hair follicle. At the base of the hair follicle is the dermal papilla. The dermal papilla is feed by the bloodstream which carries nourishment to produce new hair. The dermal papilla is a structure very important to hairgrowth because it contains receptors for male hormones and androgens. Androgens regulate hairgrowth and in scalp hair Androgens may cause the hair follicle to get progressively smaller and the hairs to become finer in individuals who are genetically predisposed to this type of hair loss.
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 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.
The Hair Growth Cycle
Hair follicles grow in repeated cycles. One cycle can be broken down into three phases.
- Anagen - Growth Phase
- Catagen - Transitional phase
- Telogen - Resting Phase
Each hair passes through the phases independent of the neighboring hairs.
Anagen Phase - Growth Phase
Approximately 85% of all hairs are in the growing phase at any one time. The Anagen phase or growth phase can vary from two to six years. Hair grows approximately 10cm per year and any individual hair is unlikely to grow more than one meter long.
Catagen Phase - transitional phase
At the end of the Anagen phase the hairs enters into a Catagen phase which lasts about one or two weeks, during the Catagen phase the hair follicle shrinks to about 1/6 of the normal length. The lower part is destroyed and the dermal papilla breaks away to rest below.
Telogen Phase - resting phase
The resting phase follows the catagen phase and normally lasts about 5-6 weeks. During this time the hair does not grow but stays attached to the follicle while the dermal papilla stays in a resting phase below. Approximately 10-15 percent of all hairs are in this phase at an one time.
At the end of the Telogen phase the hair follicle re-enters the Anagen phase. The dermal papilla and the base of the follicle join together again and a new hair begins to form. If the old hair has not already been shed the new hair pushes the old one out and the growth cycle starts all over again.
Hair Dimensions
People describe their hair as being thick or thin, coarse or fine. What they are usually talking about is the amount of coverage their hair gives to the head. This coverage depends on two things: how many hairs there are, and the thickness (diameter) of each hair shaft.
People vary a lot in how many hairs they have, and also in how closely together they grow. The 'average' person has around 100,000 hairs, but people with very dense hair may have as many as 150,000.
The diameter of the hair shaft varies too. It is usually around 57-90 µm in Europeans. This is much less than in Asians, in whom it can be 120 µm. (These are general figures, representing a wide range of values.)
Hair that is both dense and thick looks completely different from that of someone who has fewer and finer hairs.
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 grow before it falls out. Anagen lasts between three and seven years in most people.
- A hair grows at a mean rate of about 1 centimetre a month. This can be longer or shorter as shown in the pictures below.
- 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 or even behind.
