Hair structure

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.
A hair follicle is a tiny cup-shaped pit buried deep in the fat of the scalp.
Inside the hair follicle lies the hair bulb. At the base of the hair bulb is the dermal papilla.It is a structure of actively growing cells. 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.

The dermal papilla is well supplied with minute blood vessels, and the blood passing through them nourishes this growing region.
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.In the mid-follicle region 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.

Hair bulb

The hair shaft 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.
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.

The outer layer of 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 cuticle scales lie along the surfaceof the hair like tiles on a roof

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.
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! Healthy hair fiber has a tensile strength around 180-190 MPa. This means hair is almost as strong as copper wire of the same diameter.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. 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 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. typical hair is composed of 50% carbon, 20% oxygen, 17% nitrogen, 6% hydrogen, and 5.0% sulphur. Trace amounts of magnesium, arsenic, iron, chromium and other metals and minerals are also present in the hairs.