Hair Biology

Hair Biology Facts

 

The main constituent of hair is the protein keratin. Keratin is a structurally strong and remarkable protein that is resistant to wear and tear. It can be found in feathers, claws, nails and more importantly the biology of hair.

Similar to other proteins, keratin is comprised of large molecules, consisting of much smaller units called amino acids, which are held together in chains under strong chemical bonds.

Hair comprises of 10-13% of water, fats, the pigment melanin, vitamins and traces of zinc and other metals, all of which are essential components.

Each strand of hair consists of three vital layers.

 

1)   An innermost layer or medulla which is only present in large thick hairs.

2)   The middle layer known as the cortex, which provides strength, colour and the texture of hair.

3)   The outermost layer is known as the cuticle. The cuticle is thin and colourless and serves as a protector of the cortex.

 

 

 

The hair follicle

 

The Hair follicle is a tiny cup-shaped skin organ deep in the fat of the scalp that produces hair. Attached to the follicle is a sebaceous gland, a tiny sebum-producing gland found everywhere except on the palms, lips and soles of the feet. The follicle is supplied with vital nutrients from the minute blood vessels. The temperature around the follicle is regulated to normal body temperature and is not affected by external changes.

The hair follicle can be divided into two important regions.

 

a) 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.

 

 

New cells are continuously produced in the lower section 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 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.

At the base of the hair follicle is the dermal papilla which contains receptors for male hormones and is feed by the nutrient rich bloodstream to produce new hair. It is the dermal papilla (DP) which directs and dictates the embryonic generation of a hair follicle and it also retains this instructive ability throughout the life of the hair follicle.

Interestingly, DP cells are almost unique in maintaining their embryogenic regenerative properties in adults making them potentially attractive for investigation into organ/limb regeneration and similar studies. The bigger the DP, the more cells it has and the thicker the hair fibre becomes.

Special cells in the hair bulb produce the pigment that colours 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 with it.

 

 

 

b) 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. In many teenagers, a massive surge in hormone levels leads to raised grease production, thus producing greasy hair.

 

Structure of the hair shaft

 

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. The fibre-matrix combination is extremely strong and resistant to stretching and twisting.

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, this ultimately determines hair colour.

The outer layer of the hair is called the cuticle. It consists of between six and ten overlapping layers of long
cells along its full length (imagine the tiles on a roof). Each of these cells or scales is about 0.3 micrometres thick and around 100 micrometres long, and about 10 micrometres across. Cuticle cells become progressively flatter as they get older.

A healthy cuticle provides more than just a protective layer. Intact cuticle cells are smooth and glossy, and reflect light from their surfaces. This characteristic, combined with the pigment contained in the cortex, gives hair its glossy and shiny appearance. Incidentally, although black hair reflects less light than blond, it actually shines more due to the contrast between the hairs.

When contraction of tiny muscles called the arrectores pilorum or arrector pili occurs, the hair becomes rigid as the muscle pulls on the epidermis; this is what many people experience when the hairs stand up on the back of your neck. These same muscles also produce goose-pimples when you experience changes in temperature from hot to cold, or become frightened.

The total number of hair follicles for an adult human is estimated at 5 million with 1 million on the head of which 100,000 alone cover the scalp. In humans, the only external regions of skin devoid of hair follicles are the palms of the hands and soles of the feet.

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