Hair

The primary component of hair fiber is a protein called keratin. Proteins are long molecules made up of amino acids linked end to end. Keratin proteins form the cytoskeleton (miniature skeleton within all cells) is a major component of the cells in the epidermis, hair and nails.

Keratin accounts for 30% of the cellular protein of living epidermal cells and increases to 85% in dead cells of the epidermis. Up to 95%, by mass, of the hair fibre is due to keratin. Two types of keratin exist, soft and hard. Soft keratins are found in the skin while hard keratins are found in hair fibers and are highly resistant to breakdown by digestive enzymes.

Some cosmetics manufacturers claim to make hair products that contain keratin to strengthen hair or promote hair growth. However, the cells in hair follicles make all the keratin protein that they require from amino acids supplied from the diet. Eating keratin protein does not help hair growth. Eating hard keratins as found in hair fiber is pointless as they cannot be broken down and absorbed. The keratins simply pass straight through the gut. However, it is extremely rare in Australia to have a deficiency in amino acids so severe that it affects hair fiber quality. Some hair lotions claim to have keratin but topically applying keratin to hair also has no long lasting effect. The keratins are mostly washed away in the shampooing process.

The beginning of a hair is made up of cells, known as keratinocytes, which are bulging with keratin. The cells lengthen and then die, leaving behind protein bundles. Intertwining of keratin molecules form long coils which accumulate in hair. Keratinocytes that produce the hair fiber actually engulf nearby melanocytes, cells containing pigment, and hence give colour to the hair fiber. As the keratinocytes move up towards the skin surface they become flattened and are arranged in layers.

The hair fibre looks simple when seen with the naked eye. However three distinct parts exist.

The cuticle, which is made of dead, flattened epithelial cells overlapping each other.
The cortex which is just inside the cuticle and is composed of a strong protein called keratin and the pigment melanin that gives hair its colour.
The medulla is found at the centre of the cortex and contains soft, cuboidal, keratinised cells.

 
Numerous, minute charges exist along the length of the protein chains. These attract each other and hold the chains close together. Water however, is attracted to these charges and penetrates into the hair fiber and disrupts the attraction between the protein chains. This causes the chains to move apart slightly. For this reason hair absorbs its own weight in water. Before any chemical processing is done at the hairdresser the hair is often soaked in water so the protein chains move apart allowing the chemical treatment to penetrate deeper into the hair fiber.
Keratin, like all proteins, is made up of amino acids bonded end to end. Keratin however, has more of the amino acid cysteine than other proteins. This feature allows keratin to form very strong fibers. You see cysteine contains sulfur and is able to make strong cross links from one protein chain to another. These cross links are known as disulfide links and prevent the chains from moving past each other and hold them in a definite shape.

What is the major protein found in hair?

What are the three layers of a hair fibre composed of?

Why is water readily absorbed by hair fibers?

Why is keratin such a strong protein?

Why is hair washed before being chemically treated?