Human hair

Hair structure

Hairs are elongated structures made up of a protein called keratin. It is the same protein that is a component of which nails are made up. Like other proteins, keratin has large molecules made up of smaller units of amino acids. The amino acids joined together in a chain, like beads on a string form the structure of the hair.

The diameter of a hair fiber varies depending on person to person; Normally it is about 0.05 to 0.09 millimeters thick.

Outermost layer of the skin is known as epidermis. Each hair grows from an indentation on the epidermis. The hair has two parts: the hair follicle and the hair shaft.


Hair Follicle

The hair follicle is the root point in the scalp from where the hair grows. It is a tiny cup-shaped pit buried in the fat of the scalp.

The terminal part of the hair follicle seated within the skin is called a hair bulb. The hair bulb formed by actively growing cells which produce long, fine and cylindrically shaped hair fibers. The hair bulb contains some special cells that produce the pigment that gives the hair its color. This pigment is called melanin and the cells producing it are known as melanocytes. Interestingly receptors of the male hormones – known as androgens, are located on the cells of this structure.

Each hair bulb has a dermal papilla at its vase containing a vessel tuft which is essential for nourishment of the growing hairs. Over this the skin, internal and external root sheaths cover these hair follicles. The external root sheath of a hair follicle is continuous along with the epidermis. Few important glands are functional near the hair follicles, of these glands one is the sebaceous gland, which produces and secretes the natural oils lubricating hairs, namely sebum.

Hair Shaft

Hair shaft is the part of the hair seen above the skin. It consists of dead cells that have converted into keratin and binding material along with small amounts of water. This structure explains why we do not feel any pain while our hair is being cut.
The hair shaft has three layers, medulla, cortex and cuticle. Medulla is seen only in large and thick hairs. The middle layer cortex is made of keratin fibers. The strength, color and texture of a hair fiber are a result of the cortex layer of the hair shaft. Finally the outermost layer of the hair shaft is the cuticle. This is a thin, colorless is made up of six to ten overlapping layers of long cell remnants and protects the cortex.

Types of hair

There are 3 types of hair growth seen viz. vellus hair, terminal hair and intermediate hair on human body.

Vellus Hair
Vellus hairs are centimeter or two length short hairs. A vellus hair contains little or no pigment and is colorless since its hair shaft does not have a melanin layer. Vellus hairs are fine and soft however not of cosmetic importance.

Terminal Hair
Terminal hairs are large, darkly pigmented hair fibers with a medulla in the innermost part.

Hair growth cycle

Each hair follicle life cycle consists of long period of growth followed by a relatively short period of rest. During this rest period the hair does not grow even though it is still attached to the hair follicle. Hair is shed after the resting phase and a newer hair begins to grow marking the beginning of a new hair growth cycle.

A genetic predisposition of each hair follicle following a particular pattern of growth and rest exists. Hair follicles are programmed to stop producing hair and spend more time in the resting stage with the progression of age.

The growth cycle of a hair has 3 stages:

1. Anagen (Growing) Stage
This is the name for the growing period of a hair follicle. The anagen stage for the hair follicles in the scalp typically lasts about 3 to 5 years.

2. Catagen (Intermediate) Stage
At the end of the growth period, hair follicles prepare themselves for the resting phase. This transition period of a hair follicle from growth to rest is called the catagen stage. This stage of the hair growth cycle usually lasts about 1 to 2 weeks or so. During the catagen phase the deeper portions of the hair follicles start to collapse.

3. Telogen (Resting Or Shedding) Stage
This is the resting period of a hair follicle. It is usually 3 to 4 months in length and at the end of this period older hairs that have finished their life will fall out and newer hairs will begin to grow.

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