Hairs are elongated keratinized structures. Keratin is a special protein, which is resistant to wear and tear. It is the protein that also makes up the nails. Like other proteins in the body, keratin is also a large molecule made up of smaller units called amino acids. The amino acids are joined together in a chain, like beads on a string.
The diameter of a single hair fiber varies from person to person; but it is usually around 0.05 to 0.09 millimeters.
The epidermis is the outermost layer of the skin. Each hair arises from an indentation on the epidermis. The hair has two parts: the hair follicle and the hair shaft.
The hair follicle is the point from which 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 is the structure formed by actively growing cells. These cells produce the long, fine and cylindrically shaped hair fibers. Here in the hair bulb, there are some special cells, which produce the pigment that gives the hair its color. This pigment is called melanin and the cells producing it are known as melanocytes. We also know that receptors for the male hormones - androgens, are located on the cells of this structure.
At the base of each hair bulb is the dermal papilla containing a vessel tuft. Thus, it is essential for the nourishment of the growing hairs. Within the skin, internal and external root sheaths cover the hair follicles. The external root sheath of a hair follicle is continuous along with the epidermis. There are also some glands adjacent to the hair follicles. The most important one of these glands is the sebaceous gland, which produces and secretes the natural oils lubricating hairs, namely sebum.
The part of the hair seen above the skin is called the hair shaft. The hair shaft is made up of dead cells that have turned into keratin and binding material, together with small amounts of water. This structure explains why we do not feel any pain while our hair is being cut.
The hair shaft is formed by three layers. The innermost layer of the hair shaft is named the medulla. It is seen only in large and thick hairs. The middle layer of the hair shaft is called the cortex, made of keratin fibers. The strength, color and texture of a hair fiber are provided by the cortex layer of the hair shaft. The outermost layer of the hair shaft is the cuticle. This thin and colorless layer made up of between six to ten overlapping layers of long cell remnants, serves as a protection to the cortex.