Hair is the protein filament that provides the body with insulation, protection, and communication. It extends the sense of touch beyond the skin. The hair on the scalp grows in three phases, including anagen, catagen, and telogen. These cycles alternate with each other.
Hair follicles are stocking-like structures in the epidermis that provide the base for the hair. Each follicle contains a small section of dermis and a cluster of special cells. The cells within the follicle divide rapidly and grow to form the hair shaft.
Hair is composed mainly of keratin, a strong protein. Keratin is insoluble in water, so it provides hair with waterproofing properties. Keratin also acts as a component of the skin and nails. There are two types of keratin, namely alpha and beta keratin. Alpha keratin is the strongest type. When it hardens, it becomes a rigid filament called the hair shaft. Besides its important role in forming the structure of the hair, keratin helps the hair to be elastic.
Hair has the ability to change in texture, color, thickness, and length. Hair can be straight, wavy, or curly. This depends on its follicle and shape. For example, hair that has a curly structure tends to be thick and coiled. On the other hand, a strand of hair that is thin and straight can be difficult to see against a contrasting background.
Hair is constantly shed and replaced. As new cells develop, they push upward and replace the old ones. In humans, this process is controlled by an arrector pili muscle, which makes the hair stand on end.
When the hair follicle starts to develop, specialized cells known as melanocytes inject pigments into the developing hair shaft. They also produce lipids and proteins that coat the shaft and make it stronger. Pigments for hair color are protected by translucent cuticle cells.
At the base of the hair bulb, there is a vascular system that supplies blood and minerals to the root. Sebaceous glands produce oils to protect the hair shaft. Unlike the scalp, hair on the other parts of the body such as the eyebrows, lashes, and toenails are not covered by sebaceous glands.
A layer of amorphous substance, known as the pith, is at the center of the hair shaft. This part is a part of the follicle that is composed of rods of microfibrils and macrofibrils. Some follicles also contain an artery that provides nourishment to the root of the hair.
Depending on the hair follicle, the hair can be thin or thick. If the follicle is oval-shaped, the hair will be oval-shaped in cross-section. The follicles that give rise to curly hairs have a secondary c-shaped curve at the bottom.
Although hair varies in density, thickness, and color, it is composed primarily of keratin. Its main functions include thermal regulation, offensive protection, and defensive protection from ultraviolet damage. Besides its important role in the body, hair can express one’s personality.
The most common interest in the hair is its growth. Hair changes in texture, shape, and thickness, and can change color as we age.