The hair follicle is an organ that lies under the surface of the skin. It extends down into the dermis layer to contain the sebaceous gland, which secretes sebum, a lipid-rich substance that protects the hair and helps moisturize the scalp. The growth of hair takes place at the base of the follicle, where blood vessels carry nutrients to the cells inside. Once these cells start to produce new hair, they grow into strands.
Hair is made of two kinds: lanugo and vellus. The first is downy and slender, and begins growing in the third or fourth month of fetal life and is shed before birth or soon afterward. It covers most of the body except the palms and soles, and only the underside of fingers.
Hairs differ in colour and pattern, and serve multiple purposes, including sexual recognition and camouflage. Among mammals, hairs are often used as sensory organs in nocturnal animals. Porcupine quills serve a similar purpose. Warm-blooded mammals develop hair in order to prevent heat loss.
Hair is also a sign of group membership. According to Jablonski, hair on the head was advantageous for pre-humans, since it helped protect the scalp from intense UV rays from the African sun. Others believe that having hair on the head would have made humans more sexually mature. Another interesting aspect of hair is that a hairy shoulder is desirable for survival, since the head contains the brain and is especially vulnerable at birth.
Hair follicles contain cells that help the body heal after a wound. Because they are located closest to the wound site, they can quickly travel to the wound and work with white blood cells to fight the infection. The hair follicle is made of layers of cells that make a tube-like structure that houses hair.
Inflammation of the hair follicles can be caused by several conditions. Some of these include folliculitis and acne. Inflammation in the scalp may also be caused by a fungal infection. This infection can lead to hair loss. People with hirsutism may also experience hair loss.
Hair growth and development is a cyclical process that occurs on a periodic basis. The mature follicles go through growth phases, regression phases, and rest periods. Each phase is unique and has different durations. Age, nutrition, and hormonal levels can affect how long each phase lasts. In the anagen phase, mitotic activity takes place in the secondary epithelium.
The hair germ is a cluster of epithelial cells. The cuticle layer of these cells binds the hair shaft to the hair follicle. Later, in stages three and four, the hair peg is developed and surrounded by mesenchymal cells, which transform into a fibrous sheath. Later, cells of the hair matrix form the hair shaft and the outer root sheath. Later, the outer root sheath begins to be keratinized and a bulge forms at the base of the isthmus.