Hair is a thin, flexible structure that grows from every mammal’s skin. It is made mainly of a protein called keratin. Hair is much like horns and nails, both of which also are composed of keratin. Hair can be found on the head, arms and legs of animals, as well as on their tails. Some, such as the hair on a lion’s mane, are very thick and stiff, and may help them defend themselves against a predator. Hair can also be soft, like the hair on a porcupine’s face. In these forms, it helps an animal feel its environment. Some mammals also have whiskers, which act as sensory sensors.
The human hair shaft is composed of two or three layers. The outermost layer, called the cuticle, is a translucent protective layer of flattened cells that overlap like tiles on a terra-cotta roof. The cuticle protects the softer inner layers from damage and keeps the hair shiny. It also minimises the movement of moisture in and out of the underlying cortex, keeping the hair at a healthy hydration balance. Chemical processes and other environmental factors can lift the cuticle, causing the hair to become dull or dry and brittle.
Beneath the cuticle is the cortex – the middle part of the hair that gives it its natural color. The cortex is made of long proteins that coil around each other, a little like the curly cord on a telephone. The proteins are covered by pigments, which give the hair its color. The cortex is held by a cell membrane complex, which also helps keep the proteins in their natural coils and arranged evenly along the shaft of the hair.
In addition to the polar amino acids that form strong disulfide bonds, the proteins in a hair are also made of weaker hydrogen bonding between keratin molecules. These hydrogen bonds break when the hair is wetted, which allows water to flow into the keratin and soften it. As the hair dries, the hydrogen bonds are reformed, but the protein fibers still remain in their coils. This is the reason that hair tends to frizz when it gets wet and then dries. Hair care products that contain glycerin can help prevent this.
The follicle, which is located beneath the skin in the area of a hair, contains living cells that multiply and push old cells out of the way. As a result, each hair has a dead end, or the root, and a live, growing tip, or the shaft. The follicle is attached to a sebaceous gland that produces oil, which lubricates the hair and makes it shine. The gland can sometimes produce too much oil, which causes a person’s hair to look greasy. Hair also contains a small nerve bundle that receives input from the skin and muscle, and sends signals to make the hair stand on end. This function is likely vestigial, and may have helped pre-humans conserve heat in cold climates or cut a larger silhouette to intimidate rivals or would-be predators.