Hair is the tough fibrous tissue that envelops and protects the body’s skin. It helps to keep the head warm and also acts as a signal of sexual maturity and fertility in humans. In some mammals, such as hedgehogs and porcupines, hair is adapted into hard spines called quills that provide protection against predators.
In human beings, hair is a protein called keratin. It’s similar to the keratin that makes up our fingernails and the outer layer of the skin. Keratin is a tough material that is not easily damaged or ripped. There are three layers that make up the visible part of a hair: the cuticle, cortex and medulla.
Most of the hair is anchored below the surface in a pit (follicle) in the skin. This follicle is surrounded by a circle of skin cells that is constantly going through mitosis (cell division) to produce new hair. The cells in the follicle include specialized ones called keratinocytes (cells that make a fibrous protein called keratin). There are also special pigment-producing cells (melanocytes) located near the hair follicle. These produce the coloration of hair.
The follicle is fed by blood vessels and by sebaceous glands that secrete oil. A muscle (arrector pili) in the scalp causes some of the hairs to stand up at certain times. This is an evolutionary adaptation to reduce the amount of UV light reflected from the scalp and face.
Once the hair is rooted, it grows out of the follicle and into the skin. The growing hair pushes out a small section of the epidermis at the base of the follicle and then keratinizes. This gives the hair its strength and structure. As it grows, the hair may curve or straighten, depending on its shape in the follicle and the surrounding environment. It may even get colored or scented, such as when it is sprayed with perfume or dyed a bright shade of blue or pink.
Whether the hair is fine, medium or coarse depends on the shape of the follicle: circular follicles produce straight hair, while oval follicles produce wavy or curly hair. The thickness of the hair depends on how many keratinocytes are produced in the follicle.
Hair is made up of bundles of coiled proteins, with the strands being held tightly together by the matrix of keratin. These proteins are surrounded by a translucent outer layer of cuticle cells that help to protect it from the outside world.
There is a central area in the hair called the medulla, which is soft and spongy. It is not very important to hair health, but it provides some protection from injury. It is also the source of the chemical pigments that give hair its natural color.