Hair, that scaly, shiny, often colorful accoutrement that covers much of the body, has many functions. It serves as a defense against the elements (the hair on the head keeps the skull warm, for example), an aid in thermoregulation and sensory input. It also provides an opportunity to express one’s personality and sense of style through color, length, texture, and shape.
Hair is an external appendage that comes from a specialized type of skin cell called the hair matrix and its surrounding tissue. When hair is growing, cells in the matrix divide and differentiate to form different layers. This is what gives hair its unique appearance.
A strand of hair grows from an opening in the scalp or skin called a follicle (fol-LEE-kul). Each follicle is in the shape of a cylinder with a rounded bottom embedded in the skin. Each follicle has an oil gland called a sebaceous gland. Oil made in these glands helps to keep the scalp and hair hydrated.
The top layer of the follicle is the epidermis, which contains a pigment called melanin and protects against the sun. Below that is the dermis, which contains a protein called keratin. The keratin gives the hair its strength and elasticity. The dermis is then covered by a layer of cuticle (koo-tie-lay) cells. This layer is a multi-layered structure that is designed to shield the cortex and medulla, which are responsible for color and texture, from environmental stressors.
In the middle of each hair is the medulla, a soft, spongy mass of cells that makes up most of the weight of the hair. Coarse hair has a medulla, while finer hair does not. The medulla also plays a role in determining the strength and flexibility of the hair.
A hair shaft (hah-skul) is formed by the medulla and cuticle. It is surrounded by a bundle of fibers that give it its shape and resistance to stress. A cross section of a hair shaft shows that it is made up of a bundle of multi-layered, flattened cells that are rolled up into rope-like filaments.
The center of the hair shaft is hollow and sometimes contains a medulla, but usually not. The center of the hair is made up of an electrolytically controlled ionic bond (called a salt or disulfide bond) and cystine bonds, which are very strong. These bonds are impacted by pH balance and other factors and they break when the hair experiences stress. The salt and cystine bonds then reform to help the hair retain its strength and elasticity.