Hair is a filamentous (silk-like) usually pigmented outgrowth of the skin (except for lips, fingertips, palms, soles, and glans penis). Each hair grows from an individual hair follicle in the deeper layer of the skin (dermis). Each strand is composed of non-living cells called keratin. Hair has a protective sheath of overlapping cells known as the cuticle. Hair’s color, strength, and flexibility are determined by the makeup of the cortex and medulla and the condition of the cuticle.
Hair carries social and cultural significance in many cultures around the world. Hair length and style may indicate group membership for better or worse; for example, Oliver Cromwell’s followers cropped their hair close to the head in an act of rebellion during the English Civil War, earning them the nickname “Cropheads.” Hair is also a symbol of mourning and is covered with turbans by women in some Muslim cultures. In addition, religious practices dictate rules on hair and body grooming.
The hair follicle is a long tube in the shape of a cylinder with a rounded bottom embedded in the skin. It is fed by nerves that come from the surrounding dermal network and are comprised of both sensory afferents and autonomic sympathetic nerves. Sensory information from the hairs enhances tactile sensation, and the autonomic nerve innervation controls the contraction of the tiny muscles that make a hair stand on end. This likely served a vestigial function related to fur; standing the hairs probably helped trap air, conserve heat in cold climates, and intimidate rivals or would-be predators.
Each strand of hair starts at the base of the follicle, where stem cells divide rapidly to form a rod of tissue in the epidermis. As the rods of cells grow upward, they are cut off from their supply of nourishment and start to harden, a process called keratinization (kair-eh-tuh-neh-ZAY-shun). The hardened cells become part of the hair shaft. Hair is essentially made of protein-rich keratin, and the dead cells that comprise it are filled with and surrounded by a hard mineral coating known as sclerene (say: sher-sen-uh).
As a result, each strand of hair has a brittle outer covering called the cuticle, which is very flexible when healthy. Chemicals and environmental damage can weaken the cuticle, resulting in dry and brittle hair. The inner core of the hair, called the medulla, is soft and contains air spaces. The medulla is responsible for most of a person’s hair’s color, and it has a very high level of elasticity due to the presence of keratin microfibrils. The medulla is the key to hair’s ability to bounce back from damage and remain healthy and strong.