Handbook of hair in health and disease

The hair follicle is a sensing, reactive, dynamic and complex skin appendage that offers many essential functions and requires a specific molecular environment to remain fully functional. Apart from providing an insulating layer in animals, hair serves as an aid in camouflage for survival and provides sensory, tactile information about the environment. Hair may also be of importance in attracting mates by offering insight into an individual’s health and vitality through its color, distribution, or quality. Hair follicles are formed from epidermal-dermal interactions and modeling during the first trimester in humans. Hair growth is a unique and complex process that involves continuous cycles of growth, resorption, rest and renewal. The hair follicle is constantly exposed to the environment and suffers direct damage from the sun, the air, and water. Exposure to foreign chemicals may also cause damage to the hair shaft and follicle. There are numerous hair diseases and the hair follicle is involved in over a thousand systemic diseases including genetic disorders. The social function of hair is the foundation for a multibillion dollar industry focused on presenting, augmenting, preserving, or removing hair.