Hair is one of the defining characteristics of mammals. Apart from areas of glabrous skin, the body is covered with follicles that produce thick terminal hair and fine down hair (vellus). The most well-known interest in this human appendage pertains to its growth, appearance and care. However, it is important to recognize that the structure of the hair is an essential biomaterial that provides protection and nutrition to the follicles and to the surrounding dermis. Hair is composed of a fibrous, helical protein known as keratin. The keratin in hair is highly insoluble in water and provides a protective barrier against physical insults and chemical damage, as well as being an important source of nutrients such as amino acids and minerals.
In the first months of life, a thin layer of downy, slender hairs called lanugo develops on the surface of the skin. This is shed before or shortly after birth. After puberty, a thicker, more heavily pigmented form of hair, termed body hair, grows in the armpits, genital regions and, in males, on the head. This is supplemented by shorter, more delicate eyelashes and eyebrows that serve as tactile sensors and play an important role in non-verbal communication. In some mammals, hairs have been modified into hard spines, such as those of porcupines and lion’s manes, that offer limited protection from predatory attacks.
The shape and size of the hair shaft determines its morphology, allowing it to take on a variety of forms. A round hair shaft produces straight hair, while a shaft that is oval or other shapes produce wavy and curly hair. Hairs are anchored to their follicular sheath by an inner root sheath and, in some cases, a follicle that contains glands, most importantly the sebaceous gland that produces the oil (sebum) that lubricates and protects the hair.
The length of a hair is determined by its follicular growth phase and, in humans, by the genes that control this process. A hair can also be influenced by the environment and lifestyle. It is also affected by disease, most notably the itching and flaking caused by eczema, seborrheic dermatitis and psoriasis.
In many societies, the color, length and appearance of a person’s hair is associated with a variety of cultural and social aspects. It is often used to identify gender, race and ethnicity, sexuality, worldview and socioeconomic status. In addition, the style and appearance of a person’s hair can be changed for aesthetic reasons.