Hair is one of the most visible parts of a person’s body. It is found throughout the world in varying shapes and sizes, but it is most common on the head, arms and legs. It helps to keep the body warm, protects eyes from dust and debris, and adds style to a person’s appearance. It is also a source of identity, with varying social norms and cultural traditions regarding its length and style. The specialized anatomical structure of hair includes three morphological components: the cuticle, the cortex and the medulla. It is composed of a tough protein called keratin, which contains sulfated amino acids linked by cysteine disulfide bonds. It is also rich in sulfur, allowing it to be stiff and strong.
Humans grow hair from follicles located in the dermis of the skin. Hair grows from these follicles in two forms: vellus hairs and terminal hairs. Vellus hairs are fine, downy hairs that are found almost everywhere on the body except for areas of glabrous skin, such as the palms of the hands, soles of the feet and undersurfaces of the fingers and toes. Vellus hairs are not permanent, and they shed and regrow at a steady rate.
Terminal hair is coarser and darker than vellus hair, and it is the type of hair that most people think of when they hear the word “hair.” It is long, thick and shiny, and it grows from a large, round, flat-topped hair follicle in the dermis of the skin. The follicle is a hollow tube in the shape of a cylinder with a rounded bottom, which extends into the surface of the skin and outward through an open hole in the epidermis.
A hair follicle grows when stem cells in the base of the follicle divide rapidly. As the follicle grows, it pushes old cells upward to form the shaft of the hair. The cell walls of the follicle are eventually replaced by the protein keratin, which is made of amino acid chains that link together to form hard strands that stick out from the skin.
As the hair grows out of the follicle, it bends and twists until it is at the desired length. The strands of hair may then be shaped by cutting or brushing, and the whole is covered by a layer of sebum.
The style and color of a person’s hair can convey information about their gender, age and ethnicity. It can even be used to make judgments about a person’s sexuality, worldview and socioeconomic status. As a result, the presence of hair is an important part of personal identity in many societies around the world. It is also a prominent feature in certain religious ceremonies and may be used as trace evidence during forensic investigation.