Hair, a protein-based fiber, has many roles in the human body including protection, thermoregulation, sensory input and communication. It is also an important feature of our identity and appearance. In humans, hair can be straight, wavy, curly or brown, black, red and white. It can be trimmed or cut, or chemically altered and dyed in different ways to change its color or texture. Hair can be damaged by too much heat, such as from blow dryers or use of hot curling or flat irons. It is also susceptible to certain conditions like dandruff and psoriasis. It may even reveal certain underlying health issues like vitamin deficiencies, infections or hormone imbalances.
The hair follicle, the primary structure from which hair grows, is a multi-cellular, dermal papillae stem cell that undergoes a series of cycles including anagen (growth), catagen (transition) and telogen (rest). The hair growth cycle depends on a balance between rapid cell division and differentiation as well as apoptotic signaling. During the anagen phase, hair cells divide and differentiate into keratinocytes, which are then pushed toward the skin surface and keratinized to form the hair shaft. Hair can be either vellus or terminal hair, depending on sexual differentiation. The vellus hair is juvenile and is shed to make way for the more mature, thicker terminal hair.
While hair is mostly composed of protein, it also contains some water and lipids, which can be hydrated with shampoos or other treatments. The condition of your hair is an important indicator of the health of your scalp and skin, which in turn can indicate underlying health issues such as nutritional deficiencies or hormonal imbalances.
Most hair is colored by a pigment called melanin. There are two types of melanin: eumelanin and pheomelanin, which give the different colors of our hair. Eumelanin is found in the outermost layer of the hair shaft and pheomelanin is produced by cells in the basal layer. The amount of melanin in the hair is influenced by the sun exposure, age and genetics. The pigment melanin has a protective function against UV damage and helps to maintain the integrity of the scalp and the hair itself.
Specialized hairs in the nose and ears (ear lobes) and around the eyes (eyelashes) protect these areas by trapping dust particles and other microbes, and prevent sweat from dripping into the eye. These specialized hairs are also richly innervated and can serve sensory as well as communicative functions.
Hair is an inherited characteristic that originated in the common ancestor of mammals, synapsids, about 300 million years ago. However, it is not known at what point in synapsid evolution this trait was acquired as it did not appear in fossils until much later. It is also unclear at what point hair became a dominant feature of mammalian species. Nevertheless, it remains an essential part of the mammalian body, and serves several key functions in humans and other mammals, such as thermoregulation and communication.