Hair is a protein filament that grows from follicles found in the dermis layer of your skin. Apart from areas of glabrous skin, most of the rest of the human body has hair (also called seta or trichoma). Hair is one of the most characteristic features of mammals and serves several important functions. These include thermal regulation, camouflage, communication and defense. Hair also provides sensory input extending beyond the surface of the skin. For example, the hairs in the nose and ears and around the eyes prevent sweat from dripping into the eyes and irritating them.
Hair growth is a complex process. Each strand of hair begins in a small saclike hole, called a hair follicle, in the dermis of your skin. The follicle is lined with a sebaceous gland, which produces lipid-rich oil called sebum that moisturizes and protects your hair. At the bottom of the follicle is an area that contains a group of cells that multiply to form new hair cells. These produce the long chains of proteins called keratin that make up your hair.
The keratin proteins are hard and flexible, and they consist of a core layer with a honeycomb-like structure, called the cortex, and an outer coating called the cuticle. The cuticle has a smooth appearance and limits friction between the hair shafts. It is a waterproof, oleophilic membrane with a lipid bilayer that repels water and protects the hair from damage. The cuticle is also a protective shield that shields the hair from the environment.
Behind the cuticle is a layer of flattened, overlapping cellular units called the medulla, which determines many of the mechanical properties of hair. The medulla is a soft, spongy mass of tissue that extends to the base of the hair root. The medulla is not present in all hairs, but it is usually visible in coarse or thick hair. The medulla is filled with melanocytes, special cells that produce the melanin pigment that gives hair its color.
A third layer of cells, called the periderm, surrounds the medulla. The periderm helps to maintain the shape of your hair, and it also contains the glands that produce sebum.
The outermost layer of the hair, which consists of scale-like overlapping epithelial cells, is called the epidermis. The epidermis is a specialized type of cell that acts as a barrier to external substances and protects the underlying cells. The epidermis also provides a smooth surface for the hair, and it can reflect light to give it a shiny appearance.
Behind the cuticle and periderm is the cortex, which makes up most of the hair shaft and is responsible for its strength and flexibility. The cortex is filled with a cellular cement that is rich in lipids and proteins, including keratin. The keratin in the cortex has a high sulfur content and consists of long keratin chains that are oriented parallel to the length of the hair. These keratin chains are joined together by disulfide bonds, which form strong crosslinks that contribute to the strength and resilience of hair.