Hair serves many purposes, including protection, thermoregulation and sensory input. It also helps to distinguish members of a species or social group. People can choose how to style their hair, and it is often an important part of a person’s personality. It can be worn slicked back and smooth, or down and wavy. People may dye their hair to change its color or add highlights. It is also often cut or trimmed, shaved, or cut into patterns. The article will explore the structure and development of hair, as well as how it can be manipulated to change its appearance.
Hair is made from a protein called keratin (kair-ahtin). It is also found in nails and the outer layer of skin. Keratin gives hair strength and resilience. Hair is composed of three layers: the cuticle, the cortex and the medulla. The cuticle is the outermost layer, and it is made of overlapping scale-like cells. The cortex is the middle layer that contains tightly packed keratin fibers. The medulla is the innermost layer that contains blood vessels that supply hair with nutrients. Hair follicles are attached to small muscles that can make hair stand up, and they have nerves that sense movement of air near the scalp.
A person’s hair color is determined by the amount of pigment melanin they have. There are two types of melanin: eumelanin and pheomelanin. Eumelanin is the dark pigment that makes up most of a person’s hair, and it is produced by follicles on the scalp and in the skin of the arms and legs. Pheomelanin is light brown and is made by follicles in the eyebrows, nose and ears and around the eyes. As people age, they produce less eumelanin and more pheomelanin, which is why their hair often turns gray or white.
The hair follicle sits underneath the skin’s surface and has an oil gland called a sebaceous (sih-BAY-shis) gland. This gland produces lipid-rich sebum that helps keep hair and skin moisturized. Each follicle grows hair by producing new cells that grow and then die, pushing the hardened older cells up through the pore to the skin’s surface as a strand of hair.
Each strand of hair has a protective covering called the cuticle, which is made from overlapping scale-like cells. This covering protects the underlying proteins. Underneath the cuticle is a matrix of keratin fibers that give hair its strength and resilience. The top of the strand is called the epidermis, and it has tiny sensory nerves that can perceive movements of the air, much more so than the skin surface over it.
The hair strand then grows out of the hair follicle and onto the epidermis, where it becomes surrounded by more melanin and a thin layer of fatty acids that help to keep it soft. Hair strands may be straight or wavy, curly or kinky. They can be smooth or frizzy, but they are usually softer than skin and rougher than nails.