Hair is the defining feature of many mammals, including humans. It is a series of threadlike outgrowths of the outer layer of skin (epidermis) that make up an animal’s coat, or pelage. Some species have thick, dense fur; others are nearly hairless. In humans, most of the hair is found on the head and shoulders, but a small amount can be seen elsewhere on the body.
Hair grows from a tiny, saclike hole in the skin called a hair follicle. Each hair follicle contains a cluster of special cells that produce and mature into the hair. As the hair cells grow, they push up through the follicle, where they are nourished by blood vessels until they reach the surface of the skin. The hair follicles have a round shape and are located close to the scalp.
Once a hair has reached the surface of the skin, it is no longer alive. It is made up of a bundle of multi-layered, dead cells filled with and surrounded by a protein called keratin. Keratin is also the main component of nails and the outer layers of skin. In addition to keratin, hair has several other proteins, fats, and minerals that contribute to its color and texture.
As the hair grows out of the follicle, it undergoes a process called keratinization, which causes it to harden. This process is what gives hair its characteristic feel and strength. The hair shaft is composed of a core of cells that contain the majority of its mass and a surrounding sheath of flat, overlapping cuticle cells. The sheath of cuticle cells protects the keratinized cortex from environmental stresses and alterations, and it helps the hair retain its moisture.
The hair cell sheaths are lined with a hydrophobic lipid membrane that allows for light reflection and limits friction between the cell walls. The sheaths are further protected by a thick, transparent layer of protein known as the epidermis. The epidermis is also a primary site for the formation of natural oils.
A spongy, translucent layer of tissue known as the medulla is present in some hairs but not in all. The medulla provides some of the structural integrity of the hair, but its main role is to regulate the flow of ions and nutrients into the cortex.
Hair is the most durable and flexible fiber of the human body, but its strength and resilience depend on its physical and chemical properties. It is important to keep your hair healthy by eating a balanced diet and using proper shampoos and conditioners. Hair can be damaged by environmental factors such as sun exposure, chlorine in swimming pools, and high temperatures. In addition, certain illnesses and hormonal changes can affect the growth and condition of your hair.
The most noticeable thing about your hair is its appearance and length, but the fact that it is made of a protein – keratin – and is actually dead tissue makes it unusually strong for its size. In humans, hair has a regenerative process that allows it to grow back after it is cut or injured.