The part of a hair below the surface of the skin rests in a bag-like structure called the follicle. The root, which is the lowest section of a hair, enlarges at the end into a soft, light-coloured structure called the hair bulb.
Hair develops from the cells of the hair bulb, which divide rapidly.
The cells of the hair bulb move upward as new cells begin to form beneath them. A structure called the papilla projects into the bulb at the base of the follicle. It contains an artery that supplies the blood necessary for the rapidly growing cells. As these cells move higher, they are cut off from their supply of nourishment and start to form a hard protein called keratin. The formation of keratin is called keratinization, and the hair cells die as it occurs.