Typical mammalian hair consists of the shaft , protruding above the skin, and the root , which is sunk in a follicle , or pit, beneath the skin surface. Except for a few growing cells at the base of the root, the hair is dead tissue and is composed of keratin and related proteins.
Human hair is formed by rapid divisions of cells at the base of the follicle.
As the cells are pushed upward from the follicle's base, they harden and undergo pigmentation. The hair on our scalps and in our eyebrows and eyelashes are different from other bodily hairs. The hair on our heads grows a healthy .5 inch per month, and long scalp hairs have an average life of 3 to 5 years. Before any permanent color can be deposited into the hair shaft, the cuticle , or outer layer, must be opened. The insoluble formula then reacts with the cortex to deposit or remove the color.