Hair Structure: hair is composed of a strong structural protein called keratin.
Each strand of hair consists of three layers:
1. an innermost layer or medulla.
2. The middle layer known as the cortex. This part gives the hair strength and shape. It also contains the pigment that gives the hair its color.
3. The outermost layer is known as the cuticle. The cuticle protects the cortex.

Structure of the hair root: below the surface of the skin is the hair root, which is enclosed within a hair follicle. At the base of the hair follicle is the dermal papilla. The dermal papilla is fed by the bloodstream which carries nourishment to produce new hair.

The Hair Growth Cycle: hair follicles grow in repeated cycles. One cycle can be broken down into three phases.
Anagen Phase – growth phase: approximately 80-90% of all hair is in the growing phase at any one time. Hair grows approximately 10cm per year. This phase can vary from 2 to 7 years.
Catagen Phase – transitional phase: at the end of the Anagen phase the hair enters into a Catagen phase which lasts about 2 weeks, during the Catagen phase the hair follicle shrinks to about 1/6 of the normal length. The lower part is destroyed and the dermal papilla breaks away to rest below.
Telogen Phase – resting phase: the resting phase follows the Catagen phase and normally lasts about 5-6 weeks. During this time the hair does not grow but stays attached to the follicle while the dermal papilla stays in a resting phase. Approximately 10-15% of all hair is in this phase at any one time. At the end of the Telogen phase the hair follicle re-enters the Anagen phase. The dermal papilla and the base of the follicle join together again and a new hair begins to form.