Hair Growth Cycle
Hair on the scalp grows at a rate of about 6 inches/ year. At any given time, a random number of hairs will be in various stages of growth and shedding. There are three stages of hair growth: catagen, telogen, and anagen.
The anagen phase is known as the growth phase. It begins in the papilla and can last from two to six years. The span at which the hair remains in this stage of growth is determined by genetics. The longer the hair stays in the anagen phase, the faster and longer it will grow. During this phase, the cells in the papilla divide to produce new hair fibers, and the follicle buries itself into the dermal layer of the skin to nourish the strand. About 85% of the hairs on one's head are in the anagen phase at any given time.
The catagen phase, also known as the transitional phase, allows the follicle to renew itself. It lasts for about two weeks. During this time, the hair follicle shrinks due to disintegration and the papilla detaches, cutting the hair strand off from its nourishing blood supply. Ultimately, the follicle is 1/6 its original length, causing the hair shaft to be pushed upward. While hair is not growing during this phase, the length of the terminal fibers increases when the follicle pushes them upward.
During the telogen, or resting phase the follicle remains dormant anywhere from 1-4 months. 10 to 15% of the hairs on one's head are in this phase of growth at any given time. In this phase the epidermal cells lining the follicle channel continue to grow as normal and may accumulate around the base of the hair, temporarily anchoring it in place and preserving the hair for its natural purpose without using the body's resources.
The anagen phase begins again once the telogen phase is complete. The dermal papilla and the base of the follicle join together again and a new hair begins to form. If the old hair has not already been shed the new hair pushes the old one out and the growth cycle starts all over again. The process results in normal hair loss known as shedding.