A hair follicle is part of the skin that grows hair by packing old cells together. Inside the follicle the sebaceous gland is found, a tiny oil-producing gland found everywhere except on the palms and soles of the feet. The thicker density of hair, the more sebaceous glands are found. At the end of the hair, tiny blood vessels form the root, around the root there is a white structure called a bulb, which is visible on plucked healthy hairs.

At the base of the follicle is a small structure that is called the papilla. This is the actual cell-producing structure where the hair strand is constructed. The papila is shaped like a cone, protruding into the bulb of the growing hair.

Also attached to the follicle is a tiny bundle of muscle fiber called the arector that is responsible for causing the follicle and hair to be come more perpendicular to the surface of the skin, and causing the follicle to protrude slightly above the surrounding skin. This process results in goose bumps or goose flesh . Stem cells are located at the junction of the arector and the follicle, which are principally responsible for the ongoing hair production during the Anagen stage.

Hair grows in cycles of various phases. Anagen (hair growth), catagen (involuting) and telogen (resting). Normally up to 90% of the hair follicles are in anagen phase while, 10-14% are in telogen and 1-2% in catagen. The cycle's length varies on different parts of the body. For eyebrows the cycle is completed in around 4 months while it takes the scalp 3-4 years to finish, this is reason hair on eyebrows have a fixed length, while hair on the head seems to have no length limit. Growth cycles are controlled by chemical signals like epidermal growth factor.

Hair growth cycle times

• Scalp:
  • anagen phase, 2-3 years (occasionally much longer)
  • catagen phase, 2-3 weeks
  • telogen phase, around 3 months

• Eyebrows etc:
  • anagen phase, 4-7 months
  • catagen phase, 3-4 weeks
  • telogen phase, about 9 months