Hair follicle
A hair follicle is part of the skin that grows hair by packing old cellss 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.
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.