A sac from which a hair grows and into which the sebaceous (oil) glands open.

The follicle is lined by cells derived from the epidermal (outside) layer of the skin.

Each follicle normally goes through a five-year cycle of growth and rest, with about 90% of the follicles growing hair at any one time, averaging about six inches (15 cm) of growth per year.

Derived from the Latin word follis, "bag".

Excerpts of information about hair from several sources

Hair follicles are one of the few immunoprivileged parts of the body; that is, they are protected from the immune system so the body doesn't treat them as foreign and attack them.

Researchers thus wondered if they might be transplanted from one person to another without triggering an immune response and therefore rejection.

Hair follicle cells, donated from the arm of a male scientist, were implanted into the arm of a female scientist.

A few weeks later, she grew large, thick, dark hairs, unlike her own, in the area of the transplant.

In what is currently a standard hair transplant, follicles are transplanted from one part of the patient's scalp where hair is present, to another where hair is sparse.

In other words, more hair is not being created, but just spread around.

The amount of coverage that can be obtained depends on how many active hair follicles still remain.

On average, the total number of hair follicles for an adult human is estimated at 5 million; with one million on the head, of which 100,000 alone cover the scalp!

At the base of the hair follicle are sensory nerve fibers that wrap around each hair bulb.

Bending the hair stimulates the nerve endings allowing a person to feel that the hair has been moved.

One of the main functions of hair is to act as a sensitive touch receptor.

Sebaceous glands are also associated with each hair follicle that produce an oily secretion to help condition the hair and surrounding skin.

Human hair consists of the hair shaft, which projects from the skin's surface, and the root, a soft thickened bulb at the base of the hair embedded in the skin.

The root ends in the hair bulb.

The hair bulb sits in a sac-like pit in the skin called the follicle, from which the hair grows.

At the bottom of the follicle is the papilla, where hair growth actually takes place.

The papilla contains an artery that nourishes the root of the hair.

As cells multiply and produce keratin to harden the structure, they're pushed up the follicle and through the skin's surface as a shaft of hair.

Each hair has three layers: (1) the medulla at the center, which is soft; (2) the cortex, which surrounds the medulla and is the main part of the hair; and (3) the cuticle, the hard outer layer that protects the shaft.

Hair grows by forming new cells at the base of the root.

These cells multiply to form a rod of tissue in the skin.

The rods of cells move upward through the skin as new cells form beneath them.

As they move up, they're cut off from their supply of nourishment and start to form a hard protein called keratin in a process called keratinization (ker" uh tuh nuh ZAY shun).

As this process occurs, the hair cells die.

The dead cells and keratin form the shaft of the hair.

Each hair grows about one-fourth inch (about six millimeters) every month and keeps on growing for up to six years.

The hair then falls out and another grows in its place.

The length of a person's hair depends on the length of the growing phase of the follicle.

Follicles are active for two to six years; they rest for about three months after that.

A person becomes bald if the scalp follicles die and no longer produce new hair.

Thick hair grows out of large follicles; narrow follicles produce thin hair.

The color of a person's hair is determined by the amount and distribution of melanin in the cortex of each hair (the same melanin that's found in the epidermis).

Hair also contains a yellow-red pigment; people who have blonde or red hair have only a small amount of melanin in their hair.