The Anatomy of Hair

There is a lot more to the structure and function of hair than meets the eye. Hair helps transmit sensory information and is a huge part of our general appearance. There is hair on all the major visible surfaces of the body, and hair is the only body structure that is completely renewable without scarring.

The Origin of Hair

A developing fetus has all of its hair follicles formed by the 22nd week of development. At that time, there are roughly 5 million hair follicles; one million of those are on the head and 100,000 are on the scalp. This is the largest number of hair follicles you will ever have as follicles are never added during our lifetime. As the size of they body increases as we get older, the density of the hair follicles on the skin decreases.

The Structure of Hair

Hair has two separate structures: the follicle in the skin and the shaft.
The follicle is a stocking-like structure that contains several layers. At the base of the follicle is a projection called a papilla that contains capillaries that feed the cells. The living part of the hair is the bottom part of the papilla called the bulb. This is the only part fed by the capillaries. The cells in the bulb divide every 23-72 hours, faster than any other cells in the human body.
The follicle is surrounded by two sheaths, an inner and outer sheath, that protect and mold the growing hair shaft. The inner sheath follows the hair shaft and ends below the opening of a sebaceous gland (oil gland). The outer sheath continues all the way up the gland. A muscle called an erector pili muscles attaches below the gland to a fibrous layer around the outer sheath. When this muscle contracts, it causes the hair to stand up.
The sebaceous gland is important because it produces sebum, which acts as a natural conditioner for the hair. Sebum production decreases in women as they age; it decreases in men, also, but not quite as much.
The hair shaft is made up of dead, hard protein called keratin in three layers. The inner layer is called the medulla. The next layer is the cortex, and the third (and outer) layer is the cuticle. Most hair conditioning products attempt to affect the cuticle. Also, there are pigment cells that are distributed throughout the cortex and medulla giving the hair its color.

Hair Growth

Hair on the scalp grows approximately a half inch per month. Unlike other mammals, hair growth and loss in humans is random and not seasonal or cyclical. At any given time, a random number of hairs will be in various stages of growth and shedding. The three stages of growth are catagen, telogen and anagen.
The catagen phase is a transitional stage and 3% of all hair is in this phase at any given time. This phase lasts for 2-3 weeks, and during this time, hair growth stops and the outer root sheath shrinks and attaches to the root of the hair. This is the formation of what is known as club hair.
The telogen phase is the resting phase and accounts for 10-15% of all hairs. Telogen lasts about 100 days for hair on the scalp and much longer for hairs on the eyebrow, eyelash, arm and leg. During the telogen phase, the hair follicle is completely at rest and the club hair is completely formed. Pulling out a hair in this phase will reveal a solid, hard, dry white material at the root. Typically, 25-100 telogen phase hairs are shed each day.
The anagen phase is the active phase of the hair, and in this phase, the cells in the root of the hair are dividing rapidly. A new hair is formed and pushes the club hair up the follicle and eventually out. Some people have difficulty growing their hair beyond a certain length because they have a short active phase of growth.

Hair Shape

The amount of natural curl hair has is determined by its cross-sectional shape. Hair that is most similar to a circle is straight, and hair that is flattened and elliptical is curly or kinky. The cross-sectional shape also determines the amount of shine that hair has. Straighter hair is typically shinier because sebum from the sebaceous gland can travel down the hair more easily. The curlier the hair, the more difficulty the sebum has traveling down the hair; curlier hair may seem drier and duller than straight hair.