How Does Your Hair Grow?

Hair can be one of a person's most distinctive features, and many of us devote considerable time to hair care and styling. Understanding how hair grows can help you make informed decisions about care and styling. The biological structures and processes at work in hair growth are also fascinating in their own right, and they provide an interesting example of how specialized and sophisticated the human body can be, even in its most minor details.

Parts of the Hair

Hair fiber is made of keratin proteins, which also are found in our fingernails and toenails, as well as the horns, claws, hooves, beaks and other parts of many animals. Every strand of hair contains three layers. The outside layer is a thin, colorless protective coating known as the cuticle. Beneath the cuticle lies the cortex, which is thicker and contains the melanin that gives hair its color. The shape of the cortex also determines whether hair is curly, wavy or straight. Within the cortex, the center of the hair is the reflective medulla, which gives hair its shine and tone.

Parts of the Follicle

Every hair of the body is anchored by a follicle, a specialized structure in the skin that surrounds the root of the hair and drives its growth. Each follicle is connected to sebaceous glands that secrete sebum, an oil that helps to protect and moisturize the hair and skin. Arrector pili muscles connected to the follicle can make the hair stand up, a process you can observe when you get goosebumps.

Types of Hair

Hair follicles are found in the skin covering the entire body except for the lips, palms and soles of the feet. Different types of hair look and act differently, as you can easily see by comparing the hair on your arms, eyebrows and scalp. Very fine, light, short hair is known as vellus hair. Terminal hair, such as the hair found on the scalp and on men's faces, is generally darker, thicker and longer.

Growth Phase 1: Anagen

Hair growth consists of three cyclical phases. During the anagen phase, hair grows at a rate of approximately a half-inch per month for a period of two to eight years. The duration of anagen determines how long a person's hair can grow; the wide variation explains why some people can grow hair up to 4 feet long, while others may have trouble exceeding 1 foot.

Growth Phase 2: Catagen

When growth ceases, a transitional catagen phase of two to four weeks follows. During catagen, the follicle shrinks, loses its blood supply and eventually disintegrates. Approximately 10 to 15 percent of your hair is estimated to be in catagen at any given time.

Growth Phase 3: Telogen

The final phase of hair growth, telogen, lasts for two to four months while the hair remains attached to the follicle but does not grow. When anagen begins again following telogen, the existing hair detaches and is lost, making way for new growth. This process accounts for the 50 to 100 hairs a person naturally loses (often while showering or brushing hair) in an average month.

References

• Hair Growth
• How Does Hair Grow?