What is hair? |

Structure and Functions

Humans grow three kinds of hair. The downy hair that covers the fetus is lanugo. Soon after birth, it is replaced by vellus (or villus) hair. Vellus hair covers the entire skin surface except for the palms of the hands and the soles of the feet. It is fine, short, nearly colorless, and slow-growing.

The thick, pigmented hair on the head, eyebrows, and eyelids is terminal hair. Somewhere between 65 and 95 percent (by weight) of terminal hair is protein. Other components include water, fats (lipids), trace elements (minerals), and pigment. Because proteins twist into complex three-dimensional shapes held together by chemical bonds, hair is both rigid and flexible. Terminal hair replaces villus hair on the genitals at puberty. Called pubic hair, it is coarse and curly. Also at puberty, terminal hair begins to grow in the armpits and on the faces of males.

The shaft of a single hair has three layers. The outer casing is the cuticle, made of overlapping layers of proteinaceous material. Inside the cuticle lies the cortex, a column of cells containing keratin, the same protein that hardens tooth enamel and fingernails. The central core of the hair is the medulla. Also called the pith, it is made of small, hardened cells snared in a web of fine filaments.

Hair grows from a tiny pouch below the skin’s surface called a follicle. At the bottom of the follicle lies the papilla, an upward-growing finger of connective tissue. The papilla forms the root of the hair shaft. The actively growing part of the hair shaft is the hair bulb. The cells that generate the hair lie just above the hair bulb. As soon as hair cells are manufactured, they harden and die, forming the hair shaft.

Tiny blood vessels around each follicle supply nutrients. Sebaceous glands that open into the follicle produce the oily sebum that lubricates hair and skin. In the papilla of the follicle, melanocytes produce melanin, the same pigment that gives skin its color. There are two kinds of melanin: Eumelanin makes hair black or brown, while pheomelanin makes it red or blond. Melanin is deposited in the cortex of the hair shaft of terminal hair, giving it its color.

Hair helps insulate the body. Arrector pili muscles at the base of the follicle elevate hair in response to environmental stimuli, including cold. The high sulfur content of keratin gives it heat-retaining properties. Hair also retards water loss from the body. Body hair augments the sense of touch. Hair’s movement facilitates the detection of light touches and slight temperature changes.

Terminal hair cushions the head against blows and protects the scalp from sunburn. Eyelashes keep dirt, insects, and foreign objects out of the eyes. Eyebrows keep sweat from running down into the eyes. The hair inside the ears is coated with the waxy substance cerumen that traps dirt and prevents infections. Hair in the nose filters dust and bacteria from the air.

Hair growth and replacement occur in three stages. Anagen is the active growth stage. It lasts from two to six years. During catagen, lasting about two weeks, the lower segment of the hair follicle breaks down. A “club hair” separates from the papilla and falls out. Then, during telogen, the follicle “rests.” Telogen lasts several weeks or months. At any one time, about 80 to 90 percent of the hairs on the head are in anagen, 3 to 4 percent are in catagen, and the remainder are in telogen.

Disorders and Diseases

While a loss of fifty to one hundred scalp hairs per day is normal, alopecia, or noticeable hair loss, occurs in nearly one-third of women and two-thirds of men. Women typically notice a general thinning of the hair, while men usually experience male pattern baldness:
loss at the hairline and crown first, followed by loss at top of the head. The cause is neither a loss of follicles nor a cessation of hair growth. Instead, follicles gradually shrink and become less active, producing shorter, finer vellus hairs. Three interacting factors—heredity, hormones, and aging—cause the change in follicles. Genetic programming controls the age when hair loss begins and how fast it progresses. Male hormones (even in women) must be present for balding to occur.

No drug can reverse baldness in its later stages. However, minoxidil (trade name Rogaine) slows the rate of hair loss or promotes regrowth in about 25 percent of men and 20 percent of women. It received the approval of the Food and Drug Administration (FDA) in 1988 and became available without a prescription in 1996. Another drug, finasteride (trade name Propecia), was approved in 1997 for use in men only.

Surgical alternatives are available for those bothered by baldness. To transplant
or graft hair, surgeons remove segments of scalp from the sides and back of the head (where follicles are less sensitive to hormones) and transfer them to the top. To perform a scalp reduction, surgeons cut away a portion of hairless scalp, then stitch the remaining scalp together, reducing the total area of baldness. Another alternative is flap surgery. A flap of hair-bearing scalp is turned to cover the spot where bald scalp has been removed.

Alopecia areata is the loss of hair in round patches, usually on the scalp, beard, eyebrows, or pubic area. It is thought to be an autoimmune disease. The immune system “mistakes” the hair follicles for invading disease agents and attacks them, reducing their size and decreasing hair production. The disorder may result from fever, stress, surgery, allergies, crash diets, burns, scalds, and tumors. Other possible causes include radiation exposure, an overactive or underactive thyroid gland, liver or kidney disease, and illnesses ranging from influenza to scarlet fever. A deficiency of iron, zinc, or certain vitamins is the cause in some cases, as is chemotherapy for cancer. Hair loss is also a side effect of many drugs. Alopecia areata is seldom serious or permanent, but it can be disturbing. A doctor may prescribe drugs to combat it, but it generally resolves itself within a year.

Hirsutism, excessive hair growth on the face or body, can cause concern, especially for women. Although it can be triggered by tumors, diseases of the ovaries or adrenal glands, contraceptive pills, hormonal drugs, or anabolic steroids, the most common cause is the menopause. As production of the female hormone estrogen declines, the relative concentration of male hormones (produced naturally by the adrenal glands) rises, causing dark hairs to appear on the upper lip, chin, and cheeks. Shaving, tweezing, waxing, sugaring, or using depilatory creams and lotions removes hair temporarily. Electrolysis, in which a needle inserted into the follicle delivers a current that destroys the follicle, removes hair permanently. In severe cases, doctors may prescribe drugs that block the action of male hormones to treat hirsutism in women.

Perspective and Prospects

Since ancient times, people have sought to prevent or reverse the balding process. The Egyptians of the sixteenth century BCE prescribed a blend of iron, red lead, onions, and alabaster, along with prayers to the sun god. In 420 BCE, the Greek Hippocrates, often called the founder of modern medicine, recommended a mixture of opium, horseradish, pigeon droppings, beetroot, and spices.

The concept of surgical hair transplantation arose in the early nineteenth century, and in the twentieth century reliable techniques were developed. Japanese physicians pioneered hair transplantation and grafting in the late 1930s and early 1940s, but it was not until the 1980s that procedures yielding cosmetically acceptable results were achieved in the United States. Drug treatments for baldness were discovered by chance as side effects. Minoxidil originally treated hypertension; finasteride ameliorated prostate enlargement.

Attempts to remove unwanted hair have roots in prehistory. Sharpened rocks and shells used for hair removal have been found in archaeological sites dating back twenty thousand years. The ancient Sumerians invented tweezers, and the Egyptians buried razors and arsenic-based depilatories with their dead. Native American men tweezed facial hair with clamshells, and North American colonists in the seventeenth century used caustic lye to burn hair away. In 1903, the American inventor King Gillette marketed the first razor with a disposable blade. Jacob Schick introduced the electric shaver in 1931.

In the 1960s, lasers were first used to heat and disable hair follicles over large areas, and hair removal entered the arena of medical practice. Early lasers emitted a continuous wave that risked overheating and skin damage. The invention of a switching device allowed light energy to enter the follicle in controlled pulses. In 1995, the FDA approved the first laser device for hair removal. Most developed since that time use water or gel to cool the skin and laser light to target the melanin in the hair. Other strategies are being investigated, and studies of such variables as beam width, pulse duration, and delivery rate may result in laser hair removal treatments that are safe, effective, and permanent.

Basic research on the nature and action of the immune system may lead to treatments for autoimmune disorders, including alopecia areata. The cloning of individual, hair follicles may facilitate hair transplantation. Gene therapy could, in theory, be used to alter the genetic control of follicles, preventing inherited baldness entirely.

Bibliography

Anderson, Richard R. “Lasers in Dermatology: A Critical Update.” Journal of Dermatology 27, no. 11 (November, 2000): 700–705.

Brynie, Faith Hickman. 101 Questions About Your Skin That Got Under Your Skin . . . Until Now. Brookfield, Conn.: Twenty-first Century Books, 1999.

Burns, Tony, et al., eds. Rook’s Textbook of Dermatology. 7th ed. Malden, Mass.: Blackwell Science, 2004.

Dunn, Rob. "Hirsute Pursuits." New Scientist 214, no. 2869 (June 16, 2012): 44–47.

Freedberg, Irwin M., et al., eds. Fitzpatrick’s Dermatology in General Medicine. 7th ed. 2 vols. New York: McGraw-Hill, 2008.

Kuntzman, Gersh. Hair! Mankind’s Historic Quest to End Baldness. New York: AtRandom, 2001.

Unger, Walter P., Ronald Shapiro, Robin Unger, and Mark A. Unger, eds. Hair Transplantation. 5th ed. New York: Informa Healthcare, 2011.

Wood, Debra. "Alopecia." Health Library, October 31, 2012.