Hair today, gone tomorrow? Teasing out details of hair growth

Science News, Oct 20, 2001 by Damaris Christensen

From the strength-giving locks of the biblical Samson to the elaborate updo of a geisha and the neon, spiked styles of punk rockers, hair remains an important part of a person's self-image and one of the most obvious forms of self-expression.

People may cut their hair, dye it, perm it, and style it. But unless they resort to wigs and hair transplants, they are stuck with the quantity of hair that nature gives them--and takes away. That fact of life becomes especially vexing when people lose their hair to chemotherapy, diseases that attack hair follicles, or simply aging. And there are few options for hirsute women, who have an overabundance of hair and may even grow beards.

At the moment, scientists don't fully understand the molecular orchestrations that underlie hair growth. That hasn't stopped them from providing a few drugs for hair-related problems. The two medications now available to treat hair loss, known commercially as Rogaine and Propecia, were developed after researchers fortuitously observed hair growth as a side effect of drugs designed for treating hypertension and enlarged prostates. Likewise, the only drug on the market that slows hair growth came as a spin-off of a search for an anticancer drug.

In the past few years, however, researchers have begun to tease out the molecular signals that cause hair to grow and fall out. Unraveling such signaling, they hope, will lead to new ways of addressing the needs of people with either too little hair or too much.

"Hair disorders aren't necessarily important from a life-or-death situation ... but we are defined by how we look, in terms of gender and of youth," says Ricardo Azziz, a specialist in hair disorders at the University of Alabama at Birmingham. "Our job is not only to make people survive but to give them a better quality of life."

Whether it's the light, downy hair on a woman's arm, the short, curly hairs on a man's chest, or the longer hairs on either gender's head, each hair grows from a tiny, cell-lined skin indentation called a follicle. By adulthood, the skin hosts all of the follicles it ever will have naturally.

A hair follicle consists of three concentric cylinders. The central cylinder, the hair fiber, is created by the rapid growth and death of cells at the follicle base, which make proteins such as keratin (SN: 8/25/01, p. 124). The outermost cylinder is known as the outer root sheath, a structure that separates the hair follicle from the surrounding skin. The middle cylinder, the inner root sheath, shapes and guides the hair as it grows outward. A person's hair will be straight if this middle cylinder is round and will be curly if the cylinder is flattened.

An eyebrow hair grows slowly and falls out after just a couple of months. In contrast, a hair on a person's head can extend many feet long because the follicles there stay in the growth stage for 6 to 10 years.

One theory has it that such long-term growth cycles operate on an internal clock that's independent of the seasons or temperature. Some scientists suggest the timer is set by the dermal papilla, a structure containing dividing cells at the base of the hair follicle. Others argue that the clock controlling hair growth is part of a bulge lying just to the side of the hair follicle. Still others doubt whether this timer exists at all.

Over the past 50 years, scientists have pieced together the basic steps in the hair cycle anywhere on the body. The initial step, anagen, provides the active growth of the hair fiber, during which cells at the base of the follicle rapidly divide. In catagen, the hair follicle stops producing the fiber and regresses, shrinking dramatically. Telogen is a resting stage.

The process of shedding the hair is also a distinct stage, argue Kurt S. Stenn of the Skin Biology Research Center at Johnson and Johnson in Skillman, N.J., and some of his colleagues. They call it exogen. Stenn notes that "shedding is probably the most important aspect of hair growth from a patient's view." But other scientists aren't convinced that hair loss requires a unique step; most hold that hair falls out during telogen.

Normally, the hair cycle continues throughout a person's life. In most people not showing hair loss, about 90 percent of the hair follicles on the head are in anagen at any given time. Anagen can last for a few weeks to a decade, catagen for 14 to 21 days, and telogen for 1 to 3 months.

Even bald people have hairs on their scalp, but those hairs are unusually fine and short. Furthermore, the proportion of hair follicles in the regression and resting stages increases. Androgens, or male sex hormones, are behind much of such hair loss in men.

Unusual hair loss or growth may be a sign that something else is wrong. Loss of hair may signal thyroid disorders or lupus, an autoimmune disease. About half of women who have abnormal facial hair have higher-than-normal concentrations of androgens in their blood, a condition that may signal reproductive disorders like polycystic ovary syndrome (SN: 7/8/00, p. 31).