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Each hair on your head is made beneath the surface of your skin in a little bulbous structure called a follicle. There, a clump of cells called the papilla, at the base of the follicle, produces the keratinous cells that become a strand of hair. The papillae get good supplies of food and oxygen, since they are well furnished with blood vessels, on which the growth and health of every hair depends. When, for any reason, circulation to your scalp is decreased or interfered with, the papillae get fewer nutrients and less oxygen than they need, and your hair suffers. The function of a follicle is to produce keratin, just as your pancreas produces insulin or your stomach hydrochloric acid. The follicle also contains an oil gland, which produces oil to coat each hair and to protect it from water loss. How efficiently and how well it does this depends on a number of things such as the level of androgenic and estrogenic hormones in your system, your genetic inheritance, and your general health.

You are born with more than 90,000 follicles. This number doesn’t change. If the amount of hair on your head changes, it is because some or most of these follicles are not working properly or have shut down, not because they disappear or because you don’t have enough.

Each strand of hair, or hair shaft, can be divided into three basic layers: the outside, which is called the cuticle; the medulla, at the center; and the cortex, made up of complicated amino-acid chains, in between. The cuticle serves as your hair’s protective coating: It guards against excessive evaporation of water (just as the stratum corneum does for your skin). It is made up of a transparent, hard keratin formation that is itself layered.

These layers overlap, like the tiles on a roof or fish scales. When they lie flat and smooth against the hair shaft, the hair shaft refracts light beautifully and your hair looks shiny. When they are peeling or damaged or raised, each hair doesn’t catch the light, so your hair lacks sheen and looks flat and dull. The cuticle provides 35 percent of your hair’s elastic strength.

The threadlike cortex, just beneath the cuticle, contains the pigment granules, which give your hair its color. The cortex is softer than the cuticle, yet it provides 65 percent of the hair’s elastic strength. It is also the thickest part of the hair. If the amino-acid chains that make up the cortex break up as a result of too harsh treatment from hair dyes, dryers, highly alkaline shampoos, or overprocessing, then you end up with weak and brittle hair that splits easily and breaks off. The most common manifestation of poor cortex condition is the familiar split ends.

The hair shaft’s innermost layer, the medulla, is made up of very soft keratin, and in many people there is even a hollow center. It appears to transport nutrients and gases to the other layers of the hair and may be the means by which your hair is so rapidly affected by changes in your body’s condition. But as yet not a great deal is understood about the biological functions of the medulla.

The Three-Stage Cycle of Growth

Hair follicles are the most efficient metabolizers of any organs in the body. This is what makes hair growth possible. They and the hairs they produce function on a three-part growth cycle that lasts from two to seven years. It is important to understand this growth cycle, because understanding it can dispel many of the fears women have that something is wrong when they look at their hairbrush and discover a number of hairs in it. Hair loss is continuous and is a normal part of the cycle. Without it there would be no new hair growth.

During the first part of a hair’s growth cycle—called the anagen phase —the papilla proliferates keratin at a rapid rate as the follicle expands and imbeds itself deeply in the vascular scalp to provide the oxygen and nourishment needed for growth. During this anagen phase, which lasts between two and six years (depending on your genetic makeup, general health, and the hormone balance in your body), your hair continues to grow from the follicle very much as toothpaste is squeezed out of a tube. The anagen phase is longer when you are young than at the age of fifty or sixty, but no matter what your age, eventually it has to come to an end to make ready for the next phase: the transitional catagen stage, which lasts only a few weeks. During catagen, the follicle’s metabolism slows down, the follicle contracts, and the papilla’s production of keratin stops. This is not a sign that something has gone wrong but, rather, that the growth of that particular hair has run its course.

It is ready to be shed, so soon it enters the last, or telogen, phase of the cycle. Now the follicle rests in its contracted state—rather like an animal hibernating—until, in about three months, the hair it contains is physically dislodged from it by normal activity such as combing or washing. The loss of this hair triggers the follicle to enlarge again, and it heads back into the anagen phase, where it produces yet another hair. And so the cycle continues throughout your life.

At any one time, about 85 percent of your hairs will be in the anagen phase and the rest in either telogen or catagen. Luckily, each hair begins life separately, at a different time from the others, or one could end up bald for three months every two to six years. As it is, your hair tends to be shed relatively rapidly in the autumn as more of the follicles head into the telogen stage, and to grow rapidly in the summer.

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