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.