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.