Emily Ang, age 10, of Calgary, Alta., Canada, for her question:
WHAT IS HAIR MADE OF?
We can say that your hair is made mostly of keratin, tinted with melanin though this sounds rather uninteresting. The more detailed story of the hair and how it grows is very interesting and full of surprises. For example, you might never suspect that it is related to a porcupine's quills and the horn on the nose of a rhino.
The wonderful, waterproof skin that covers your entire body produces your hair. Here and there it grows shortish hairs, such as lashes and eyebrows. But the longest and loveliest hairs grow from skin on your head. Each hair grows from a clever little root embedded in the scalp. It keeps on growing until it reaches its full length. Then it falls out and the root sprouts a replacement.
The root is a pale blob of special cells. It is fed by a tiny blood vessel, from which it takes chemicals to build a tough, horny material called keratin, plus some brown coloring material called melanin. It builds these ingredients into boxy, little cells and stacks them together to make a fine string.
As the string grows longer, it pokes up through the skin and becomes a hair. Its cells are boxy shells, with no nerves to feel pain and no blood vessels to do any more growing of their own. The hair gets longer as the buried roots add more cells and push the old cells up from below.
To see the interesting cells in a hair, we must cut a cross section and enlarge the whole thing under a microscope. The cross section may be round, slightly flat or very flat. In any case, the outer edge is a layer of tiny overlapping cells that look somewhat like scales. This layer is called the hair's cuticle.
Under the cuticle is a thick layer of boxy cells, shaped liked cigars and packed very closely together. This is the hair's cortex and usually it is colored with melanin. When hair turns white, the melanin is lost.
In the center of our cross section there is a core of loosely packed cells, separated by tiny air spaces. This is called the medulla. It may or may not contain a trace of brown melanin.
Hairs that are flat tend to grow unevenly and come forth with waves or kinky curls. When the cross section is round, there are no natural curves and the hair is straight. All melanin is the same color. The only difference is that blond hair has just a trace, brown hair has more and black hair has lots of the same coloring material.