Each hair on our body grows from a hair follicle, a tiny, saclike hole in our skin. At the bottom of each follicle is a cluster of special cells that reproduce to make new hair cells. The new cells that are produced are added on at the root of the hair, causing the hair to grow longer. The living tissue that makes our hair grow is hidden inside the hair follicle. The shaft, the part of a hair that we see, is made of cells that aren't living anymore. That's important to know when we are messing with coloring or perming or straightening our hair. If we cut ourselves, our skin can heal, since it's living tissue. If we damage our hair, it can't heal. We just have to do what little we can to repair the damage or cut the damaged hair off and wait for more hair to grow back.
Each hair shaft is made up of two or three layers: the cuticle, the cortex, and sometimes the medulla. The cuticle is the outermost layer. Made of flattened cells that overlap like the tiles on a terra-cotta roof, the cuticle protects the inside of the hair shaft from damage.
To feel the cuticle, just pinch a single long hair between our fingers starting up near the root. Pull the hair between our fingers and feel how slick and smooth it is. As we move from root to tip, we're running our fingers in the same direction as the cuticle layers. Start at the tip of the hair. In this direction, the hair may feel rougher; it may squeak as it passes between our fingers. We're running our fingers against the grain, and we're bumping into the edges of all those flattened cuticle cells.
It's handy to know how different conditions affect this protective layer on the outside of each hair. Chemists talk about solutions that are acidic (like vinegar or lemon juice) and ones that are alkaline (like a mixture of water and baking soda). In an acid solution, the cuticle cells shrink and harden. In an alkaline solution, the cuticle cells swell up and soften.
Just to confirm the response of the cuticle to acidic and alkaline solutions, try soaking one strand of hair in water and lemon juice and another in water and baking soda and rinse. When they dried, the hair from the lemon juice bath felt smoother and looked shinier. (Another triumph for scientific understanding.)
Underneath the cuticle is the cortex, which is made up of long proteins that twist like the curly cord on a telephone. Try stretching a hair and you'll find that it's elastic -- it stretches before it breaks. When we stretch a hair, we are straightening the coiled proteins in the cortex. When we release the hair, the proteins coil up again. The pigments that give our hair its natural color are tucked among these protein strands and protected from the elements by the translucent layer of cuticle cells.
When we get split ends, we're seeing the cortex at its worst. We've worn away the protective cuticle on the tips of our hairs with harsh treatment like hard brushing or too much sun and water. Without the cuticle, the fibers of the cortex fray like the strands of a rope. Since the cortex can't heal itself, the only way to get rid of split ends is to cut them off.
In the center of some hairs is the medulla, a soft, spongy mass of tissue. Coarse hair generally has this layer, while fine hair usually doesn't. The presence or absence of a medulla doesn't have much to do with how our hair behaves when we wash or color or curl it, however, so we don't have to worry about it.
To fill the gaps between the protective cuticle cells and to keep our hair shiny and flexible, glands adjacent to the hair follicle produce a kind of natural hair conditioner called sebum. Unfortunately, that sebum, which is oil, also makes dirt stick to your hair. When we shampoo our hair, you wash away this protective oil and the dirt that clings to it.
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