|MadSci Network: Medicine|
I'm not really sure what you mean by "the bottom layer" of your friend's hair, but I know why she has really thick hair. Every hair has a follicle that is embedded in the scalp. You can think of it like a very tiny flower pot, and each hair is like a flower that grows out of the pot. People have different DENSITIES of hair follicles on their head. For example, some have more hair follicles (and therefore more hairs) per square centimeter than other people, ands therefore their hair is called "thicker". Now, everyone has hairs that fall out all the time. It's just that we have thousands and thousands of hairs, so losing only a few per day is not really noticeable. Each hair follicle is programmed to grow a hair of a certain length (say 10 centimeters). Once the hair reaches that length, it falls out and a new hair starts growing. In this way new hairs keep replacing the ones that fall out. However, if there is damage to the follicle, then a new hair will not replace the one that falls out, and baldness is the result. The other thing I should mention is that different people's hair follicles are programmed to grow hair of different lengths. For example, even if I never cut my hair (which I tried doing once) it would never get longer than my shoulder blades before falling out. Some people, however, can grow hair that goes down to their ankles (but this is rare). The most interesting thing about hair for me is that there is such a great diversity of it. So many colors, styles, and thicknesses. Then again, there's nothing wrong with baldness...just look at Michael Jordan.
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