People grow hair over quite large areas of their body. The most obvious point is the head, but with varying degrees of thickness, it can pop out almost everywhere. Cultures vary as to hair on the head. Some find facial hair attractive and people make a feature of it. In men, conventions vary as to beards and moustaches. Some shave with enthusiasm, others have spectacular beards. On the scalp in both sexes, everything is possible from complete shaving to shoulder length (and beyond). Others cover up hair with turbans and hijabs. Religion and different standards of modesty prevail. Wherever people live, hair is an integral part of their identity and deeply woven into their local cultures. But all this is under control. The real question is the biology. What makes hair grow and why does it stop?
Let us start with a simple statement. Each hair grows out of a follicle and, on the average human head, there are 100,000 follicles. That is a lot of roots and helps to give you a measure of seriousness when you hear that between 50 and 100 hairs randomly fall out every day. When everything is working well, once a hair drops, a new one begins to grow in its place. Thus, there is a continuous cycle of growth, rest and then drop. This takes between two and three years per hair so, at any one time, about 10% of the scalp is at rest and the rest is growing. Why have hair at all? It is mainly protective, shielding the skin from the sun and offering some cushioning against blows and cuts. It also helps us to feel our surroundings. Inside the follicle, the cells regenerate to maintain growth. If anything interferes with this regeneration and the hair loss increases to 150 or more hairs per day, the results will slowly appear. In women, the loss continues to be random, resulting in a general thinning of the hair. In men, a characteristic pattern of baldness tends to emerge.
Each follicle is fed by a system of capillaries that supplies nutrients including amino acids, minerals and vitamins. If the arteries in the neck and through the head are in good shape, the blood will bring the right mix to sustain growth. The key to growth are the androgens, hormones that prompt sexual development. Testosterone is the dominant hormone in in men. In some, there is an inherited trait to converted testosterone into dihydrotestosterone (DHT). DHT causes hair loss. In women, dehydro-epiandrosterone (DHEA) and the smaller quantities of testosterone can be converted into DHT. Propecia prevents the conversion of testosterone into DHT and so allows each follicle to continue growing normally. So long as people begin the treatment early enough, further hair loss will be prevented and some follicles will begin growing again. However, conventional wisdom says that propecia is for men only. Women, it seems, must find other remedies.