Free Never Mind the Bullocks, Here's the Science by Karl Kruszelnicki

follicles, you know when your hair brushes against something.
    The hair follicle is special—it’s one of the very few organs in an adult that regenerates in a cyclical fashion.
Colour of Hair
The colour of hair usually comes from a pigment called melanin. (Once again, as is always the case with the human body, everything is more complicated than first thought. There are actually different types of melanin: eumelanin is the most common and gives the shades from brown to black, and pheomelanin gives the yellow-blond and red colours.)
Black hair contains lots of melanin, while light brown and blond hair contains less. And the hair of people with albinism contains no melanin at all.
But red hair is special. Not only does it contain melanin, it also contains some iron. So ‘rusty’ is actually a scientifically correct nickname for someone with red hair.
    Shaft’s the Name
    Cutaway of the microscopic structure of a human hair shaft.

And, a diagram of a hair follicle.

Baldness
About 40% of men go bald.
In males, baldness is programmed by the male sex hormone, testosterone. The testosterone gets converted into dihydrotestosterone (DHT). In youth, DHT puts hair on males. But in later years (and we don’t know why), DHT has the completely opposite effect. It seems to shut down the follicles that have lots of DHT receptors. In fact, some men with very high levels of testosterone go bald at an early age. (Surprisingly, not only do some follicles have the receptors for DHT, they carry the enzyme that converts testosterone to DHT. This makes them ‘Suicide Follicles’, carrying the means of their own loss of function.)
A hair follicle has only a limited number of cycles of growth and rest before it stops working – probably around 25 or so. (Once again, we don’t know why.) The hair product company L’Oréal studied ten men over a 14-year period in the 1980s and 1990s. They shaved off the 200-300 hairs in just one single square centimetre on each man’s head, and counted how many of the hairs grew back, and how quickly they did so. They found that men who tended to go bald early had hair follicles with very short cycles of growth and rest. It seems to be a case of Live Fast and Die Young for some cases of baldness.
Getting back to testosterone, if a male is castrated at a very early age (i.e. before puberty), he never makes any large amounts of testosterone. So the castrated male, a eunuch, will never go bald.
    Hair 102
    Each hair follicle manufactures its hair shaft from the base (unlike plants, which do most of their growing at the tip). Hair follicles have a long growing period, followed by a short resting period.
    The follicle grows hair for up to five years in women, but only three years in men. In that time, a single hair can grow 75 cm in a woman, but only 45 cm in a man (roughly 10-15 mm per month). This is why women can grow their hair longer than men can.
    After the growing period comes to an end, the follicle takes a rest for about three months and shrivels up. The root of the hair is now very shallow, only about half a millimetre below the surface. At that stage, the hair shaft can be easily pulled out by a comb, or a strong wind, or by rubbing against something. At any given time, about 10% of the hair follicles are resting.
    The hair follicle then wakes up and starts growing a new hair. This new hair pushes out any old hair still stuck in the follicle.
    We still don’t know the working of the mechanism that stops these cycles from being synchronised in the hair follicles. But it’s good that they are not synchronised. If they were, all of our hair would fall out at the same time, instead of at the steady rate of 50-100 hairs each day.
    Hair Tangles—A Knotty Problem
    It was a French physicist who first took this new look at hair.
    He needed a problem for his students. In fact, this preliminary study of his was originally aimed at high school students, to get them interested in science. This is why it’s more