Christopher Solomon in The New York Times:
The average human head contains around 100,000 hairs. Each is connected to a follicle, which can hold one to five hairs. “It’s basically its own organ,” Dr. Mostaghimi said of a scalp follicle. “It has its own stem cells. It regenerates.”
Typically, men’s hair loss occurs because of an increase in an enzyme in the scalp that converts testosterone to a more potent form, called dihydrotestosterone (or DHT), Dr. Mostaghimi said. The reasons that one man might have more DHT than another are not well understood, but it has a genetic component. When men have too much DHT in their scalp, the hormone initiates a complex process that leads to hair miniaturization, in which hairs and follicles begins to shrink. (This is why men frequently have finer hair or even peach fuzz where they are balding.) This hair loss occurs in a predictable sequence: first around the temples, then at the crown of the head, where increased levels and activity of the offending enzyme and its modified testosterone are found, Dr. Mostaghimi said. Hence the phrase “male-pattern baldness.”
More here.