Follicles at the back and sides of the head are far less sensitive to DHT, the hormone that shrinks follicles on top. They are on a different genetic setting, which is why a man can lose the crown entirely and keep a full band around the back for life.
Transcript
Look at almost any bald man, and you'll see the same thing. Gone on top, still there around the back and sides. That isn't styling. Follicles at the back are far less sensitive to the hormone that shrinks the ones on top. They're on a different genetic setting. That single fact is the reason hair transplants are possible at all. You're not adding hair. You're moving the hair that was never going to leave.
That single fact is what makes hair transplantation possible at all: the surgeon is not adding hair, they are moving hair that was never going to leave. It is also why a doctor looks at the pattern of loss before anything else — where hair is holding tells them as much as where it is going.