The cause of male pattern baldness is no longer a mystery. Research has shown that when the male hormone - testosterone is exposed to 5-alpha-reductase, an enzyme in the hair follicle, it produces another hormone called dihydrotestosterone (DHT), which causes permanent hair loss.
This hormone (DHT) causes the hair follicle to shrink and grow a finer hair that is lighter in color, shorter and less deeply rooted than its predecessor. Most often, the hair in the balding region will continue to grow at an increasingly slower rate until hair growth ceases completely.

The pattern of progression, and the extent and speed of male pattern baldness, is largely dictated by genetics, hormones and age.
For men, this permanent type of baldness generally follows specific patterns. Even in the most extensive cases, a horseshoe shaped fringe of hair remains. This donor fringe is insensitive to the balding process and will last a lifetime. This is because this hair is genetically programmed in the root, or hair follicle, to be resistant to the effects of DHT.

The hair in this donor area provides the basis for surgical hair transplantation. These hairs, when transplanted into thinning or balding areas, will take root and grow, and continue to grow for the rest of a person's life.
This is also reffered to as the "principle of donor dominance". That is, the transplanted hair will grow in its new location as it would have in its original donor location.