Androgenic alopecia develops when the hair follicle (the place under the skin where hair grows from) experiences a reduction in size, as well as a time reduction in the active growth phase. this translates into a simple fact: more and more of the hair follicles will spend time in the resting state where hair is shed once the state is completed. Fortunately, androgenic alopecia does not develop in all hair follicles at the same time. This is why some part of the scalp seems to be losing more hair than the other.
What causes androgenic alopecia? As boys grow up, their hair follicles are exposed to androgen (any hormone that controls the appearance and development of masculine characteristics) that their body has synthesized. Once hair follicles are exposed to androgen, they become "androgen sensitive" and androgenic alopecia can develop latter in time. Stopping the hair follicles from interacting with androgens seems to prohibit any chance of androgenic alopecia development.
To put it in a more detailed explanation there are two types of androgen that need to be considered: testosterone and DHT (dihydrotestosterone). An enzyme called 5-alpha-reductase converts testosterone to DHT. Once DHT is formed, it has the ability to bind to any androgen receptor in hair follicles. This binding reduces follicle activity, shrinks the hair follicle which in turn slows the hair growth cycle, reduces the diameter of the hair shaft and over times hair growth ceases.
For females, the mechanism of the condition is similar to that of the males. the source of the androgen is the combined activity of the ovaries and the adrenal glands. Hair loss in women can many times be directly related to pregnancy or menopause and their effects.
Why do some people seem to have more extensive hair loss than others? recent research suggest that the answer has something to do with the differences in the concentration of 5-alpha-reductase in skin and hair follicles of people who have androgenic alopecia. It is possible that some people just produce more of this enzyme, which in turn converts more testosterone to DHT, which will result in greater hair loss. Also, some people just develop greater sensitivity of androgen receptors in their hair follicles. This means that a certain concentration in others with less sensitive androgen receptors in the follicles.