They say a Prostaglandin 2 (PGD2) is more plentiful in the bald areas of the head, causing hair follicles to shrink.
Prostaglandin is a class of unsaturated fatty acids. They're involved in the contraction of smooth muscles as well as the control of inflammation and body temperature. Researchers tested mice too, and found the same Prostaglandin during their hair shedding phase.
So now, the doctors think if they can find a away to block PGD2 from binding to a receptor on the hair follicle, they could stop baldness in both men and women.