<prompt> As menstruation ends, estrogen levels in the blood rise rapidly. What is the source of the estrogen?Corpus luteumDeveloping folliclesEndometriumStromal cells of the ovaries  <response>: In the nonpregnant female, the only significant source of estrogen is ovarian follicles or corpus luteum. Menstruation begins when the corpus luteum degenerates. Menstruation ends when developing follicles secrete estrogen sufficiently to raise circulating concentration to a level that stimulates regrowth of the endometrium 