RU486-induced menses in cynomolgus monkeys: uniformity of endometrial sloughing

A progesterone (P) antagonist (RU486) was administered to cynomolgus monkeys in the midluteal phase (cycle day 21 to 24), alone or in combination with P. Vehicle only was administered to other monkeys that served as controls. On cycle day 25, hysterectomy was performed on all of the monkeys. The endometrium was studied histologically in fundal, medial, and isthmic regions. RU486 induced menses in all treated monkeys; but the amount of residual endometrium was slightly higher in primates that received RU486 plus P. The effect of RU 486 on the endometrium was surprisingly homogeneous throughout the uterus. The findings demonstrate that administration of RU486 in the luteal phase induced a nearly uniform and homogeneous sloughing of the endometrium. Whereas concurrent P treatment did not prevent induction of menstruation, the depth of endometrial shedding was less than with RU486 alone.