Vascular changes in the human endometrium following the administration of the progesterone antagonist RU 486

Eleven healthy women were assigned to one of two groups. They received 50 mg RU 486 orally per day either on cycle days 7 to 10 (preovulatory group n = 5) or on cycle days 20 to 23 (postovulatory group, n = 6). An endometrial biopsy was taken on the fourth day of the RU-treatment in the preovulatory group and on the second (n = 2) or fourth (n = 4) treatment day in the postovulatory group. Biopsies from 34 untreated women representing matched samples from early and mid preovulatory phase (n = 10) and mid and late postovulatory phase (n = 24) were used as control. The ultrastructure of the endometrial capillaries was investigated by morphometric methods. The administration of RU 486 during the preovulatory phase did not modify the vascular structure. However, when given in the postovulatory phase, necrosis occurred in the capillary endothelial cells with and without regressive changes of the adjacent stroma. The area and diameter of the capillary lumen and the area of the adventitia was smaller than in the control material (p less than 0.01). The result of the study suggests that RU 486, when administered in the postovulatory phase, directly affects the capillary vessels of the endometrium.