Morphologic response of endometrium to a progesterone receptor antagonist, RU486, in monkeys

Fifteen castrate cynomolgus monkeys were assigned to one of two groups. All monkeys received estradiol (E2) capsules to mimic follicular phase E2 levels for development of a proliferative endometrium. In study I, three monkeys received RU486 on day 13 for 3 days; two others were given vehicle. On day 16, endometrium was removed. In study II, the remaining ten monkeys received progesterone capsules, beginning on day 12, to form a secretory endometrium. On day 29, six monkeys were given RU486 and four acted as controls. Endometrium was removed after 6, 18, and 32 hours. All tissue samples were prepared for light-microscopic evaluation. The results indicated that exogenous E2 alone stimulated intracellular stromal development; RU486 had weak progestational effect on proliferative endometrium, as evidenced by increase in the area occupied by glands; and RU486 had strong antiprogestational activity on secretory endometrium, illustrated by regression in the upper endometrial zones within 32 hours after a single injection of RU486.