Displacement of receptivity for nidation in the rat by the progesterone antagonist RU 486: a scanning electron microscopy study.

In the rat, the period of receptivity for nidation (implantation window) is characterized by specific change of the apical surface of the luminal uterine epithelium, such as the presence of large ectoplasmic projections (pinopodes). Under the conditions of this study, fully developed pinopodes appeared during normal pseudopregnancy on day 5 only. When RU 486 (5 mg/kg s.c.) was given once on day 1, the chronological limits of the appearance of these projections were changed. Fully formed pinopodes were observed on days 6-8 instead of day 5, indicating a displacement and enlargement of the receptive period. This property of the progesterone antagonist could be of clinical interest if it could be used to enlarge the 'implantation window' in women after replacement of in-vitro fertilized eggs.