Differential effects of 5α-norethisterone on the histomorphology of the oviduct and uterus of the pregnant rabbit

In this study, the histomorphological changes induced by 5α-norethisterone (5α-NET), a reduced metabolite of the contragestational postcoital agent norethisterone, in the oviduct and the uterus of the pregnant rabbit were determined. Adult fertilized rabbits were treated daily with 5α-NET (1.0, 1.5, 2.5, and 5.0 mg/kg/day) during 7 consecutive days, starting from the first day after coitus. Twenty-four hours after the last administration, the histological analysis of the oviduct and the uterus was performed. It was observed that in the infundibulum-ampullae region as well as in the isthmus of the oviduct, the number of nonsecretory cells (PAS-negative) were decreased, whereas the number of secretory cells (PAS-positive) were increased significantly after 5α-NET administration. The proportion of glandular tissue in the uterus markedly diminished in relation to that of the stromal tissue. This indicates an inhibition of the endometrial transformation observed during normal pregnancy. Interestingly, the highest doses of 5α-NET (2.5 and 5.0 mg/kg/day) induced necrosis in the uterus but not in the oviduct. These results suggest that the molecular antiprogestational effects previously observed after 5α-NET administration are also related to changes in the histomorphology of both the oviduct and the uterus of the pregnant rabbit.