Development of an animal experimental model to study the effects of levonorgestrel on the human endometrium

This study was designed to develop an animal model to test the response of endometrium to local progestin delivery. Proliferative human endometrium was subcutaneously grafted in two groups of SCID mice that received, 2 days before, a subcutaneous estradiol (E-2) pellet and, for half of them, an additional implant of levonorgestrel (LNG). Mice were sacrificed 1, 2, 3 or 4 weeks after endometrial implantation and grafts were histologically analysed. Proliferation, steroid hormone receptors, blood vessels and stromal decidualization in both groups (E-2 and LNG) were immunohistologically evaluated and compared with proliferative endometrium and endometrium from women with an LNG intrauterine device. Grafts presented normal morphological endometrial characteristics. The expression of progesterone receptors was significantly decreased in glands and stroma of the LNG group as compared with the E-2 group at all times. A significant decrease was also observed in the stromal expression of estrogen receptor-alpha in the LNG group. At 4 weeks, the mean cross-sectional area of vessels was significantly higher after LNG treatment. These morphological and immunohistochemical characteristics are similar to those observed in women treated with local LNG. This mouse model might facilitate further investigations needed to understand the mechanisms responsible for the breakthrough bleeding frequently observed in progestin users.