The rat as an animal model for endometriosis to examine recurrence of ectopic endometrial tissue after regression

The effects of pregnancy and ovariectomy on the growth of endometrial implants were examined in rats with experimentally induced endometriosis. Ectopic implants regressed completely after ovariectomy and we were unable to detect any viable endometrial cells in histological examination of the implant sites. Administration of 17β-estradiol cypionate to ovariectomized rats with regressed endometrial implants led to recurrence of the regressed implants. Animals with experimentally induced endometriosis were fertile and the number of embryos on day 10 of pregnancy was not different from the control group. Ectopic implants which regressed during pregnancy recurred 1 month after parturition. It is concluded that endometrial cells may survive at the implanted site even after apparent complete morphological regression, which has the potential to grow into an implant.