Reproduction Abstracts

Introduction: Progesterone (P) has an unusual role in didelphid marsupials in that pheromonally induced levels of pro-estrous P trigger ovulation. Work in our lab reveals that serum P levels are significantly higher on pregnancy day 3 in the didelphid, Monodelphis domestica, plummet to extremely low levels during pregnancy day 5, and return to day-3 levels on day 9, before gradually declining as pregnancy advances. Because this precipitous decline coincides with the highest levels of embryonic mortality during the 14-day gestation period in M. domestica, we investigated the possibility that these unusual events may be related.

Methods: We compared uterine histological samples from days 1, 3, 5, 7, 9, 11 and 13 of pregnancy as well as samples from non-pregnant females. To understand the pattern of progesterone receptor (PR) expression in the uterus during pregnancy, we used HRP-mediated immunoassay of histological samples from these pregnancy stages.

Results and discussion: Our results indicate that P levels fluctuated in a manner coincident with that of thickness of the endometrial epithelium, both parameters being highest on days 5 and 9. Additionally, on day 9, the uterine stroma as well as the uterine glands in them were at their thickest. PR was clearly detectable in the cytoplasm and stroma of uterine gland cells at all pregnancy stages, declining gradually as pregnancy progressed, with one Xion: day 5. PR expression was extremely faint, if at all detectable, at this pregnancy stage. Low PR expression would thus seem to exacerbate the effects of low P on day 5 of pregnancy, suggesting that low or absent progesterone signaling may have adverse effects on embryonic survival.