Progesterone signaling during pregnancy in the lab opossum, Monodelphis domestica.

To investigate subtle pregnancy-associated changes in the lab opossum, Monodelphis domestica, an induced ovulator, we compared pregnant with non-pregnant and pseudopregnant animals with regard to serum P4 levels and progesterone receptor (PR) expression. Using video-verified, time-mated lab opossums as sources of biological material, we compared ovaries, uteri and sera obtained on odd-numbered days of the 14.5-day pregnancy in this animal. Females that mated successfully but did not produce embryos were classified as pseudopregnant. P4 levels differed significantly between pregnant (N = 21) and either non-pregnant (N = 3) or pseudopregnant (N = 3) opossums, but not between the non-pregnant and pseudopregnant groups. A significant decline in serum P4 occurred between pregnancy days 3 and 5, coinciding with an elevated probability of pregnancy failure between days 5 and 9. PR was detected in the nuclei of uterine-gland epithelial cells on pregnancy days 5 and 7 as well as variably in the corpora lutea (CL) of animals on pregnancy days 3-11. PR expression in the CL suggests that P4 may be autostimulatory in lab opossums and that certain levels of this steroid are required during normal pregnancy. The significant day-3 drop in P4 may explain why pregnancy failure in this polyovular metatherian is likeliest to occur between days 5 and 9, an interval during which the extended period of blastocyst morphogenesis and expansion occurs. Taken together, these results suggest that P4 may have unrecognized signaling roles not only in pregnancy but perhaps embryonic development as well in the lab opossum.