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.