Early cellular changes and circular muscle contraction associated with the induction of decidualization by intrauterine oil in mice.

Intrauterine instillations of oil or saline distended the uterus in ovariectomized mice treated with progesterone + oestrogen to sensitize the uterus to a decidualizing stimulus. Saline does not induce decidualization, and therefore uterine distension per se is not the trigger to decidual induction. Oil induces decidualization, but does not involve gross damage to the epithelium, penetration of oil into the stroma or release of epithelial lipid into the stroma. Instillation (oil, saline or sham) induced a contraction of the circular muscles along the length of the uterus which closed the uterine lumen, expelled most of the oil and located the remainder primarily in the antimesometrial cleft of the lumen. Progesterone inhibited longitudinal muscle contraction and facilitated circular muscle contraction. These effects are discussed in relation to the spacing and implantation of blastocysts.