The hormonal control and morphology of blastocyst invasion in the mouse uterus.

When mature and immature uteri from ovariectomized mice were cultured in chemically defined media, blastocyst invasion occurred in the presence of progesterone but not in media containing only oestradiol. The invaded stromal tissue did not decidualize unless the uteri were taken from mice pretreated with progesterone and oestradiol. Fibrillar material was, however, concentrated in the stromal tissue adjacent to the invasive trophoblast. Neither progesterone nor oestradiol, had any ultrastructural effect on the endometrium of cultured uteri from ovariectomized mice, other than closure of uterine lumina in the presence of progesterone. Embryogenesis became abnormal, probably owing to the failure of formation of a suitable implantation chamber.