Changes in the endometrium of mice after the induction of implantation by actinomycin D.

Actinomycin D can induce a small number of implantations in pregnant mice undergoing progestin-induced delayed implantation following ovariectomy. However, the response of the uterus to the blastocyst is considerably retarded compared with the response observed when implantation is precipitated by oestradiol. With the electron microscope the attachment reaction between the trophoblast and uterine epithelium is evident about 48 h after administration of the drug. However, the differentiation of the luminal surface of the epithelial cells in areas of uterus distant from a blastocyst (2nd stage of closure), which normally accompanies implantation, and can be induced by oestradiol in progesterone-treated animals, is not seen. Thus actinomycin D, although allowing implantation to proceed, does not completely mimic the actions oestradiol on the progesterone-treated uterus.