The antibiotic, actinomycin D, prevents the normal implantation of blastocysts (Psychoyos, 1967), and delays the development of the decidual cell reaction (Finn & Martin, 1972), probably due to an effect on RNA synthesis. The first stage of implantation, i.e. increased vascular permeability, as shown by the extravasation of the dye Pontamine Sky Blue, takes place normally, and the growth and development of the blastocyst continues. The endometrial epithelium, however, does not degenerate and decidualization of the stromal cells fails to occur at the normal time, with the result that the blastocyst is surrounded by an intact epithelium (Finn & Bredl, 1973).
The object of the present work was to investigate, with the electron microscope, the effect of the drug on the initial attachment of the trophoblast to the uterine epithelium in pregnant mice (the attachment reaction; Nilsson, 1966), and on the hormone-induced closure of the uterine lumen in ovariectomized mice.
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