Delayed Nidation in the Rat Induced by Autografts of the Hypophysis with a Case Report of “Asynchronous Implantation”

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Abstract
A delay in nidation of embryo lasting 3 to 18 days was obtained by autografting the anterior pituitary gland to the kidney in rats on the 2nd day of pregnancy. Ovo-implantation was induced in these rats at will; by the daily injection of 1 [mu]g of estrone. Implantation only rarely occurred, and it was followed immediately by death of the embryos in rats hypophysectomized on the 2nd day of pregnancy and treated daily with 4 mg of progesterone and 1 [mu]g of estrone starting the 9th day of pregnancy; it never occurred when treatment was not started until the 14th day. The daily use of 1 [mu]g of estrone in lieu of the progesterone-estrone treatment starting on the 9th day or 14th day of pregnancy failed to induce nidation in hypophysectomized untreated rats. A case of implantation of 1 embryo previous to estrone treatment and nidation of 8 additional embryos after the estrone in a rat bearing a pituitary autograft is reported. It is proposed that this phenomenon be called "asynchronous implantation" rather than the misleading terms superfetation and super-implantation.