Pseudopregnancy in the Rabbit after Stimulation by Human Chorionic Gonadotropin

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Abstract
The development of HCG-stimulated pseudopregnancy in the rabbit was studied with histological, electronmicroscopic and biochemical methods. This included the fate of the secretion unfertilized ova on the one hand and an analysis of the pseudopregnant endometrial secretion on the other. The unfertilized ova degenerate during pseudopregnancy, over half of these ova can be found in the uterus between the 4th and the 11th day after injection of HCG, the others are probably aborted. Endometrial secretion appears much sooner during the HCG-induced pseudopregnancy than in a normal pregnancy. The picture of maximal secretory capacity is maintained in the pseudopregnant cavum epithelium at least as long as in a normal pre-implantation pregnancy, i.e. up to the 7th day post coitum. The 11-day endometrium is no longer secretory active.During the secretory peak the pseudopregnant cavum epithelium exhibits a great number of apical protrusions, just as in normal pregnancy. With the electron microscope it was possible to demonstrate these protrusions as cell compartments rich in cytoplasm. The development of the pseudopregnant endometrial protein secretion is not identical with the normal one.The present results of differences between normal pregnancy and pseudopregnancy after stimulation with chorion gonadotropin provide further indications that the still unimplanted rabbit blastocyst must connect itself to the maternal regulating circuits before implantation. This is to adapt the intra-uterine environment to the growth and developmental requirements of the embryo and in order to synchronize the former with the embryonic system. The reactions of the external developmental factors do not proceed fully autonomously, but must to some extent be triggered off by the embryo itself.

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