Quantitative Studies on Deciduoma Formation and Implantation in the Lactating Rat1

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Abstract
Deciduoma formation and the capacity for ovum implantation were used as an index of uterine sensitivity during the luteal phase following postpartum ovulation. This luteal phase, which was rigidly controlled, was initiated by suckling stimuli (2 or 8 young/litter) and/or cervical stimuli (mechanical or mating). In all groups, the magnitude of the deciduoma response was greatest following knife-scratch trauma of the endometrium on Day 4. The endometrium by Day 6 showed either a reduction or an absence of sensitivity. The increase in the size of the suckling litter to 8 young not only reduced the magnitude of the deciduoma response to the Day 4 trauma, but prolonged the state of uterine sensitivity as well, thereby resulting in a bimodal sensitivity curve. It was the second portion of this curve, however, that best correlated with the ability of blastocysts to induce decidua formation. Even with a controlled environment, delays in implantation from 2 to 13 days were noted with this degree of suckling stimulation. Castration of rats suckling 8 young was performed on Day 3 of pregnancy, and a ratio of 1 μg estrone/2000 μg progesterone was administered daily thereafter. Under these conditions all blastocysts initiated implantation on Day 4. These results suggest that individual variations exist in the temporal aspect of the mechanism (s) whereby ovarian estrogen secretion may be held to suboptimal levels as a result of intense suckling activity. Once the estrogen appears, it is believed to continue at levels less than those associated with the pseudopregnancy initiated by cervical stimuli. This may account for the prolonged state of uterine sensitivity. It is suggested that different methods for deciduoma induction may yield different sensitivity curves. (Endocrinology75: 883, 1964)