Effects of the embryo upon endometrial estrogen synthesis in the rabbit

To analyze for the possible embryonic effects upon endometrial estrogen synthesis (estrone, estradiol, estrone sulfate and estradiol sulfate), endometrial samples from each horn of unilaterally tubal-ligated estrous, pseudopregnant and pregnant rabbits were incubated with [3H] progesterone or [3H] androstenedione on Days 0, and 3 to 8 postcoitum. No endometrial estrogen synthesis was detected on Day 0, or on Days 3 to 5 of pseudopregnancy but low levels of estrogens (less than 0.7 pg/g wet wt) were detectable from androstenedione metabolism when individual samples were pooled (n=4 per day) for Days 6, 7 and 8 of pseudopregnancy. Low levels of estrogen synthesized from the pseudopregnant endometrium may represent a latent capability without the required stimulation for active synthesis. In pregnant animals, incubation of the gravid horn endometrium with either substrate produced similar trends of estrogen synthesis, in that estrogen levels increased at Days 4 to 5 (P less than 0.01), stabilized until Day 7 when levels decreased and then further increased on Day 8 (P less than 0.05). Estrogen synthesis by the endometrium of the nongravid horn was generally lower (P less than 0.05) than the gravid horn and stabilized or decreased as pregnancy progressed. Increased estrogen synthesis by the endometrium from pregnant rabbits and particularly from the gravid horn denotes a possible relationship between the developing embryo and endometrial estrogen synthesis.