Is estrogen required for implantation in the ferret?

A series of experiments was designed to further test the hypothesis that ferret corpora lutea synthesize and secrete estrogen between Days 6 and 8 of pregnancy, and that this estrogen is required to initiate implantation of blastocysts on Day 12. Corpora lutea, removed on Day 8 of pregnancy contained significant quantities of testosterone. Incubation of aliquots of the same luteal tissue for 4 h significantly elevated estrogen levels above those of nonincubated controls. Peripheral plasma estrogen levels exhibited a slight increase on Day 8 over those observed on Day 6 of pregnancy (7.9 +/- 3.4 vs. 4.1 +/- 1.1 pg/ml). However, continuous release of estradiol from Days 6-8 from two different sizes of Silastic capsules failed to induce implantation in ovariectomized-progesterone treated ferrets, whereas this same treatment was compatible with nidation in intact ferrets. Administration of the aromatase inhibitor, androsta-1,4,6-triene-3,17-dione (ATD) on Days 5-8 of pregnancy prevented implantation of blastocysts on Day 13. Simultaneous administration of estradiol and ATD failed to reverse the inhibitory effect of ATD. Results of this study confirm that luteal tissue of ferrets possesses at least one aromatizable androgen which is converted to estrogen under physiological conditions. However the data do not support the hypothesis that estrogen is required for implantation in the ferret.