Summary. Early embryo development and implantation were arrested in ferrets passively immunized with a mouse monoclonal anti-progesterone antibody injected intraperitoneally at 72 and 96 h post coitum (p.c.) or at 72 h p.c. only. In control ferrets injected with mouse serum or 0·9% NaCl, implantation sites were found in all mated females; autopsies were carried out at Day 14 p.c. A total of 34 unimplanted embryos were recovered from the reproductive tract of antibody-treated ferrets and none of these had progressed to the blastocyst stage.
When ferrets were treated with antibody at 72 h p.c. and autopsies were carried out at Day 6p.c., only 1 of 29 embryos recovered had progressed beyond the 4-cell stage in 4 females. In 4 control animals most embryos recovered at Day 6 were at the morula (32%) or blastocyst (28%) stage. Embryos from ferrets treated with antibody were therefore developmentally arrested when recovered 72 h after antibody administration.
Plasma progesterone concentrations were ∼ 6-fold higher in antibody-treated ferrets with unimplanted embryos (711 ± 132 nmol/l; 223 ng/ml) compared with control pregnant females (102 ± 4 nmol/l; 32 ng/ml) at Day 14 p.c. The results are consistent with the hypothesis that the normal course of pregnancy is arrested as a result of antibody binding of progesterone in the circulation, presumably causing a decrease in the amount of progesterone available to target cell receptors, and that heterologous anti-progesterone antibody blocks normal cleavage and embryonic development at an early stage before cavitation.