The binding of progesterone in different parts of the rabbit uterus during implantation.

Progesterone receptors, both nuclear and cytosolic, were determined in the embryonic and inter-embryonic segments of the rabbit uterus at 6, 7 and 8 day post-coitum. At day 6 postcoitum a higher concentration of nuclear receptor in the embryonic segment was observed compared with that in the inter-embryonic segment. A reverse situation was observed in the case of cytoplasmic receptors. On the 7th day post-coitum, no significant alteration in the concentration of either kind of the receptors was observed. However, on day 8, a higher concentration of both nuclear and cytosolic receptors at the embryonic site was observed compared to that in inter-embryonic segment. Since receptors are influenced only in the immediate vicinity of the blastocyst, it can be suggested that the blastocyst plays a role in the induction of its own implantation. Further, at day 8 increase in receptor concentration at the embryonic site may be related to the presence of decidual tissue at this site.