Effects of relaxin on the intrauterine distribution and antimesometrial positioning and orientation of rat blastocysts before implantation.

An i.v. infusion of porcine relaxin was administered to rats from the afternoon of Day 4 of pregnancy to the morning of Day 6, a time by which implantation has normally occurred. Implantation sites were irregularly distributed and confined to the cranial half of each uterine horn. Histological sections of uteri fixed by vascular perfusion with osmium tetroxide revealed that the blastocyst was no longer invariably positioned antimesometrially within the lumen and that embryonic disc orientation was often abnormal. A reduced decidual cell reaction was observed around several of the implanting blastocysts.