Objective
To determine whether preimplantation embryos are targets for relaxin secreted from the corpus luteum of the menstrual cycle. Design: Rhesus monkey oocytes obtained from females undergoing controlled ovarian hyperstimulation were inseminated, and the resulting embryos were cultured in medium with or without recombinant human relaxin (20 ng/mL) for 8 days. Setting: Research laboratory. Animal(s): Rhesus monkey. Intervention(s): Controlled ovarian stimulation to obtain oocytes for in vitro-produced embryos that were cultured with or without human recombinant relaxin. Main outcome measure(s): Rate of blastocyst development, percentage of blastocysts, and inner cell mass/trophectoderm cell ratio were measured on day 8 of culture. The presence of relaxin receptor (RXFP1) messenger RNA in eight-cell embryos was observed by array hybridization. Result(s): RXFP1 receptor expression was localized to the inner cell mass of blastocysts, as shown by immunohistochemistry. The percentage of embryos that developed to blastocyst and the inner cell mass/trophectoderm cell ratio was unchanged with relaxin supplementation; however, the relaxin-treated embryos developed into blastocysts significantly sooner than untreated embryos.