Effects of relaxin on the endometrial stroma. Studies in mice

The current study was designed to elucidate the effects of relaxin, administered systemically to mice in combination with estrogen and medroxyprogesterone, on the endometrial stroma, with use of structural, ultrastructural, and immunocytochemical methods. Female, sexually mature virgin mice were placed into 4 groups and treated with estrogen, estrogen + progestin, estrogen + relaxin, and estrogen + progestin + relaxin in sequence. The results obtained show that relaxin causes a striking increase in the protein-synthesizing machinery of endometrial stromal cells, leading them to resemble closely the decidualized cells of rodent pregnant uterus. Relaxin also induces in the same cells the expression of laminin, which is regarded as a reliable marker of decidual transformation of endometrial stromal cells in mice, and which is needed for trophoblast to adhere to and invade the endometrial stroma. Moreover, relaxin seems to synergize with the steroids in promoting a growth response of the endometrial stromal cells. The above findings allow for relaxin to be regarded as an agent capable of promoting decidual transformation of endometrial stromal cells and, possibly, to participate in the events that lead to blastocyst implantation.