MONOCLONAL-ANTIBODIES SPECIFIC FOR RAT RELAXIN - X - ENDOGENOUS RELAXIN INDUCES CHANGES IN THE HISTOLOGICAL CHARACTERISTICS OF THE RAT VAGINA DURING THE 2ND HALF OF PREGNANCY

This study employed morphometric analysis to evaluate changes in the h istological characteristics that accompany relaxin-induced growth and softening of the vagina during the second half of rat pregnancy. There were three treatment groups (N = 4/group). Five milligrams of a monoc lonal antibody for rat relaxin, designated MCA1, were injected iv dail y on days 12-21 of gestation to treatment group MCA1. Control groups r eceived either 5 mg of monoclonal antibody for fluorescein (MCAF; mono clonal antibody control) or 0.5 mi PBS (vehicle control). Vaginas were removed on day 22 of pregnancy, fixed in 10% neutral-buffered formali n, and embedded in paraffin. Tissue sections (5 mu m) were stained wit h Gomori's trichrome to visualize collagen, or orcein to visualize ela stin. Measurements were performed with a light microscope equipped wit h a video camera connected to a computer, Within the vaginal stroma, t he density of collagen fiber bundles was lower, the length of elastin fibers was shorter, and the cross-sectional area and wall thickness of arteries were greater in relaxin-replete control rats than in relaxin -deficient MCA1-treated rats. These relaxin-induced changes in the str oma appear to account, at least in part, for the hormone's softening e ffect on the vagina. Within the epithelium, there were approximately 2 -fold more basal and mucus-secreting cells in relaxin-replete control rats than in MCA1-treated rats. The relaxin-induced accumulation of ep ithelial cells appears to contribute to vaginal growth. We conclude th at relaxin plays a role in preparing the vagina as well as the cervix for rapid and safe delivery in pregnant rats.