Uterine growth-promoting effects of relaxin: a morphometric and histological analysis

The hormone relaxin has been implicated in the regulation of several processes in the reproductive tract during pregnancy and parturition. This study investigated the uterine effects of relaxin in immature and mature ovariectomized, estrogen-primed rats using morphometric and histochemical analysis. Rats were sprayed at 30 or 70 days of age and given estrogen (5 micrograms) 7 days later. After a week, they received an injection of porcine relaxin (100 micrograms) and were killed 6 h later; controls received vehicle alone. Histological sections were obtained from 7 levels of each uterine horn, and the volumes of endometrium and myometrium were calculated by use of a Zeiss Videoplan Computer Image Analyzer. In immature animals, relaxin treatment doubled uterine weights during the treatment period, and cross sections from relaxin-treated animals exhibited significant increases in the areas of both the myometrium and endometrium, 150% and 130% respectively. Mature animals were less responsive to relaxin although they also exhibited significant increases in uterine weight (31%), myometrial volume (29%), and endometrial volume (22%). With the use of Masson's Trichrome stain for collagen, we observed that relaxin alters the connective tissue framework of both endometrium and myometrium; control uterine collagen appears highly organized and dense with compact collagen fibers, whereas the collagen of relaxin-treated uteri is loosely arranged and disorganized with widely separated collagen fibers. Relaxin-stimulated uteri exhibited significantly greater vascularization, as evidenced by the size of arteries and veins in the vascular region between the circular and longitudinal muscle layers. Increased vascularization and uterine blood flow may be one mechanism involved in relaxin's uterotropic effect and is being investigated further.(ABSTRACT TRUNCATED AT 250 WORDS)