Hormonal control of the cervix in pregnant gilts. IV. Relaxin promotes changes in the histological characteristics of the cervix that are associated with cervical softening during late pregnancy in gilts.

We recently described relaxin-induced changes that occur in both the physical properties and the biochemical composition of the uterine and vaginal portions of the cervix during the last third of gestation in the gilt. This study employed morphometric analysis to examine both the changes that occur in the histological characteristics of the uterine and vaginal portions of the cervix between days 80 and 110 of pregnancy in intact gilts and the effects of relaxin on these changes in ovariectomized gilts given progesterone to maintain pregnancy. There were four treatment groups: intact day 80 control gilts, sham-ovariectomized day 110 control gilts, ovariectomized progesterone-treated day 110 gilts, and ovariectomized progesterone- plus relaxin-treated day 110 gilts. The histological characteristics of the uterine portion of cervices obtained from intact controls on day 110 differed markedly from those of intact controls on day 80; there was a reduction in the density and organization of collagen fiber bundles, a reduction in the density of smooth muscle fiber bundles, and an increase in amorphous ground substance. The histological characteristics of cervices removed on day 110 from ovariectomized gilts given progesterone only did not differ from those of day 80 controls. After replacement therapy with progesterone plus relaxin, however, the histological characteristics of the cervix on day 110 did not differ from those in intact day 110 controls. Additionally, this study demonstrates that the relaxin-dependent changes are more dramatic in the uterine than in the vaginal portion of the cervix. These findings are consistent with and extend our earlier findings that relaxin-dependent changes in the physical properties and biochemical composition of the uterine portion of the cervix far exceed those in the vaginal portion of the cervix during late pregnancy in the gilt. We conclude that the relaxin-dependent changes in the histological characteristics of the cervix described in this report may contribute at least in part to the cervical softening that occurs during late pregnancy in gilts.