Marked growth and softening of the uterine portion of the cervix occur during the last third of the 115-day gestation period in the gilt. These changes in the cervix are temporally correlated with elevated blood levels of relaxin, estrogen, and progesterone. We recently demonstrated that relaxin plays a major role in promoting both the growth and softening of the cervix that occur in pregnant gilts. The roles of estrogen and progesterone in these cervical changes remain poorly understood. Accordingly, this study determined the influence of relaxin, estrogen, and progesterone, individually and in combination, on cervical growth and softening in gilts. Fifteen days after ovariectomy, six to nine nonpregnant, sexually mature gilts were assigned to one of the following eight treatment groups: ovariectomized controls, relaxin treated, estrogen treated, progesterone treated, estrogen plus relaxin treated, progesterone plus relaxin treated, estrogen plus progesterone treated, and progesterone plus estrogen plus relaxin treated. Treatment was given for 10 days, with doses of relaxin (0.5 mg, four times daily), estradiol benzoate (1 mg, twice daily), and progesterone (50 mg, twice daily) selected to provide blood levels resembling those between days 100-110 of gestation. The growth, softening, and histological characteristics of the cervices were determined. Treatment with relaxin significantly increased the growth and softening and altered the histological characteristics of the uterine portion of the cervix in the absence of steroid treatment. Estrogen treatment alone increased cervical growth, but when given in combination with relaxin, estrogen did not augment relaxin's ability to increase either cervical growth or softening. Progesterone alone had little or no effect on the growth or softening of the uterine portion of the cervix. Unexpectedly, when given in combination with relaxin, progesterone augmented markedly relaxin's effects on softening and alteration of the histological characteristics of the cervix. In conclusion, this study is consistent with recent reports that relaxin promotes both the growth and softening of the cervix in the pig. Additionally, this study provides evidence that estrogen and relaxin act independently to promote the growth of the cervix in gilts, whereas progesterone acts in concert with relaxin to promote softening of the cervix and a reduction in the organization and density of cervical collagen fibers. This study provides evidence that relaxin, estrogen, and progesterone all very likely play important roles in the modifications of the cervix that occur during late pregnancy in the gilt.
