RELAXIN INCREASES THE ACCUMULATION OF NEW EPITHELIAL AND STROMAL CELLS IN THE RAT CERVIX DURING THE 2ND HALF OF PREGNANCY

Both cervical and vaginal growth are relaxin dependent during rat preg nancy. We recently reported a relaxin-dependent 1.5-fold increase in c ervical and vaginal DNA content from midpregnancy until term. This fin ding indicated that relaxin probably promotes cervical and vaginal gro wth at least in part by promoting cellular proliferation. The objectiv e of this study was to identify and quantify cells in the cervix and v agina that proliferate during the second half of rat pregnancy in resp onse to relaxin. Primiparous pregnant rats were ovariectomized or sham ovariectomized (group C; n = 8) on day 9 of pregnancy (D9). Ovariecto mized rats mere then treated with physiological doses of progesterone plus estrogen (n = 7) or progesterone, estrogen, and porcine relaxin ( n = 7). Cellular proliferation was determined by continuously administ ering a low dose of 5-bromo-2'-deoxyuridine (BrdU) via miniature osmot ic pump from D9-D22. On D22, cervices and vaginas were collected, fixe d in formalin, paraffin embedded, and serially sectioned (4 mu m) Adja cent serial sections were either immunostained for BrdU to assess cell proliferation or stained with hematoxylin to determine total cell num ber. Cell proliferation was evaluated by counting BrdU-positive nuclei and total nuclei in the same area on adjacent sections. Cell counts w ere determined using computerized digital morphometric analysis at X57 5. In control rats, nearly 75% of the epithelial cells and 55% of the stromal cells within the cenix at term had proliferated during the sec ond half of pregnancy. The accumulation of approximately half of the n ew cells was relaxin dependent. Within the cervical stroma, relaxin in creased the accumulation of cells associated with blood vessels and al so the number of isolated cells (probably fibroblasts). Relaxin did no t appear to affect smooth muscle cell proliferation in the cenix. In c ontrast to the cervix, a minority of vaginal epithelial cells (45%) an d stromal cells (20%) proliferated during the second half of pregnancy . Although relaxin appeared to have a tendency to increase the accumul ation of new vaginal epithelial and stromal cells, morphometric analys is did not provide support for such an effect. In conclusion, this stu dy demonstrates that relaxin promotes a marked increase in the accumul ation of new epithelial cells and stromal cells within the cervix. The relaxin-induced increase in new epithelial and stromal cells probably contributes to relaxin's effects on growth and remodeling of the cerv ix that are required for rapid and safe delivery.