Cellular proliferation and fibroblast growth factors in the corpus luteum during early pregnancy in ewes

To determine the relationship between cellular proliferation and the presence of FGF-1 and FGF-2 in the ovine corpus luteum (CL) during early pregnancy, ewes received an intravenous injection of bromodeoxyuridine (BrdU) 1 h before slaughter (n = 3/day) on day 12 after estrus (nonpregnant) or on days 12, 18, 24 or 30 after mating (pregnant). The labeling index (LI; number of BrdU-labeled nuclei expressed as a percentage of total nuclei) of each CL was determined by immunohistochemistry and subsequent image analysis. FGF-1 and FGF-2 were immunolocalized by using specific antibodies, and indirect immunoperoxidase detection. Moreover, FGF-2 was immunolocalized by using a primary antibody and fluorescein isothiocyanate (FITC)-labeled secondary antibody, and immunofluorescence was quantified by using an interactive laser cytometer and image analysis. Results demonstrated that the LI was similar for CL of nonpregnant and pregnant ewes on day 12 (4.27+/-0.23 vs 5.10+/-0.14%) and decreased (P < 0.05) from days 12-30 of pregnancy (2.73+/-0.08, 2.02+/-0.09 and 1.70+/-0.04% on days 18, 24 and 30, respectively). FGF-1 was present in the cytoplasm of large and a few small parenchymal luteal cells, and the distribution and intensity of staining was similar for nonpregnant and pregnant ewes on day 12 as well as across days of pregnancy. In contrast, FGF-2 immunoreactivity was present only in luteal nonparenchymal cells and interstitial areas and was greater (P < 0.05) for pregnant than nonpregnant CL on day 12 (2.34+/-0.12 vs 0.14+/-0.01%). Although FGF-2 immunoreactivity decreased (P < 0.01) from days 12-30 of pregnancy (0.70+/-0.04, 0.22+/-0.01 and 0.06+/-0.02% on days 18, 24, and 30, respectively), it was highly correlated (r = 0.99, P < 0.01) with luteal LT. We therefore suggest that FGF, and especially FGF-2, play a role in luteal cell proliferation or turnover during early pregnancy, and may thereby contribute to the maintenance of luteal function, which is critical for the successful establishment of pregnancy.