BRDU LABELING OF FETUS OOCYTES IN THE RAT

To analyze dynamically the development of small follicles in the immature rat, a cohort of follicles was set up by labelling a certain group of oocyte's nuclei in the embryonal stage. The pregnant rats were trans-abdominally injected with bromodeoxyuridine (BrdU) in the 17th day of pregnancy. The litters were sacrificed at 1, 2, 3 and 4 weeks after birth to localize BrdU in the ovaries and the livers by immunohistochemistry with anti-BrdU antibody. The BrdU staining was evident mainly in the nuclei of oocytes in primordial follicles through 1 to 4 weeks, while live cells were not stained after the 2nd week, indicating that the fetal oocyte escaped the dilution of BrdU uptaken due to arrest of its mitosis. The primordial follicles occupied 100% of the BrdU-labelled follicles at the lst week, 98.8% the 2nd week, 90.9% the 3rd week, and 88.5% the 4th week. However, the majority of antral follicles were unlabeled though only a few vesicular follicles showed BrdU incorporation even at the 4th week. This result suggests about 10% in a given cohort of primordial follicles were developed into the next growing follicles during this period.