BRDU LABELING OF FETUS OOCYTES IN THE RAT

To analyze dynamically the development of small follicles in the immat ure 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 tra ns-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 immunohistoch emistry 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 o f its mitosis. The primordial follicles occupied 100% of the BrdU-labe lled 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 we re unlabeled though only a few vesicular follicles showed BrdU incorpo ration even at the 4th week. This result suggests about 10% in a given cohort of primordial follicles were developed into the next growing f ollicles during this period.