Mitoses of existing corticotrophs contribute to their proliferation in therat pituitary during the late fetal period

We studied the proliferation of pituitary corticotrophs quantitatively by l abeling the proliferating cells with bromodeoxyuridine (BrdU) and carrying out immunocytochemistry for ACTH in rat fetuses at 19.5 days of gestation. In addition to labeling proliferating cells with a single injection of BrdU , we used double BrdU administrations at 9:00 and 19:00 for a more sensitiv e detection of proliferating cells. With this double administration, the nu mber of cells labeled with either BrdU or both BrdU and ACTH increased by 1 .75 and 2.3 times, respectively, compared with the single BrdU injection. T he labeled cells further increased when the sections were stained for the p roliferating cell nuclear antigen (PCNA) instead of BrdU. The number of cel ls labeled with PCNA or both PCNA and ACTH was 1.37 and 1.68 times that of the cells labeled with either BrdU or both BrdU and ACTH, respectively. The ratio of BrdU/ACTH-labeled cells or PCNA/ACTH-labeled cells to all cortico trophs was 13.6% and 24.3%, respectively, much higher than the ratios in fe tuses having a single BrdU injection (6.6%). These results indicate that th e mitosis of existing corticotrophs contributes greatly to their increase d uring the late fetal period.