Abstract
Proliferating cell nuclear antigen (PCNA) was evaluated as a marker of cell proliferation in formalin-fixed rat liver tissue through a comparative study with the thymidine analogue 5-bromo-2'-deoxyuridine (BrdU). The comparison was conducted through the introduction of a dual immunohistochemical procedure that allows the simultaneous detection of the two antigens. The results of this study suggest that although statistically similar indexes for each can be achieved, what has been reported to be the "S-phase fraction" of PCNA-labeled nuclei is significantly different from the population of cells marked by BrdU. The data also suggest that the reason for this difference is that the "S-phase fraction" of PCNA-labeled nuclei is the population of cells in late G1- and early S-phases. BrdU, by comparison, is incorporated into cells only during DNA synthesis. Therefore, although BrdU and PCNA labeling techniques may both be effective for evaluating cell proliferation rates, it must be recognized that labeling indices derived from each are not entirely synonymous. The method presented here for the simultaneous labeling of PCNA and BrdU antigens may have utility in studies of cell cycle perturbations.