Measurement of ploidy and cell proliferation in the rodent liver

In most investigations of cell proliferation in vivo, the population under study consists of mononuclear diploid cells that undergo replication via normal complete division cycles. Because the phenomena associated with the cell cycle are sequential, only one is normally measured and it is usually adequate to quantify the proliferative activity in one of two ways. The first involves labeling the cells undergoing semi-conservative DNA synthesis with a radioactive DNA precursor, preparing autoradiographs of histological sections, and counting labeled nuclei. The other commonly studied parameter of cell proliferation is mitotic activity. The livers of rats and mice, unlike those of other mammals, consist mainly of hepatocytes that contain two classes of cell with respect to nuclei and several ploidy classes. These classes of hepatocytes arise as the result of modified cell division cycles. The peculiar cytological composition of the rodent liver has, until recently, caused difficulties in the measurement and interpretation of cell ploidy and cell proliferation by the above methods. Flow cytometry and fluorescence-activated cell sorting used in conjunction with quantitative fluorescent stains for DNA and fluorescently labeled antibodies to bromodeoxyuridine have permitted the rapid and precise quantification of cell proliferative activity in the rodent liver. Studies using these techniques have revealed that proliferative activity of hepatocytes may occur in different subpopulations of cells depending on the kind of toxicological injury inflicted on the animal.