Quantitation of apoptotic bodies in rat liver by in situ end labelling (ISEL): correlation with morphology

Nongenotoxic hepatocarcinogens have been proposed to inhibit hepatocyte apoptosis, preventing the removal of DNA-damaged cells. Conclusive proof of this hypothesis has been hindered by the lack of a marker or stain for apoptosis suitable for high throughput counting. A method is described for the detection of apoptotic bodies (AB) in paraffin sections of rat liver using an in situ end labeling (ISEL) technique that detects DNA damage. Results of this AB quantitation were compared with routine hematoxylin-eosin (H&E)-stained sections. The number of apoptotic bodies was enhanced by the withdrawal of the rodent nongenotoxic hepatocarcinogens and hepatomitogens nafenopin (naf) or cyproterone acetate (CPA) after 7 days of daily dosing. In rat livers 24-96 hr after cessation of dosing with naf, and in control livers, an AB index of approximately 0.1% of hepatocyte nuclei was seen when stained by H&E or ISEL. However, livers examined 48 hr after cessation of 7 days of daily dosing with CPA had an AB index of approximately 1% hepatocyte nuclei when stained with H&E, but only approximately 0.4% hepatocyte nuclei using the ISEL technique. Thus, CPA withdrawal from the hyperplastic liver generated a wave of apoptosis in contrast to naf withdrawal where little was seen up to 96 hr after withdrawal. The differing kinetics of CPA and naf clearance may explain this discrepancy. Less apoptosis was detected by the ISEL method following CPA withdrawal; this could arise if the stage of apoptosis labeled by ISEL is shorter than the morphologically recognizable stages (using H&E). The ISEL method for the evaluation of AB indices is useful in parallel with H&E, although more validation is required before it can be used routinely for the quantitation of AB in tissue sections.