Quantitative histological and histochemical studies on the occurrence and stages of controlled cell death (apoptosis) during regression of rat liver hyperplasia.

Hyperplasia of the rat liver can be induced by cyproterone acetate (CPA). The fate of this hyperplasia after cessation of CPA treatment has been studied and the following findings were obtained: Liver DNA content decreased by about 25% within a few days after CPA withdrawal. In histological sections some hepatocytes showed degenerative changes. Among these, small membrane bounded bodies ("apoptotic bodies"; ABs) with or without chromatin were most numerous. Their incidence coincided with the phase of DNA elimination. Inflammatory reactions were not observed. Their small size and occurrence in clusters suggests that many of these ABs are formed by fragmentation of dying hepatocytes. Liver DNA was prelabelled with 3H-thymidine. Autoradiographic evaluation showed that many hepatocytes contained labelled nuclei, but unlabelled ABs. This finding strongly suggests that ABs, after the fragmentation stage, can be phagocytized by intact hepatocytes. About 80% of all ABs were found within hepatocytes. Extracellular ABs (early stage) contained no or very few active lysosomes. Intracellular ABs were sometimes surrounded by lysosomes, while others were in various stages of digestion. These observations suggest that the lysosomes of the phagocytizing hepatocytes degrade intracellular ABs, whereas intraapoptotic lysosomes seem to be inactive until the late stages of this degradation. Hepatocytes that did not replicate during CPA-induced liver growth appear to die off preferentially after CPA withdrawal. Retreatment with CPA greatly reduced the number of ABs within 4 h. Phenobarbital, another stimulus of liver growth, had the same effect. These findings suggest that the present type of cell death can be inhibited by growth stimuli and is therefore a controlled event serving to eliminate an excess of cells, rather than a manifestation of toxic injury to hepatocytes. The findings also suggest that this type of cell death and elimination is a rapid process completed within a few hours. It is concluded that cell death under the present experimental conditions probably occurs through apoptosis.