Cytotoxicity phase of diethylnitrosamine-induced hepatic neoplasia in medaka.

Adult Oryzias latipes were exposed to 50 mg of diethylnitrosamine per liter of water for 5 wk and then transferred to clean water for an additional 15 wk. Response of the liver during the first 6 wk were analyzed by enzyme histochemistry and by high-resolution light and transmission electron microscopy. After 1 wk, cytotoxicity was apparent at the light microscopic level by piecemeal necrosis and phagocytosis apoptosis by adjacent hepatocytes and resident macrophages. Spongiosis hepatis and inflammation, found as early as wk 3, were not widespread until wk 6. Glycogen depletion and multifocal increases in gamma-glutamyl transpeptidase were found as early as 3 wk. At 5 wk, macrophage infiltration and aggregation and hepatocyte lysosome proliferation were revealed by an increase in cells staining for acid phosphatase. In addition, a subpopulation of macrophages stained positively for glucose-6-phosphate dehydrogenase during wk 6. Other histochemical biomarkers (Mg2(+)-ATPase, DT-diaphorase, uridine diphosphoglucuronyl dehydrogenase) were not altered. Mitotic figures were rare for the entire 6-wk period. At the ultrastructural level, necrotic alterations of some hepatocytes were seen within 24 h. Within 48 h, an apparent reduction of hepatocyte glycogen and cell volume characterized the majority of hepatocytes; this was accompanied by an increase in interhepatocytic space and the length and complexity of the hepatocyte microvillous projections found in the space of Disse. Lipid vacuolar inclusions inhabited space previously occupied by glycogen. Margins of hepatocyte nuclei were irregular, and mitochondria were condensed and their shape altered so that crescentric and elongated profiles were abundant. Lysosomes and residual bodies were increased after 1 wk. The cytoplasmic processes delineating spongiotic lesions were identified as originating from Ito cells. After 4 wk, apparent proliferation of smooth endoplasmic reticulum and retention of transport lipid within its cisternae were seen. The toxic depletion of hepatocytes and the attendant altered cellular environment are discussed in relation to cell-to-cell interactions and the possible contribution of stromal and extracellular matrix changes to liver regeneration and neoplasia.