CHANGES IN CATALASE AND GLUCOSE-6-PHOSPHATASE DISTRIBUTION PATTERNS WITHIN OVAL CELL COMPARTMENT AS POSSIBLE DIFFERENTIATION MARKERS DURINGVIRAL HEPATOCARCINOGENESIS IN WOODCHUCKS

Cytochemical analysis at the ultrastructural level was performed to ch aracterize expression of catalase and glucose-6-phosphatase (G6Pase) a ctivity as possible differentiation markers in oval cells proliferatin g during hepatocarcinogenesis induced in woodchucks by chronic infecti on with the woodchuck hepatitis virus (WHV) and additional treatment w ith aflatoxin B-1 (AFB(1)). Oval cells from WHV-carriers treated with AFB(1) showed two types of catalase-positive organelles: 1) microperox isomes appearing as small strongly osmiophilic bodies corresponding to those present in biliary cells from control woodchucks, 2) peroxisome s with a hepatic staining pattern resembling those of mature hepatocyt es but lacking a nucleoid. While in oval cells penetrating into the pa renchyma a catalase-positive reaction product was restricted to rare m icroperoxisomes, in close vicinity to the portal tract about 30% of th e oval cells produced peroxisomes with a hepatic staining pattern, ind icating the existence of two different populations within the oval cel l compartment. Peroxisomes with a hepatic staining pattern formed clus ters and exhibited pleomorphism with marked variation in shape and siz e, the size sometimes coming up to that of normal hepatocellular perox isomes. Serial sections revealed the complex organization of these per oxisomes. They consisted of several interconnected segments forming a peroxisomal reticulum. These findings are consistent with the hypothes is that a subpopulation of oval cells represents committed precursor c ells capable of differentiating into hepatocytes. Activity of G6Pase w as not demonstrable in this subpopulation of oval cells and became pos itive only in transitional cells. Differential expression of catalase and G6Pase activity in a stepwise fashion within the oval cell compart ment appear to mark differentiation of oval cells into hepatocytes. Th us, elevated expression of catalase may be a useful early marker for t he distinction of different subpopulations of oval cells committed to hepatic cell lineages before definitely changing their phenotype, wher eas expression of G6Pase activity seems to begin later, accompanying m orphological changes towards the phenotype of mature hepatocytes.