Europ. J. Cancer, Vol. 15, pp. 189 196 0014-2964/79/0201-0189502.00/0 3;) Pergamon Press Ltd. 1979. Printed in Great Britain Histochemical Differences Between So-Called Megalocytosis and Neoplastic Preneoplastic Liver Lesions Induced by N-Nitrosomorpholine* or HENRYK S. TAPER]-+ and PETER BANNASCH§ ]-Department of General Pathology and Neuropathology, University of Louvain, avenue E. Mounier, 52 (ANPG 52.60) B- 1200 Bruxelles, Belgium and §Division of Cytopathology, Institute of Experimental Pathology, German Cancer Research Center, D-6900 Heidelberg, W. Germany Abstract--Differences in histochemical pattern were found between the so-called megalocytes and preneoplastic and neoplastic hepatocytes after short-term administration of sublethal dose of N-nitrosomorpholine in rats. Megalocytes were poor in glycogen and intensely pyroninophilic. The activity of alkaline and acid nucleases was normal or increased. On the other hand the clear and acidophilic hepatocytes considered to be preneoplastic showed excessive storage of glycogen and had usually decreased or absent activity of nucleases. Basophilic cells of neoplastic nodules and hepatocellular car- cinomas were practically free from glycogen and nuclease activity. The majority of megalocytes which disturbed seriously the lobular architecture of liver during the N N M intoxication, disappeared soon after the withdJ mt'al of carcinogen. The megalocytes are interpreted as the result of an interaction between toxic and regenerative events. INTRODUCTION PDLYPLOID hepatocytes with considerably en- larged nuclear and cytoplasmic volume have been described as megalocytes in different pathologic conditions. During mitosis many of the so-called megalocytes show an irregular distribution of the chromosomes. Megalocytosis was observed in the rat liver in senility [1], after partial hepatectomy of X- irradiated animals [2, 3] or after the adminis- tration of a great variety of toxic agents, including potent hepatocarcinogens, such as 3'-methyl-4-dimethylaminoazobenzene [4-6], 2-amino-2-azotoluene [7], aflatoxin B118] or N-nitrosomorpholine [9-11 ]. Most of the above mentioned authors con- sidered megalocytosis to be a reversible alter- ation. Some of them suggested that the megalo- cytosis which appears under the influence of liver poisons is due to an interaction of de- Accepted 4 July 1978. *This investigation was supported by a research visitors grant of Deutsches Krebsforschungszentrum Heidelberg, Germany and by the Ponds de la Recherche Scientifique M6dicale, Belgium. +To whom requests for reprints should be addressed. 189 generative and regenerative effects. The death of cells induced by the poison calls for re- generation of other cells which during their mitosis are also injured by the poison [11, 12]. A disturbance of mitosis was also made re- sponsible for the development of megalocytes after partial hepatectomy of X-irradiated rat livers [2, 3]. A pronounced increase in free and mem- brane bound ribosomes involving a strong cytoplasmic basophilia seems to be a cytoplas- mic expression of the marked proliferative activity of megalocytes in liver parenchyma [11]. A clear-cut morphological distinction be- tween basophilic megalocytes and basophilic cells which appear during late stages of hepa- tocarcinogenesis may be difficult or even im- possible. Therefore, it seemed interesting to learn whether histochemical differences exist between the so-called megalocytes and the preneoplastic or neoplastic cells. The most convenient experimental model for such an investigation appeared to be rat liver carcino- genesis induced by short-term intoxication with a sublethal dose of N-nitrosomorpholine (NNM).