Polyploidy is a general physiological process indicative of terminal differentiation. During liver growth, this process generates the appearance of tetraploid (4n) and octoploid (8n) hepatocytes with one or two nuclei. The onset of polyploidy in the liver has been recognized for quite some time; however, the cellular mechanisms that govern it remain unknown. In this report, we observed the sequential appearance during liver growth of binuclear diploid (2 ؋ 2n) and mononuclear 4n hepatocytes from a diploid hepatocyte population. To identify the cell cycle modifications involved in hepatocyte polyploidization, mitosis was then monitored in primary cultures of rat hepatocytes. Twenty percent of mononuclear 2n hepatocytes failed to undergo cytokinesis with no observable contractile movement of the ring. This process led to the formation of binuclear 2 ؋ 2n hepatocytes. This tetraploid condition following cleavage failure did not activate the p53-dependent checkpoint in G 1 . In fact, binuclear hepatocytes were able to proceed through S phase, and the formation of a bipolar spindle during mitosis constituted the key step leading to the genesis of two mononuclear 4n hepatocytes. Finally, we studied the duplication and clustering of centrosomes in the binuclear hepatocyte. These cells exhibited two centrosomes in G 1 that were duplicated during S phase and then clustered by pairs at opposite poles of the cell during metaphase. This event led only to mononuclear 4n progeny and maintained the tetraploidy status of hepatocytes.Polyploidy is a general physiological process that prevails in many cellular systems including plants, insects, and mammals (1). The onset of cellular polyploidization is associated with late fetal development and postnatal maturation. Advanced polyploidy in mammalian cells is indicative of terminal differentiation and senescence (2). Hepatocytes come under the former category. During growth, the liver parenchyma undergoes dramatic changes characterized by gradual polyploidization during which hepatocytes of several ploidy classes emerge as a result of modified cell division cycles. This process generates the successive appearance of tetraploid and octoploid cell classes with one or two nuclei. Thus, in the liver of a newborn rat, hepatocytes are exclusively diploid (2n), 1 and polyploidization starts after weaning. In adult rats, about 10% of hepatocytes are diploid, 70% are tetraploid, and 20% octoploid. If we consider the polyploid fraction, 20 -30% of hepatocytes are binuclear (either 2 ϫ 2n or 2 ϫ 4n) (3, 4). The degree of polyploidization varies among mammals (5) and particularly in humans, where the number of polyploid cells averages 20 -30% in the adult liver (6, 7). Interestingly, in different liver pathologies, hepatocarcinoma for example, hepatocellular growth shifts to a nonpolyploidizing growth pattern, and expansion of the diploid hepatocyte population has been found to take place (4, 7).Polyploidization is a general strategy of cell growth that enables an increase in metabolic output,...