The revised model of liver regeneration. (A) Liver regeneration after 30% PHx. The median lobe (ML) is removed and the left lobe (LL), right lobe (RL), and caudate lobe (CL) regrow. During this process, all hepatocytes enlarge, but some enter S phase and divide only rarely. As a result, hepatocytes slightly increase their ploidy, but do not change their nuclear number. (B) Liver regeneration after 70% PHx. All hepatocytes increase their size and then enter the cell cycle to undergo S phase. Some cells execute cell division to produce mainly mononuclear daughter hepatocytes, irrespective of nuclear number of mother hepatocytes. However, not all hepatocytes divide. Liver recovers its lost mass by a combination of hypertrophy and proliferation. As a result, hepatocytes increase their ploidy, but decrease their nuclear number.