Liver disorders affect 30 million people in the United States and are the 10th leading cause of death in the US. Nearly 40,000 patients will develop end-stage liver disease, resulting in 30,000 annual deaths. Liver transplantation is the most effective therapy but is severely limited by donor organ supply, thus necessitating the development of therapeutic alternatives to whole organ replacement. A better understanding of hepatocyte biology is required to improve existing approaches and innovate therapies for liver disease treatment. Hepatocytes display a range of chromosomal diversity, resulting from prevalent physiological polyploidy and aneuploidy. Most eukaryotic cells contain a diploid genome comprised of homologous chromosome pairs. Polyploidy refers to gains in entire chromosome sets, and aneuploidy refers to gains and/or losses of individual chromosomes. The underappreciated role of hepatic polyploidy and aneuploidy represents a major gap in our current understanding of liver biology. Recently, it was observed that diploid hepatocytes proliferate faster than polyploids, suggesting that the polyploid state functions as a growth suppressor to restrict proliferation by the majority of hepatocytes. Together with earlier work suggesting aneuploid hepatocytes protect in chronic liver injury, the data indicate that hepatic chromosomal diversity represents a novel form of liver heterogeneity. The central hypothesis tested is the following: Hepatocytes with altered chromosome content (diploid vs. polyploid; aneuploid vs. euploid) have context-dependent functions that optimize liver regeneration and response to acute/chronic liver injury. Aim 1 will characterize the role of diploid and polyploid hepatocytes during acetaminophen (APAP)-induced acute liver injury. Experiments will test whether diploid hepatocytes promote healing by enhanced compensatory liver regeneration, and they will identify mechanisms that regulate accelerated cell cycling. Aim 2 will determine whether disrupted tissue architecture promotes chromosome segregation errors and aneuploidy by proliferating hepatocytes. Experiments using a novel cell polarity deficiency model will test whether polarity disruptions promote hepatic chromosome segregation errors and aneuploidy. Moreover, it will be determined whether defective polarity by transplanted hepatocytes can drive this process. Aim 3 will determine whether human hepatic aneuploidy is a selectable mechanism for cell adaptation. Experiments will test if hepatocytes with advantageous aneuploidy accelerate liver repopulation and determine whether increased background aneuploidy accelerates adaptation to chronic injury. Both beneficial and pathological effects of aneuploidy will be determined. Overall, the research strategy will reveal new functions for hepatocytes with chromosome heterogeneity and uncover mechanisms that regulate their activity, which will provide new and crucial insights into liver homeostasis, diseases and treatments.