Controversies Surrounding the Origin of Hepatocytes in Adult Livers and the in Vitro Generation or Propagation of Hepatocytes

Abstract

Epithelial cells in the liver (known as hepatocytes) are high-performance engines of myriad metabolic functions and versatile responders to liver injury. As hepatocytes metabolize amino acids, alcohol, drugs, and other substrates, they produce and are exposed to a milieu of toxins and harmful byproducts that can damage themselves. In the healthy liver, hepatocytes generally divide slowly. However, after liver injury, hepatocytes can ramp up proliferation to regenerate the liver. Yet, on extensive injury, regeneration falters, and liver failure ensues. It is therefore critical to understand the mechanisms underlying liver regeneration and, in particular, which liver cells are mobilized during liver maintenance and repair. Controversies continue to surround the very existence of hepatic stem cells and, if they exist, their spatial location, multipotency, degree of contribution to regeneration, ploidy, and susceptibility to tumorigenesis. This review discusses these controversies. Finally, we highlight how insights into hepatocyte regeneration and biology in vivo can inform in vitro studies to propagate primary hepatocytes with liver regeneration-associated signals and to generate hepatocytes de novo from pluripotent stem cells.

Keywords: Differentiation; Hepatocyte Expansion; Liver Stem Cells; Ploidy; Regeneration; Tumorigenesis; Zonation.

Figure 1

Sources of liver regeneration during homeostasis. Hepatocytes in the liver are spatially organized into periportal (PP), mid-lobular (ML), and pericentral (PC) “zones” depending on their proximity to the portal or central vein. PP and PC hepatocytes execute distinct metabolic functions in the liver. An oxygen gradient is established as blood flows from the portal vein to the central vein. Wnt signals are concentrated in the PC region. During homeostasis, Axin2+ (green) and Lgr5+ (teal) cells found in the pericentral region, can regenerate the liver. They propagate from the PC to the PP region. Tert^hi hepatocytes (blue) are distributed throughout the liver lobule and can clonally expand to regenerate the liver. Msfd2a+ hepatocytes (pink) are found at the PP region.

Figure 2

Liver regeneration responses by different hepatocytes during injury. (A) During partial hepatectomy (PH) injury and injury to pericentral (PC) hepatocytes, Lgr5– hepatocytes in other liver zones convert to Lgr5+ (green) to promote regeneration and repair hepatocytes. (B) Axin2– hepatocytes showed a similar pattern of regeneration, converting to Axin2+ (light green) hepatocytes after Cre-mediated expression of diphtheria toxin A (DTA). (C) Msfd2a+ (pink) hepatocytes repopulate the liver by proliferating from the periportal (PP) to the PC region on carbon tetrachloride (CCL₄)-induced injury to PC hepatocytes. (D) Sox9+ Hnf4a+ (purple) hepatocytes contribute to regeneration in response to PH and CCL4-induced injuries. (E) 3,5-diethoxycarbonyl-1,4-dihydrocollidine (DDC), CCL4-induced injuries, and Cre-mediated expression of DTA promote proliferation of Tert^hi hepatocytes (blue) to regenerate PC hepatocytes. (F) Polyploid cells (red, 2 grey nuclei) contribute to liver recovery in response to arachidonic acid-induced (AA) PP injury and (CCL₄)-induced PC injury. During liver regeneration, polyploid cells can reduce their ploidy status (red, 1 grey nucleus).