The liver is thought to utilize facultative stem cells also known

The liver is thought to utilize facultative stem cells also known as “oval cells” or “atypical ductal cells” (ADCs) for regeneration following various types of injury. which contribute to tissue homeostasis under special circumstances. By definition facultative stem cells lack stem cell activity during normal tissue turnover but are recruited during specific types of injury to function as stem or progenitor cells (Yanger and Stanger 2011 The mammalian liver has stood as the major paradigm for regeneration via a facultative stem-cell-mediated recovery. In response to various disease states and toxin-induced injuries rodents and humans exhibit an accumulation of atypical ductal cells (ADCs)-commonly referred to as “oval cells”-within the liver parenchyma (Farber 1956 Popper et al. 1957 ADCs have a ductal morphology but their arrangement into an intricate anastomosing configuration that extends into the hepatic lobule gives them a histologic appearance that is distinct from that of normal biliary epithelial cells (BECs) (Desmet 1985 ADCs are thought to arise from BECs within the Canals of Hering structures that reside at the interface of the intrahepatic bile ducts and hepatocyte-lined canaliculi (Figure 1A) (Factor et al. 1994 Preisegger et al. 1999 This putative mode of recovery stands in contrast to liver regeneration following surgical resection which is mediated largely through cell growth and division (Miyaoka et al. 2012 Figure 1 BECs Lack Detectable Progenitor Cell Activity In Vivo Based on in vitro studies ultrastructural analyses and cell transplantation assays ADCs have been proposed to function as bipotent facultative stem cells giving rise to both hepatocytes and BECs during toxin-mediated liver injury although this issue is controversial (Espa?ol-Su?er et al. 2012 Fausto and Campbell 2003 Friedman and LERK1 Kaestner 2011 Furuyama et al. 2011 Huch et al. 2013 Malato et al. 2011 Zaret and Grompe 2008 Furthermore adult hepatocytes exhibit significant plasticity in vivo a phenomenon that may give the appearance of stem-cell-mediated differentiation (Michalopoulos et al. 2005 Tanimizu et al. 2014 Yanger et al. 2013 In order to obtain direct evidence for liver stem cell activity in vivo we labeled three distinctive cell populations in the liver-BECs hepatocytes and rapidly dividing cells-using both direct genetic and unbiased nucleoside analog-based lineage labeling tools under multiple ADC-inducing injury conditions. Our studies demonstrate that hepatocytes not ADCs serve as the major if not exclusive source for hepatocyte renewal and regeneration in the adult liver regardless of Acemetacin Acemetacin (Emflex) (Emflex) the type of injury. RESULTS BECs Acemetacin (Emflex) Do Not Exhibit Progenitor Cell Activity In Vivo BECs residing within the Canals of Hering are thought to serve as precursors of liver progenitor cells (Factor et al. 1994 Wang et al. 2003 Such Acemetacin (Emflex) BECs in both rodent and human studies are characterized as single or small groups of cells positive for the BEC marker cytokeratin-19 (KRT19) and located within the hepatic lobule separated from the larger bile ducts (Crawford et al. 1998 Roskams et al. 2004 Saxena and Theise 2004 Theise et al. 1999 To directly address the ability of these cells to differentiate into hepatocytes in vivo we crossed inducible KRT19 promoter (knockin mice (Means et al. 2008 to reporter mice (Srinivas et al. 2001 to label cells from the BEC lineage prior to injury (Figure 1A). Bigenic mice were given tamoxifen resulting in pulse labeling completely restricted to BECs as previously reported with no yellow-fluorescent-protein-positive (YFP+) cells staining for the hepatocyte marker hepatocyte nuclear factor 4 α (HNF4α) (Figure 1B top panel; Scholten et al. 2010 The efficiency of labeling was 36.2% ± 8.7% of Krt19+ cells (Figure 1B middle panel; n = 4) and included single/small groups of cells within the Canals of Hering (Figure 1B bottom panels). To ensure that labeling was not limited to a specific subset of BECs we examined the expression of TROP2 a widely used oval cell/ADC marker (Okabe et al. 2009 Yamazaki et al. 2011 and found that Krt19+/YFP+ cells and Krt19+/YFP? cells stained positively for TROP2 with equal frequency (Figures S1A and S1B available online). We further assessed the spectrum of cellular labeling by costaining with YFP and a number of other.