Hepatocyte buds derived from progenitor cells repopulate regions of parenchymal extinction in human cirrhosis

Repair of cirrhotic livers occurs, in part, by repopulation with hepatocytes through the stem/progenitor pathway. There remain many uncertainties regarding this pathway. Hepatocyte "buds" occurring in broad septa are hypothesized to be the anatomic manifestation of this pathway. Our purpose was to define a morphologic sequence of bud maturation to allow a quantitative measure of the importance of the stem/progenitor pathway in humans. Histologic sections from 37 liver resection specimens were stained with trichrome, epithelial cell adhesion molecule (EpCAM), K19, CD34, glutamine synthetase (GS), and Ki-67. Specimens were stratified by etiology (10 biliary, 22 nonbiliary, five controls) and stage. Buds were defined as clusters of hepatocytes within septa. Five levels of bud maturation (0-4) were defined by the progressive increase in hepatocyte progeny relative to cholangiocytes. Level 0 single-cell buds are K19⁺/GS⁺/EpCAM⁺/Heppar1^-. In level 1, the progeny are morphologically hepatocytes (K19^-/GS⁺/EpCAM⁺/Heppar1⁺). In level 2-4 buds, hepatocytes increase and become progressively GS^- and EpCAM^-. Associated endothelium is CD34⁺ in level 1-2 buds and becomes CD34^- near hepatic veins in level 3-4 buds. Progeny of the bud sequence may represent up to 70% of hepatocytes (immaturity index of 70%). In biliary disease, bud number is reduced in association with duct loss and cholestatic destruction of nascent buds. Conclusions: The stem/progenitor pathway, manifested anatomically by the bud sequence, is a major mechanism for repopulation of cirrhotic livers. The bud sequence reveals some critical features of hepatic morphogenesis, including that 1) the majority of distal cholangiocytes have stem-like properties, and 2) availability of bile ducts and/or venous drainage are limiting factors for regeneration.