The fate of putative multipotent, mixed GSK3235025 purchase epithelial-mesenchymal precursors is not addressed in our study, except to say that such cells are not marked by the Alfp-Cre transgene.53 Many studies of EMT in fibrosis have failed to define EMT rigorously or to differentiate between the transition to a mesenchymal (EMT) versus a myofibroblast (EMyT) phenotype. Type I collagen expression is the most direct measure of fibrogenesis, and the

literature suggests that α-SMA-positive cells are the primary effectors of fibrogenesis.15, 18, 54, 55 Nevertheless, surrogate fibroblast markers have often been used to identify EMT, most notably S100A4, despite recent data suggesting that it is nonspecific.10, 18, 38 In our study we examined the expression of four different mesenchymal markers, including S100A4, vimentin, α-SMA, and procollagen I. Their lack of colocalization with YFP in the setting of fibrosis supports the conclusion that in these models EMT does not contribute to fibrosis. The lack of vimentin colocalization is particularly interesting. Although often said to

be a nonspecific indicator of cholangiocyte damage, it is in fact a highly specific marker of the mesenchymal state and has been used as a marker of EMT in nonfibrosis contexts (e.g., embryonic development and cancer).56 The complete absence of its colocalization with YFP in our study suggests that liver epithelial cells do not transition to either

mesenchymal cells or myofibroblasts JAK inhibitor in the mouse models examined. Furthermore, our data contradict the recently proposed hypothesis that stellate cells are derived from medchemexpress epithelial progenitors.57 Rather, they are consistent with studies that have shown that stellate cells originate in the hepatic submesothelium.58-60 Although our data stand in contrast to another study that supported the concept of hepatocyte EMT,14 they complement those of Taura et al. and Scholten et al.,8, 9 which provide strong, direct evidence that EMT does not occur in rodent models of fibrosis. As definitive as these lineage-tracing data are, however, it is difficult to reconcile them with the costaining data, primarily from human tissue, which originally led to the concept of cholangiocyte EMT.3-7, 14 Although most of these studies demonstrated little or no coexpression of cholangiocyte markers with α-SMA, there was significant cholangiocyte expression of other mesenchymal markers, including vimentin and the collagen chaperone HSP47. It may be that in human livers EMT occurs in cirrhosis, a state not well modeled in rodents, and may require a florid ductular reaction, which is also poorly mimicked by rodent models. Alternatively, this discrepancy may reflect the limitations of immunohistochemistry-based lineage-tracing methodology, although this is mitigated in our study by the high efficiency of Cre recombination.