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Inflammatory injury to the pancreas results in regeneration of normal tissue and formation of metaplastic lesions of a ductal phenotype. These metaplastic ductal lesions (MDL) are called tubular complexes (TC), mucinous metaplasia, or pancreatic intraepithelial neoplasia. Because they are regularly found in chronic pancreatitis and pancreatic cancer, their formation is thought to represent a step in inflammation-mediated carcinogenesis. Despite these lesions' ductal character, their origin is controversial. All known pancreatic cell lineages have been suggested as the origin. In vitro studies suggest that differentiated cells in the pancreas remain highly plastic and can transdifferentiate as a mechanism of regeneration and metaplasia. In vivo studies suggest that islets, specifically [beta] cells, may be the cell of origin. However, in vitro studies are subject to ductal cell contamination, and previous in vivo studies interpret static data rather than direct evidence. Using genetic lineage tracing in vivo, we investigate whether transdifferentiation of [beta] cells contributes to regeneration or metaplasia in pancreatitis. RIP-CreER;Z/AP mice were used to heritably tag [beta] cells in the adult pancreas. Injury by cerulein pancreatitis resulted in regeneration of normal tissue and metaplasia with formation of two distinct types of TC and mucinous lesions. Lineage tracing revealed that none of these MDL are of [beta] cell origin; nor do [beta] cells contribute to regeneration of normal acinar and ductal tissue, which indicates that the plasticity of differentiated pancreatic islet cells, suggested by earlier static and in vitro studies, plays no role in regeneration, metaplasia, and carcinogenesis in vivo. metaplasia | pancreatitis | pancreatic intraepithelial neoplasia

Background & Aims: Chronic injury results in regeneration of normal pancreatic tissue and formation of a metaplasia of ductal phenotype. Metaplastic ductal lesions are seen in pancreatitis as well as in specimens of pancreatic cancer and are thought to represent a condition with increased risk of neoplasia. Acinar-to-ductal transdifferentiation is thought to be the source of this metaplasia. This has been suggested for flat duct-like lesions called tubular complexes and for lesions exhibiting a mucinous metaplasia. However, available studies are based on interpretation of static data rather than on direct evidence. Transdifferentiation from acinar to ductal cells has never been confirmed in the adult pancreas. Methods: Here, we use Cre-loxP-based genetic lineage tracing in vivo to investigate whether transdifferentiation of acinar cells contributes to regeneration and metaplasia in pancreatitis. Results: The results show that transdifferentiation does not play a role in regeneration of normal tissue. Acinar cells are regenerated by preexisting acinar cells and not from other cell types. Three different types of metaplastic ductal lesions are observed and analyzed. Whereas the majority of metaplastic lesions are not of acinar origin, acinar-to-ductal transdifferentiation is identified in a minority of mucinous metaplastic lesions. Conclusions: Here, we provide direct evidence that acinar-to-ductal transdifferentiation occurs in the adult pancreas in vivo. However, it accounts for only a minority of metaplastic lesions. [Copyright &y& Elsevier]