This chapter reviews evidence for the existence and identity of pancreatic progenitor and stem cells, and describes the criteria for experimental demonstration of such cells. The most rigorous definition of a stem cell states that it is a cell that, upon proliferation, produces some progeny that have the same developmental potential, as well as other progeny that have a more restricted developmental potential. In Type I diabetes, the insulin-producing β-cells that reside in the pancreatic islets of Langerhans are destroyed by autoimmune attack, and it is thought that self-renewing stem cells could provide an unlimited source of β-cells for transplantation. Such therapeutic efforts require the prospective isolation of stem cells with the potential to produce β-cells and the development of methods to direct their expansion and differentiation. From a developmental biology perspective, the role of stem cells in the pancreas is a fascinating problem. Much of the field is focused on the identification of β-cell progenitors and the characterization of their molecular requirements, but it is not known what role such cells play during pancreas maintenance and regeneration, or whether the adult pancreas contains a population of true stem cells.
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