We used cre/loxP-based genetic lineage tracing analysis to test a previously proposed hypothesis that in vitro cultured adult pancreatic β-cells undergo epithelial-mesenchymal transition (EMT) to generate a highly proliferative, differentiation-competent population of mesenchymal islet “progenitor” cells. Our results in the mouse that are likely to be directly relevant to the human system show that adult mouse β-cells do not undergo EMT in vitro and that the mesenchymal cells that arise in cultures of adult pancreas are not derived from β-cells. We argue that these cells most likely originate from expansion of mesenchymal cells integral to the heterogeneous pancreatic islet preparations. As such, these mesenchymal “progenitors” might not represent the best possible source for generation of physiologically competent β-cells for treatment of diabetes.