Figure 2 Possible cells of origin for Barrett's metaplasia. If the metaplastic process occurs by altered differentiation of a mature squamous oesophageal epithelial cell (shaded cells in grey in the mucosal layer) without requiring proliferation, it is called transdifferentiation. Alternatively, the cell of origin may be an undifferentiated cell with the capacity to form multiple‐cell lineages: a so‐called “stem cell”. These stem cells may be of tissue or bone marrow origin. The tissue‐derived stem cells may be located in the basal compartment of the interpapillary layer or in the submucosal gland duct. In either case, the trigger for columnar differentiation seems to depend on surface epithelial damage from luminal factors.