Diseases of esophageal epithelium (EE) such as reflux esophagitis and cancer are rising in incidence. Despite this, the cellular behaviors underlying EE homeostasis and repair remain controversial. Here we show that in mice, EE is maintained by a single population of cells that divide stochastically to generate proliferating and differentiating daughters with equal probability. In response to challenge with all-trans Retinoic Acid (atRA) the balance of daughter cell fate is unaltered but the rate of cell division increases. However, following wounding, cells reversibly switch to producing an excess of proliferating daughters until the wound has closed. Such fate switching enables a single progenitor population to both maintain and repair tissue without the need for a "reserve" slow-cycling stem cell pool.Murine EE consists of layers of keratinocytes. These tissue lacks structures such as crypts or glands which form stem cell niches in other epithelia . Proliferation is confined to cells in the basal layer (6). On commitment to terminal differentiation, basal cells exit the cell cycle and subsequently migrate to the tissue surface from which they are shed. Early studies suggested all proliferating cells were functionally equivalent, but recent reports propose that a discrete population of slow-cycling stem cells is responsible for both maintenance and wound healing (7)(8)(9)(10)(11). This controversy and the importance of EE in disease motivated us to resolve the proliferative cell behavior in homeostatic EE and in tissue challenged by systemic treatment with the vitamin A metabolite all-trans Retinoic Acid (atRA) or acute local wounding (12-13).To investigate cell division rates in EE we used a transgenic label retaining cell (LRC) assay ( Fig. 1C) (1, 14-15). Doxycycline (DOX) induction of Histone-2B EGFP fusion protein (HGFP) expression in Rosa26 M2rtTA /TetO-HGFP mice resulted in nuclear fluorescent labeling throughout the EE (Fig. 1D and fig. S1A). When DOX is withdrawn, HGFP is diluted by cell division, leaving 0.4% basal layer cells (561 out of 140000) retaining label after a 4 week chase (Figs. 1E and S1B). 3D imaging showed these label retaining cells * To whom correspondence should be addressed. phj20@cam.ac.uk. 5 These authors contributed equally to this work Supplementary Materials: Materials and Methods Figures S1-S13 However, 99.9% (2457 out of 2459) of LRC were positive for the pan leukocyte marker CD45 (Fig. 1E inset), comprising of a mixture of Langerhan's cells and lymphocytes (Figs. S1E and F). These findings lead to the surprising conclusion that, unlike tissues such as the epidermis, there are no slow-cycling or quiescent epithelial stem cells in EE (1, 17). Indeed, HGFP dilution in basal cells was strikingly homogeneous, suggesting that all cells divide at a similar rate of approximately twice per week (Fig. S1G).Although epithelial cells have the same rate of division, they may still differ in their ability to generate cycling and differentiated progeny. We therefore used inducible ...

Multiple cancers may arise from within a clonal region of preneoplastic epithelium, a phenomenon termed 'field change'. However, it is not known how field change develops. Here we investigate this question using lineage tracing to track the behaviour of scattered single oesophageal epithelial progenitor cells expressing a mutation that inhibits the Notch signalling pathway. Notch is frequently subject to inactivating mutation in squamous cancers. Quantitative analysis reveals that cell divisions that produce two differentiated daughters are absent from mutant progenitors. As a result, mutant clones are no longer lost by differentiation and become functionally immortal. Furthermore, mutant cells promote the differentiation of neighbouring wild-type cells, which are then lost from the tissue. These effects lead to clonal expansion, with mutant cells eventually replacing the entire epithelium. Notch inhibition in progenitors carrying p53 stabilizing mutations creates large confluent regions of doubly mutant epithelium. Field change is thus a consequence of imbalanced differentiation in individual progenitor cells.