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Yuki Sugiyama, Elizabeth Shelley, Frank Lovicu, John McAvoy; Searching for stem-like cells in the lens epithelium. Invest. Ophthalmol. Vis. Sci. 2013;54(15):470.
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© ARVO (1962-2015); The Authors (2016-present)
The lens grows throughout life by proliferation and differentiation of epithelial cells. In adults there is a population of lens epithelial cells that maintain their proliferative ability, raising the possibility of the existence of cells with a stem cell-like character. Evidence for this comes from studies that identify slow cycling (3H-TdR-label-retaining) lens epithelial cells in adult (18.5 week old) mice (Zhou et al, IOVS 47, 2997, 2006). In other tissues, Wnt/β-catenin pathway has been shown to regulate stem-like cell populations. Recently we found a small population of lens epithelial cells that showed TCF/lef reporter activity during embryogenesis. The aim of this study was to assess the existence of stem-like cells during embryogenesis and determine the TCF/lef-activity status of such cells.
BrdU was applied to pregnant mice at E14.5 and embryos were collected after either 1 hr or 4 days (E18.5). Both activation of the Wnt/β-catenin pathway and proliferation were examined by assessing LacZ expression in TCF/lef reporter mice (Mohamed et al, Dev Dyn 231, 416, 2004) at P5 after BrdU application. For this, lens epithelial cell whole mounts were incubated with X-gal substrate and then processed for immunofluorescent staining to visualise incorporated BrdU.
After the 4-day chase period, BrdU was retained in 14% of lens epithelial cells. This rate was much lower than the BrdU incorporation rate at E14.5 after 1 hr (23%), suggesting that by E18.5 some of the BrdU had diluted to undetectable levels. BrdU intensity at E18.5 was variable; some cells had stronger staining than others but the strongest label was always weaker than that of fully-labeled cells observed after the 1hr application. Thus the majority of cells that incorporated BrdU at E14.5 appeared to divide at least once by E18.5. We also found about 4% of the lens epithelial cells of P5 mice were positive for TCF/lef reporter activity. These cells were labelled by BrdU at the same rate as non-TCF/lef-positive cells after the 2-3 hrs labeling period.
Most of the lens epithelial cells, including stem like-cells (if they exist) are highly proliferative during embryonic stages. For neonates, TCF/lef-active cells appear to be in a similar proliferating mode to non-TCF/lef-active cells. In future studies it will be important to conduct longer-term chase experiments of TCF/lef-active cells to determine if any of these cells ever enter a quiescent state.
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