The limbal epithelial stem cell (LESC) hypothesis proposes that LESCs in the corneal limbus maintain the corneal epithelium both during normal homeostasis and wound repair. The alternative corneal epithelial stem cell (CESC) hypothesis proposes that LESCs are only involved in wound repair and CESCs in the corneal epithelium itself maintain the corneal epithelium during normal homeostasis. We used tamoxifen-inducible, CreER-. loxP lineage tracing to distinguish between these hypotheses. Clones of labelled cells were induced in adult CAGG-CreER;R26R-. LacZ reporter mice and their distributions analysed after different chase periods. Short-lived clones, derived from labelled transient amplifying cells, were shed during the chase period and long-lived clones, derived from stem cells, expanded. At 6. weeks, labelled clones appeared at the periphery, extended centripetally as radial stripes and a few reached the centre by 14. weeks. Stripe numbers depended on the age of tamoxifen treatment. Stripes varied in length, some were discontinuous, few reached the centre and almost half had one end at the limbus. Similar stripes extended across the cornea in CAGG-CreER;R26R-mT/mG reporter mice. The distributions of labelled clones are inconsistent with the CESC hypothesis and support the LESC hypothesis if LESCs cycle between phases of activity and quiescence, each lasting several weeks.
Bibliographical noteWe thank Drs Kevin Painter (Heriot-Watt University), Steven Morley and Richard Mort for their helpful discussion. We also thank Mr Ronnie Grant for his help with the figures, staff at BRR, University of Edinburgh, for their specialised technical services and Drs Bettina Wilm (Liverpool University), Peter Hohenstein, Richard Mort and Alison Mackinnon for kindly providing founder mice. This work was supported by the UK Biotechnology and Biological Sciences Research Council (grants BB/J015172/1 and BB/J015237/1).
- Limbal epithelial stem cells
- Lineage tracing
- Stem cell quiescence
- Stem cells