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A. Joseph, V. Shanmuganathan, J. McElveen, H.S. Dua; Limbal Stem Cells - Do We Have a Marker? . Invest. Ophthalmol. Vis. Sci. 2003;44(13):2029.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: To use known and putative stem cell markers to identify limbal epithelial stem cells (SC). Methods: Immunohistochemical analysis of sections of human adult cornea, 6-8 week old embryonic cornea and cultured corneoscleral explants (CSE) using monoclonal antibodies against p63 (proposed limbal SC marker), Ki67 (proliferating cell marker), CD34and CD133 (hematopoietic SC markers) and hTERT (telomerase, abundant in all SC). Both frozen and formalin fixed tissue were used. Results: p63: Patchy expression of p63 was seen on basal epithelial cells of both limbal and central adult cornea. Expression of p63 declined from day 0 to week 3 on cultured CSE. Epithelial cells that had migrated to cover the under surface of the explant too were positive for p63. Both layers of corneal epithelium and contiguous surface ectoderm in embryonic cornea were positive for p63. Ki67: Adult limbal and corneal epithelium was negative for Ki67. Patchy expression of Ki67 on basal epithelial cells of cultured CSE was seen at week 1 and its expression declined by week 3, correlating with the p63expression. CD34: CD34 was not expressed by adult or embryonic corneal epithelium. CD133: Membranous expression of CD133 was present in all layers of adult and embryonic corneal epithelium. hTERT: Nuclear expression of hTERT was seen in almost all the cells of adult corneal and limbal epithelium, with stronger staining on the basal layer. Conclusions: In the limbus 10% of basal cells are thought to be SC. Our findings of extensive p63 expression in the limbal and central cornea render p63 too ubiquitous a molecule to be a stem cell marker. SC are characteristically slow cycling cells; the correlation of p63 with Ki67 in cultured CSE suggests that it is present in rapidly cycling cells as well. p63 staining in cells that had migrated around the explant would imply that stem cells migrate out of their niche and retain their stemness. This is not compatible with current thinking on the influence of microenvironment on maintenance of SC. We were unable to identify limbal SC using well-established hematopoietic stem cell markers like CD34 and CD133. Similar expression of CD133 on plasma membrane protrusions in the apical surface of some developing, but not adult epithelial cells, has been demonstrated. Telomerase is an RNA dependent DNA polymerase that synthesizes telomeric DNA sequences and provides the molecular basis for immortal cells. Antibodies to hTERT did not specifically stain limbal SC. From our results we conclude that, while there is a "source cell" at the corneoscleral limbus, this is probably not a "stem cell" as seen in other systems, but is a committed progenitor or unipotent SC.
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