Potential localization of putative stem/progenitor cells in human bulbar conjunctival epithelium

Although the conjunctival fornix appears to contain the greatest proportion of stem cells, it is likely that pockets of conjunctival epithelial stem cells may also exist throughout the conjunctival epithelium. This study was to investigate the potential localization of putative stem/progenitor cells in the human bulbar conjunctival epithelium by evaluating 6 keratins and 13 molecules that have been previously proposed stem cell associated or differentiation markers. We found that cornea specific cytokeratin (CK) 3 was not expressed by the bulbar conjunctival epithelial cells. In contrast, CK4 and CK7 were expressed by the superficial cells of bulbar conjunctival epithelium. CK14 and CK15 were confined to the basal cell layer. CK19 was strongly expressed by all layers of the bulbar conjunctival epithelium. The expression patterns of molecular markers in the basal cells of human bulbar conjunctival epithelium were found to be similar to the corneal epithelium. Basal conjunctival epithelial cells strongly expressed stem cell associated markers, including ABCG2, p63, nerve growth factor (NGF) with its receptors tyrosine kinase receptor A (TrkA) and neurotrophin low-affinity receptor p75NTR, glial cell-derived neurotrophic factor (GDNF) with its receptor GDNF family receptor alpha 1 (GFRalpha-1), integrin beta1, alpha-enolase, and epidermal growth factor receptor (EGFR). The differentiation associated markers nestin, E-cadherin and involucrin were not expressed by these cells. These findings indicate that the basal cells of bulbar conjunctival epithelium shares a similar expression pattern of stem cell associated markers to the corneal epithelium, but has a unique pattern of differentiation associated cytokeratin expression.