Stem cells have several unique attributes, the key features being their potency and plasticity. They have the ability to give rise to multiple cell lineages and to transdifferentiate into totally different cell type(s) when relocated to a novel stem cell niche. Most self-renewing tissues are served by stem cells. At the ocular surface, the corneo-scleral limbus is believed to provide the niche for corneal epithelial stem cells. A large body of circumstantial evidence, both clinical and basic, supports this view. However, specific identification of limbal stem cells has proved elusive. Cytokeratin markers, vimentin, epidermal growth factor receptors, p63, and others have been used to identify epithelial cell populations at the limbus, which could harbour putative stem cells. In contrast, none of the known haematopoietic stem cell markers namely, CD34 and CD133, stain any specific subset of corneal or limbal epithelial cells. Singly or collectively, none of these markers point to any unique cell(s) that could be regarded as stem cells, supporting the notion that the corneal epithelium is served by 'committed progenitors' rather than by stem cells. Disease or destruction of the corneoscleral limbus is associated with consequential events that eventually lead to visual impairment or blindness. Conjunctivalisation and vascularisation of the corneal surface and persistent or recurring epithelial defects are hallmarks of limbal deficiency.

Normal human corneal keratocytes express all three classes of CD34. The expression of this adhesion molecule on corneal keratocytes suggests that it may have a role in keeping the keratocytes anchored in their microniche, between the collagen lamellae. The positive staining for L-selectin found by immunohistochemistry but not by Western blotting or PCR would indicate the presence of either another ligand from the selectin family or a cross-reactive epitope on corneal keratocytes.

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