Stem cells of the human cornea.

INTRODUCTION Blindness affects 50 million individuals worldwide; a significant proportion of them require a cell or tissue-based repair or replacement strategy to mend their damaged or diseased cornea. This review will focus on the epithelial stem cell (ESC) population of the cornea, where they reside, how they are identified and what alternative cells can be used as functional substitutes. SOURCE OF DATA Data for this review were collated after performing literature searches by inserting key words (cornea, limbal, stem cells (SCs), epithelium, stroma, and endothelium) into the search engine PubMed. AREAS OF AGREEMENT The prevailing notion is that corneal ESCs reside in an exclusive niche and their activation is dictated by niche-specific signals. AREAS OF CONTROVERSY Recent studies refute this hypothesis, as the central cornea of many animal species also possesses similar proliferative and clonogenic activity. The other area of controversy is in relation to the use of autologous and/or allogeneic cell therapies which are mostly contaminated with xenogeneic factors, potentially exposing the recipient to potentially harmful foreign infection. GROWING POINTS Due to the shortage of donor corneal biomaterial, alternative cellular sources are being sought, discovered and trialed. AREAS TIMELY FOR DEVELOPING RESEARCH With the exception of conjunctival and oral mucosal epithelium, no other cell type has been successfully used to treat patients with severe corneal epithelial defects. Embryonic and foetal SCs may have the greatest potential of all; however there are moral, legal, religious and scientific hurdles to overcome before they become routinely used in the clinic.