Cell Markers and the Side Population Phenotype in Ocular Surface Epithelial Stem Cell Characterization and Isolation

The ocular surface is covered by two rapidly renewing and embryologically-related linings, the corneal and conjunctival epithelia. The long-term survival of these tissues is ultimately dependent on their respective resident stem cells. In the corneal epithelium, the stem cells and their early precursors are exclusively circumscribed to the narrow vascularized limbal rim that provides epithelial precursor cells to the critically transparent central cornea. Limbal damage causes an interruption of this essential cell supply and allows the invasion of the corneal surface by the conjunctival epithelium, an event that ultimately leads to corneal scarring. The limited supply of immunocompatible tissue is a major hindrance to efforts to develop effective procedures for ocular surface reconstruction. This review describes some of the current work and strategies being developed to achieve the isolation of the limbal stem cell and define its genetic, biochemical, and functional make-up. The study of isolated ocular surface stem cells will foster basic understanding of the environmental requisites for their survival and proliferation in a self-replicative mode, leading eventually to advances in therapeutic approaches.