Perpetuation of stem cells in the eye.

In adult tissues, cell numbers are maintained through a subpopulation of cell termed stem cells, characterised in part by a high capacity of self-renewal, slow cell cycle, and resistance towards differentiation. Stem cells are capable of asymmetric division and able to maintain their position in a particular microenvironment or niche. In the cornea, epithelial stem cells are believed to reside in the basal cell layer of the limbal epithelium. We consider the question of how stem cells are perpetuated in the limbus without entering the pathway of terminal differentiation. This perpetuation could presumably be the result of extrinsic properties of the limbal zone creating a 'stem cell niche', or of intrinsic properties of the cells. For example, limbal basal cells contain four- to fivefold higher levels of epidermal growth factor receptor than central corneal basal cells, suggesting that high levels of epidermal growth factor receptor help maintain the limbal basal cells in an undifferentiated stem cell state.