Stem cells and corneal epithelial regeneration.

Self-renewing tissues such as the corneal epithelium contain stem cells which represent the proliferative reserve. Studies of cellular differentiation and proliferation suggest that corneal epithelial stem cells are localised exclusively in the basal limbal epithelium. Although regulatory factors for the amplification of corneal stem cells are unknown, serum factors such as retinoic acid might induce differentiation of stem cells to transient amplifying cells which are responsible for cell amplification. These cells are regulated by various polypeptide growth factors and extracellular calcium. Loss or malfunction of stem cells does not permit maintenance or regeneration of the corneal epithelial mass but leads to conjunctivalisation of the corneal surface. Clinically, several ocular surface disorders such as chemical burns can cause limbal damage and consecutive limbal insufficiency. Treatment for these disorders is available only by transplantation of healthy stem cells, which can be performed as both autograft and allograft.