Limbal stem-cell transplantation.

Cornea epithelium is subject to constant trauma and shedding of the surface epithelium, and replenishment is from epithelial cells beneath and peripheral to the central desquamating epithelium. The origin of the corneal epithelium appears to reside in the crypts of Vogt, where a population of "immortal" stem cells resides, possessing enormous potential for clonogenic cell division. These cells, like all stem cells, have inherent properties which enable them to accomplish error-free replication which avoids development of abnormal differentiation and cellular dysfunction; a low mitotic rate and asymmetrical DNA segregation are essential in this error-free proliferation of these relatively primitive, poorly differentiated cells. The limbal stem cells lack the keratin 3 differentiation marker typical of all the rest of corneal epithelium, but does possess keratin 19. Cell kinetic studies show that limbal stem cells, and the cells into which they evolve, transiently amplifying cells (TAC) remain in the so called "proliferative" compartment, undergoing cell division and not evolving to commitment to further specialized differentiation. These cells (stem cells and TAC) are located exclusively in the limbal region, with the stem cells in the basal epithelium and TAC occurring in the basal and suprabasal levels, extending up to the superficial layers. The corneal epithelium is then maintained by cellular proliferation of these cells which then migrate further onto the central cornea and become terminally differentiated. The lack of a vascular supply and the presence of special elements within the corneal basement membrane, and differences in vitamin A concentrations may be some of the primary factors responsible for the terminal differentiation events.