Location and clonal analysis of stem cells and their differentiated progeny in the human ocular surface

We have analyzed the proliferative and differentiation potential of human o cular keratinocytes. Holoclones, meroclones, and paraclones, previously ide ntified in skin, constitute also the proliferative compartment of the ocula r epithelium. Ocular holoclones have the expected properties of stem cells, while transient amplifying cells have variable proliferative potential. Co rneal stem cells are segregated in the limbus, while conjunctival stem cell s are uniformly distributed in bulbar and forniceal conjunctiva. Conjunctiv al keratinocytes and goblet cells derive from a common bipotent progenitor. Goblet cells were found in cultures of transient amplifying cells, suggest ing that commitment for goblet cell differentiation can occur late in the l ife of a single conjunctival clone. We found that conjunctival keratinocyte s with high proliferative capacity give rise to goblet cells at least twice in their life and, more importantly, at rather precise times of their life history, namely at 45-50 cell doublings and at similar to 15 cell doubling s before senescence. Thus, the decision of conjunctival keratinocytes to di fferentiate into goblet cells appears to be dependent upon an intrinsic "ce ll doubling clock.'' These data open new perspectives in the surgical treat ment of severe defects of the anterior ocular surface with autologous cultu red conjunctival epithelium.