Purpose: The limbus of the cornea is said to be the niche for limbal stem cells (LSCs) and the primary source of corneal epithelial maintenance. Previously, we aimed to have shown that central human epithelial cells are capable of corneal regeneration after wounding. In this study, we aimed to investigate whether central epithelial cells in human corneas have LSC properties.
Methods: Human corneal epithelial cells were separated from the central cornea and the limbus. Isolated cells were collected for sphere-forming assay, and spheres formed subsequently were analyzed using immunohistochemistry. Fluorescence-activated cell sorting (FACS) was also used to analyze epithelial cells from central cornea, limbal rim, older donors, younger donors, and dissociated spheres. These analyses were based on cell size and Hoechst 33342 dye efflux ability, and side populations and non-side populations were isolated for colony growth measurement and sphere-forming assay.
Results: Human central and limbal epithelial cells were capable of forming spheres, in a 1:2 ratio, that were positive for p63 immunolabeling. In FACS, central and limbal epithelial cells showed no significant difference in cell size and dye efflux ability. There were almost 10 times more large cells with good dye efflux ability from younger donors than from older donors, and the gated side population showed more than 4 times faster rate of colony growth than the non-side population. Dissociated sphere cells, however, did not follow a similar pattern to tissue-derived cells using FACS analysis. In these, there were more than twice as many large cells than small cells with good dye efflux ability.
Conclusions: Both limbal and central epithelial cells are capable of forming spheres in cultures that have stem cell properties. Central and limbal epithelial cells cannot be differentiated using FACS, but younger donor tissues give rise to greater numbers of large cells with high dye efflux. Therefore, results indicate that human central corneal epithelium contains cells with stem/progenitor properties, and these stem properties decline with age.