Study of stem/progenitor cells located in the posterior limbus of the eye

Abstract

Some stem-like cells that have received limited attention but may be of considerable clinical value reside in the transition zone between the corneal endothelium (CE) and trabecular meshwork (TM) at the posterior limbus of the eyes. A growing body of evidence has revealed that these cells may be able to rejuvenate the CE, TM or both. However, these stem-like cells have not been clearly defined and we have thus named them PET cells (Progenitor cells of the Endothelium and Trabeculum).

Both the CE and TM cells are unique cell types in the eye that do not normally replace themselves once they are lost in ageing or diseases, such as Fuch’s endothelial dystrophy and primary open angle glaucoma. To date, no therapy exists that targets the rejuvenation of the compromised CE or TM in diseases. Therefore, the potential to repair or replace the diseased CE or TM through a cell repopulation approach is an important area that needs to be explored. The use of PET cells for tissue rejuvenation may represent an attractive therapeutic direction for the regeneration of the CE and/or TM. As a good animal model for PET cells is lacking, we sought to identify whether bovine eyes could serve as a good large tissue model for the studies of PET cells.

The comparative anatomy of the human and bovine posterior limbus was studied using light, confocal and scanning electron microscopy. Immunohistochemical studies were performed to localize the stem cell niche for the PET cells. Sphere culture was used to isolate and amplify progenitor cells from the CD and TM respectively. A detailed characterization of the spheres and their progenies was performed with immunocytochemistry, quantitative reverse-transcription polymerase chain reaction and differentiation and functional assays.

We showed the presence of stem or progenitor cells in the bovine CE, transition zone and TM in situ. The bovine TM insert region may house a stem cell niche that is comparable to that observed in humans. The progenitor cells isolated from the bovine CE and TM that grew as floating spheres demonstrated similar phenotypes in terms of stem cell marker expression. In addition, both the CE and TM spheres were bipotent, highly proliferative and had limited self-renewal capacity. However, they showed a high propensity to differentiate back into the cell type of their tissue of origin. We speculated that the PET cells become more tissues-specific as they migrate away from their niche towards the CE or TM. This may be a reasonable explanation why the CE and TM spheres respectively adopted their original lineage upon non-directed differentiation.

Taken together, our results support the hypotheses that PET cells are present in the posterior limbus of bovine eyes and that bovine eyes may serve as a good large tissue model for the studies of these cells. The PET cells represent an attractive target for developing new treatments to regenerate both the CE and TM, thereby reducing the requirement for donor corneas and invasive treatments in glaucomatous patuents.