Corneal Stem Cells: Bridging the Knowledge Gap

Approaches toward enhancing survival probability following deep anterior lamellar keratoplasty

The greatest advantage of deep anterior lamellar keratoplasty over full-thickness corneal transplantation is the elimination of graft failure caused by endothelial rejection. Despite this advantage, a deep anterior lamellar keratoplasty graft can fail because of several factors, such as complications related to the donor-recipient interface, graft epithelial abnormalities, graft vascularization, stromal graft rejection, and recurrence of herpetic keratitis. Increased deep anterior lamellar keratoplasty graft survival is mainly built upon optimization of the ocular surface to provide a hospitable environment for the graft. Any predisposing factors for graft epithelial abnormalities, corneal neovascularization, and preexisting vernal keratoconjunctivitis should be identified and treated preoperatively. Prompt recognition and appropriate treatment of interface-related complications and stromal graft rejection usually result in good anatomic outcomes, with no detrimental effects on vision.

Keratinocyte growth factor-2 and autologous serum potentiate the regenerative effect of mesenchymal stem cells in cornea damage in rats

AIM

To investigate the healing process after severe corneal epithelial damage in rats treated with mesenchymal stem cells (MSCs) cultured with or without keratinocyte growth factor (KGF-2) and autologous serum (AS) on amniotic membrane (AM). Many patients are blind and devastated by severe ocular surface diseases due to limbal stem cell deficiency. Bone marrow-derived MSCs are potential sources for cell-based tissue engineering to repair or replace the corneal tissue, having the potential to differentiate to epithelial cells.

METHODS

The study included 5 groups each including 10 female "Sprague Dawley" rats in addition to 20 male rats used as bone marrow donors. Group I rats received AM+MSCs, Group II rats AM+MSCs cultured with KGF-2, Group III rats AM+MSCs cultured with KGF-2+AS, Group IV rats only AM and Group V rats, none. AS was derived from blood drawn from male rats and bone marrow was obtained from the femur and tibia bones of the same animals. Therapeutic effect was evaluated with clinical, histopathological and immunohistochemical assessment. MSC engraftment was demonstrated via detection of donor genotype (Y+) in the recipient tissue (X) with polymerase chain reaction.

RESULTS

Corneal healing was significantly better in Groups I-III rats treated with MSC transplantation compared to Group IV and Group V rats with supportive treatment only. The best results were obtained in Group III rats with 90% transparency, 70% lack of neovascularization, and 100% epithelium damage limited to less than 1/4 of cornea.

CONCLUSION

We suggest that culture of MSCs with KGF-2 and AS on AM is effective in corneal repair in case of irreversible damage to limbal stem cells.

Comparison of stem cell properties in cell populations isolated from human central and limbal corneal epithelium

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.

Autoradiographic study on the regenerative capability of the epithelium lining the center of the cornea after multiple debridements of its peripheral region

BACKGROUND

The epithelium lining the center of the cornea is assumed to lack stem cells.The purpose is to investigate by autoradiography the regenerative capability of the epithelium lining the central region of the rabbit cornea following seven scrapings of its peripheral lining, during several months.

METHODS

After marking the center of the cornea with a 6 mm-diameter trephine, the epithelium outside this area was scraped until reaching the corneoscleral zone. This procedure was repeated seven times on the same eye at intervals of 20 days. One day after the last scraping, (3)H-thymidine was injected intravitreally and the corneas processed for autoradiography.

RESULTS

At 2 days after injection, the corneal surface was entirely lined by an epithelium made up by two layers of squamous cells, most of them being labeled with the DNA precursor. A multilayered epithelium was visualized at the center with most of its basal cells also labeled. The limbal epithelium had at least two of its layers labeled with the precursor. At 9 days, the multilayered central unscraped epithelium exhibited labeled cells not only in the basal but also in its suprabasal layers. The labeling index (labeled nuclei/100 cells) for its basal stratum was very close to 100%. A similar feature was observed at 16 days, except that the mutilayered central epithelium was seen lining a larger area when compared to the precedent interval and that it exhibited evidences for vertical renewal.

CONCLUSIONS

The epithelium lining the central region of the cornea--where it was assumed that stem cells do not exist--exhibited capability for regeneration and self-renewal in spite of seven consecutive debridements of its periphery. No evidence was found for transposition of limbal epithelial cells to the center of the cornea during the early merger of the epithelial sliding fronts.

Regeneration of the corneal epithelium after debridement of its central region: an autoradiographic study on rabbits

PURPOSE

To investigate the proliferative behavior of the corneal and limbal epithelia after debridement on the central region of the rabbit cornea.

METHODS

After scraping a circular epithelial area, 5 mm in diameter, in the center of the cornea, ([3]) H-thymidine ( ([3]) H-TdR) was injected intravitreally, and the rabbits killed from 1 to 49 days afterward. The cornea, together with the adjacent conjunctiva, was processed for autoradiography.

RESULTS

The regenerating epithelium at the center of the cornea exhibited high frequencies of labeled nuclei when compared to controls. The mitotic indexes for the limbus were comparable in experimental and control eyes. The unique basal stratum of the limbal epithelium exhibited quick proliferation and vertical migration in all eyes. Cells that remained labeled for four weeks or more were observed throughout the corneal epithelium, including its basal stratum, and this did not depend on epithelial damage.

CONCLUSION

Corneal epithelium wounds are healed by sliding and proliferation of cells surrounding the epithelial gap without any evidence for the participation of the limbal epithelium. Daughter cells labeled with ([3]) H-TdR were visualized in all layers of the corneal epithelium up to 7 weeks after the DNA precursor injection. However, at this long interval, the only labeled cells in the limbus were in the suprabasal layers.

Morphological and autoradiographic studies on the corneal and limbal epithelium of rabbits

The investigation was centered on the morphological features of the conjunctiva-cornea transition (limbus) of the rabbit eye and the proliferative behavior of its epithelium. The eyes were processed for examination with light and electron microscopy, as well as for autoradiography after intravitreal injection of [(3)H]thymidine ([(3)H]TdR). At the sites of extraocular muscle insertion, the vascularization of the stroma extended to the peripheral cornea, and the limbal epithelium was thin with its basal stratum made up by clear cuboidal cells. In between the muscle insertions, the cuboidal clear cells, as well as the stroma blood vessels, were scarce. At the light microscope level, the basement membrane was distinct in the cornea but not in the limbus or the conjunctiva. Autoradiographs demonstrated that, at the limbus, the basal cells migrated very quickly to the suprabasal region and remained there up to the 28-day interval. Labeled cells were identified in all epithelial layers of the cornea, including the basal one, at 21 and 28 days but not in the limbal basal clear cells. The rate of renewal of conjunctival epithelium was similar to that observed for the transition with scarce clear cells. The high-resolution autoradiographs demonstrated that the basal cuboidal clear limbal cells exhibit a quick renewal and that they are not label-retaining cells. These latter ones were detected all over the corneal epithelium and in the suprabasal layers of the limbus up to 28 days, in physiological conditions, without the need of stimulation by damage to the corneal epithelium.

Graft failure: II. Ocular surface complications

Risk factors for corneal transplantation failure include both immunologic factors, such as graft rejection, corneal neovascularization, and peripheral anterior synechiae, as well as non-immunologic factors, such as ocular surface disorders (OSD) and glaucoma. This review highlights the necessity of having healthy ocular surface epithelia, tears, and eyelids. It presents different types of OSD, their underlying pathology, and their impact on native cornea and corneal grafts. In addition, a range of proposed donor and surgical factors influencing surface integrity following corneal transplant are addressed. Current medical and surgical research, both pre- and post-operative that promise to further improve the outcome of corneal grafts in the context of OSD are discussed.

Stem cells: a revolution in therapeutics-recent advances in stem cell biology and their therapeutic applications in regenerative medicine and cancer therapies

Basic and clinical research accomplished during the last few years on embryonic, fetal, amniotic, umbilical cord blood, and adult stem cells has constituted a revolution in regenerative medicine and cancer therapies by providing the possibility of generating multiple therapeutically useful cell types. These new cells could be used for treating numerous genetic and degenerative disorders. Among them, age-related functional defects, hematopoietic and immune system disorders, heart failures, chronic liver injuries, diabetes, Parkinson's and Alzheimer's diseases, arthritis, and muscular, skin, lung, eye, and digestive disorders as well as aggressive and recurrent cancers could be successfully treated by stem cell-based therapies. This review focuses on the recent advancements in adult stem cell biology in normal and pathological conditions. We describe how these results have improved our understanding on critical and unique functions of these rare sub-populations of multipotent and undifferentiated cells with an unlimited self-renewal capacity and high plasticity. Finally, we discuss some major advances to translate the experimental models on ex vivo and in vivo expanded and/or differentiated stem cells into clinical applications for the development of novel cellular therapies aimed at repairing genetically altered or damaged tissues/organs in humans. A particular emphasis is made on the therapeutic potential of different tissue-resident adult stem cell types and their in vivo modulation for treating and curing specific pathological disorders.