[Endothelial cell transplantation and growth behavior of the human corneal endothelium]

Biomaterials for corneal endothelial cell culture and tissue engineering

The corneal endothelium is the posterior monolayer of cells that are responsible for maintaining overall transparency of the avascular corneal tissue via pump function. These cells are non-regenerative in vivo and therefore, approximately 40% of corneal transplants undertaken worldwide are a result of damage or dysfunction of endothelial cells. The number of available corneal donor tissues is limited worldwide, hence, cultivation of human corneal endothelial cells (hCECs) in vitro has been attempted in order to produce tissue engineered corneal endothelial grafts. Researchers have attempted to recreate the current gold standard treatment of replacing the endothelial layer with accompanying Descemet's membrane or a small portion of stroma as support with tissue engineering strategies using various substrates of both biologically derived and synthetic origin. Here we review the potential biomaterials that are currently in development to support the transplantation of a cultured monolayer of hCECs.

Electrospun gelatin/PCL and collagen/PCL scaffolds for modulating responses of bone marrow endothelial progenitor cells

The determination of potential transplantable substrates and substitution cells for corneal endothelium transplantation may compensate for the shortage of cornea donors. Appropriate biodegradable and biocompatible tissue-engineered substratum with seed cells for endothelial keratoplasty has been increasingly studied. In the present study, electrospun gelatin/polycaprolactone (PCL) and collagen/PCL scaffolds were successfully established. Bone marrow endothelial progenitor cells (BEPCs) were cultured on these scaffolds to determine whether the scaffolds may promote the proliferation of BEPCs as well as maintain stem cell characteristics. Two variations of hybrid scaffolds, collagen/PCL (70% collagen and 30% PCL) and gelatin/PCL (70% gelatin and 30% PCL), were established via electrospinning. Microscopic structure, hydrophilicity and wettability of the two scaffolds were subsequently investigated. BEPCs were separately cultured on the scaffolds and were also seeded on glass slides to establish the control group. Furthermore, cell morphology; adherence, as determined by investigation of F-actin expression levels; proliferation, as determined via Cell Counting Kit-8 assays, Ki-67 staining and bromodeoxyuridine (BrdU) staining; and stem cell markers, as determined by cluster of differentiation (CD)-34 and CD-133 protein expression levels; were investigated. In addition, reverse transcription-quantitative polymerase chain reaction (RT-qPCR) was performed to determine gene expression. The two nanofiber scaffolds were established using electrospun techniques with expected hydrophilicity, wettability and biocompatibility. BEPCs were revealed to spread well on and strongly adhere to the collagen/PCL (70:30) and gelatin/PCL (70:30) scaffolds. Furthermore, Ki-67 and BrdU staining results revealed greater levels of positive dots on the two hybrid scaffolds compared with the control group. CD-34 and CD-133 protein staining demonstrated increased levels of fluorescence intensity on scaffolds compared with the control group. Furthermore, increased expression levels of differentiation markers, such as ATP binding cassette subfamily G member 2, leucine rich repeat containing G protein-coupled receptor 5 and CD166, were detected on both scaffolds. RT-qPCR results demonstrated that the expression of caspase-3, which is associated with apoptosis, was decreased on the two scaffolds compared with in the control group. The expression of inflammatory factors, including interleukin (IL)-1, exhibited a significant decrease on the gelatin/PCL scaffold compared with in the control group; whereas the difference between the expression level of IL-1 exhibited by the collagen/PCL group and the control group were not markedly different. Electrospun collagen/PCL and gelatin/PCL scaffolds exhibited the potential to enhance the adherence and proliferation of BEPCs. BEPCs cultured on the two scaffolds demonstrated increased stem cell characteristics and differentiation potential. Electrospun gelatin/PCL and collagen/PCL scaffolds may represent a promising substratum in tissue-engineered corneal endothelium.

Human Corneal Endothelial Cell Cultivation From Old Donor Corneas With Forced Attachment

Human corneal endothelial cells (HCEnCs) are responsible for maintaining the transparency of the cornea. Damaged or diseased HCEnCs may cause blindness. Replacement of the diseased cells with a healthy donor endothelium is the only currently available treatment. Tissue-engineering can serve as an alternative to conventional donor corneal transplantation. Due to the global shortage of donor corneas, a wide interest in the development of cultured graft substitutes and artificial corneas has increased. Availability of the old donor corneas is higher especially for research. Although it can be proposed as a valuable source for cell culture, its less proliferative capability emerges a challenge for the researchers. This article describes the use of hyaluronic acid (HA) in combination with Rho-kinase inhibitor (ROCK) Y-27632 for the cultivation of HCEnCs from older donor corneas (age > 60 years). Four conditions including and excluding HA + ROCK and its effect on early attachment rates and proliferation was studied on forty-eight corneas. It was observed that HCEnCs reach confluence within 10–15 days when cultured with HA + ROCK. This approach improves the efficiency of cell adhesion due to force attachment. HCEnCs from old donor corneas can be cultured using this method which may further lead to cell-based therapy for treating corneal endothelial dysfunction.

Stemness characteristics of human corneal endothelial cells cultured in various media

PURPOSE

To investigate stemness characteristics of human corneal endothelial cells (HCECs) cultured in various media.

METHODS

Human corneal endothelial cells were isolated using a sphere-forming assay. Cells were allowed to attach to the bottom of culture plates and were cultured in different media designated as medium A (Opti-MEM I with 8% fetal bovine serum), medium B (DMEM/F12 with B27 supplement), medium E (DMEM/F12 with epidermal growth factor [EGF]), and medium BE (DMEM/F12 with B27 supplement and EGF), respectively. Cell morphology was evaluated with an phase-contrast inverted microscope. Immunofluorescence staining and western blotting of nestin, octamer-binding transcription factor (OCT3/4), glial fibrillary acidic protein (GFAP), zonula occludens-1 (ZO-1), collagen VIII alpha2, and Na-K ATPase was performed. Cell proliferation was assessed with a cell counting kit-8 assay.

RESULTS

A few cultured cells stained with nestin. The cells cultured in medium A expressed high levels of GFAP, OCT3/4, and nestin, and higher levels of ZO-1 were expressed in the cells cultured in medium A and medium B compared with cells cultured in the other media. The cells cultured in medium A assumed a fibroblast-like shape, whereas the cells cultured in medium B and medium BE appeared as mosaics. Cell proliferation was highest in medium A compared with those cultured in the other media.

CONCLUSIONS

Cultured HCECs expressed stem cell markers, including nestin, OCT3/4, and GFAP. The expression of stem cell markers differed according to the culture media and associated proliferation rate.

Recovery of Corneal Endothelial Cells from Periphery after Injury

Background

Wound healing of the endothelium occurs through cell enlargement and migration. However, the peripheral corneal endothelium may act as a cell resource for the recovery of corneal endothelium in endothelial injury.

Aim

To investigate the recovery process of corneal endothelial cells (CECs) from corneal endothelial injury.

Methods

Three patients with unilateral chemical eye injuries, and 15 rabbit eyes with corneal endothelial chemical injuries were studied. Slit lamp examination, specular microscopy, and ultrasound pachymetry were performed immediately after chemical injury and 1, 3, 6, and 9 months later. The anterior chambers of eyes from New Zealand white rabbits were injected with 0.1 mL of 0.05 N NaOH for 10 min (NaOH group). Corneal edema was evaluated at day 1, 7, and 14. Vital staining was performed using alizarin red and trypan blue.

Results

Specular microscopy did not reveal any corneal endothelial cells immediately after injury. Corneal edema subsided from the periphery to the center, CEC density increased, and central corneal thickness decreased over time. In the animal study, corneal edema was greater in the NaOH group compared to the control at both day 1 and day 7. At day 1, no CECs were detected at the center and periphery of the corneas in the NaOH group. Two weeks after injury, small, hexagonal CECs were detected in peripheral cornea, while CECs in mid-periphery were large and non-hexagonal.

Conclusions

CECs migrated from the periphery to the center of the cornea after endothelial injury. The peripheral corneal endothelium may act as a cell resource for the recovery of corneal endothelium.

Tissue engineering of the corneal endothelium: a review of carrier materials

Functional impairment of the human corneal endothelium can lead to corneal blindness. In order to meet the high demand for transplants with an appropriate human corneal endothelial cell density as a prerequisite for corneal function, several tissue engineering techniques have been developed to generate transplantable endothelial cell sheets. These approaches range from the use of natural membranes, biological polymers and biosynthetic material compositions, to completely synthetic materials as matrices for corneal endothelial cell sheet generation. This review gives an overview about currently used materials for the generation of transplantable corneal endothelial cell sheets with a special focus on thermo-responsive polymer coatings.

Progenitors for the corneal endothelium and trabecular meshwork: a potential source for personalized stem cell therapy in corneal endothelial diseases and glaucoma

Several adult stem cell types have been found in different parts of the eye, including the corneal epithelium, conjunctiva, and retina. In addition to these, there have been accumulating evidence that some stem-like cells reside in the transition area between the peripheral corneal endothelium (CE) and the anterior nonfiltering portion of the trabecular meshwork (TM), which is known as the Schwalbe's Ring region. These stem/progenitor cells may supply new cells for the CE and TM. In fact, the CE and TM share certain similarities in terms of their embryonic origin and proliferative capacity in vivo. In this paper, we discuss the putative stem cell source which has the potential for replacement of lost and nonfunctional cells in CE diseases and glaucoma. The future development of personalized stem cell therapies for the CE and TM may reduce the requirement of corneal grafts and surgical treatments in glaucoma.

Using Basement Membrane of Human Amniotic Membrane as a Cell Carrier for Cultivated Cat Corneal Endothelial Cell Transplantation

PURPOSE

To study the feasibility of using basement membrane of human amniotic membrane (BMHAM) as a carrier for transplantation of cultivated cat corneal endothelial cells (cCCECs).

METHODS

BHMAM was obtained by enzymic digestion. cCCECs were seeded on the BMHAM and cultivated traditionally. The resulting continuous monolayer of cCCECs was transplanted onto the cat corneal graft stripped of the Descemet membrane with endothelium. To determine whether the transplanted cCCECs were vital and functional in vivo, the corneal grafts were examined by slit-lamp microscope every day for 6 weeks, and corneal thickness was measured by ultrasonic pachymetry. Either in vivo or in vitro, the cCCEC sheets on BMHAMs were examined morphologically by light and electron microscope, and the cell density was measured.

RESULTS

Seven to 10 days after seeding on the BMHAM, the cCCECs were confluent and formed a continuous monolayer with 3486 +/- 53 cells/mm(2) cell density. Like normal corneal endothelial cells, the cCCECs were almost hexagonal, squamous, and uniform in size. After transplantation, most cells were vital and functional nearly enough to maintain corneal graft thickness and transparency without rejection for at least 6 weeks. Six weeks after operation, the average thickness of the transplanted corneal grafts was only slightly greater than that before operation. Compared with that in vitro, after transplantation there was 5% to 8% reduction per week in cell density, which lasted for almost 3 weeks. After that, the average cCCEC density of corneal grafts was 2837 +/- 57 cells/mm(2) and quite stable maintained.

CONCLUSIONS

This study demonstrated that BMHAM would be an ideal alternative for corneal Descemet membrane and a cell carrier for cCCEC transplantation.

Technique for Preparation of the Corneal Endothelium-Descemet Membrane Complex for Transplantation

PURPOSE

Replacing diseased corneal endothelium with a preparation of Descemet membrane carrying functional endothelium and no stroma may be a feasible method for treating corneal endothelial decompensation. To obtain a viable donor of a Descemet membrane endothelium disc, we modified the Descemet membrane stripping technique and monitored the percentage of endothelial damage to the donor tissue preparation.

METHODS

Forty-eight human corneas were used. Cornea buttons were mounted on an artificial anterior chamber, endothelial side up. Endothelia were stained with alizarin red, examined under the microscope, and photographed at 5 different sites (microscope, x100; digital magnification, x2.83). A 6 x 7-mm rectangular piece of endothelium-Descemet membrane complex was obtained using a Grieshaber microsurgical knife and Kelman-McPherson forceps. Digital photographs of endothelia were analyzed with a computer, and the percentage of endothelial damage was calculated. Specimens were processed for hematoxylin-eosin staining.

RESULTS

Forty of 48 endothelium-Descemet membrane preparations (83.3%) were complete peels with minimal endothelial damage. Endothelial damage before and after the surgery was 1.57 +/- 2.11% and 2.61 +/- 1.77%, respectively. Eight preparations (16.7%) failed because of tearing. Multiple hematoxylin-eosin-stained sections showed the presence of endothelium with intact Descemet membrane and no stromal tissue.

CONCLUSION

We modified the technique of Melles and obtained a sheet of Descemet membrane and endothelium with minimal endothelial damage and with no remaining stroma observed. This simple technique can be used to obtain the endothelium-Descemet membrane complex in minutes. It may be useful for corneal endothelium transplantation.

EGF suppresses hydrogen peroxide induced Ca2+ influx by inhibiting L-type channel activity in cultured human corneal endothelial cells

Endogenous generated hydrogen peroxide during eye bank storage limits viability. We determined in cultured human corneal endothelial cells (HCEC) whether: (1) this oxidant induces elevations in intracellular calcium concentration [Ca2+]i; (2) epidermal growth factor (EGF) medium supplementation has a protective effect against peroxide mediated rises in [Ca2+]i. Whereas pathophysiological concentrations of H2O2 (10 mM) induced irreversible large increases in [Ca2+]i, lower concentrations (up to 1 mM) had smaller effects, which were further reduced by exposure to either 5 microM nifedipine or EGF (10 ng ml(-1)). EGF had a larger protective effect against H2O2-induced rises in [Ca2+]i than nifedipine. In addition, icilin, the agonist for the temperature sensitive transient receptor potential protein, TRPM8, had complex dose-dependent effects (i.e. 10 and 50 microM) on [Ca2+]i. At 10 microM, it reversibly elevated [Ca2+]i whereas at 50 microM an opposite effect occurred suggesting complex effects of temperature on endothelial viability. Taken together, H2O2 induces rises in [Ca2+]i that occur through increases in Ca2+ permeation along plasma membrane pathways that include L-type Ca2+ channels as well as other EGF-sensitive pathways. As EGF overcomes H2O2-induced rises in [Ca2+]i, its presence during eye bank storage could improve the outcome of corneal transplant surgery.

Quantitative evaluation of the corneal endothelium in the mouse after grafting

BACKGROUND/AIM

Corneal graft survival depends critically on the quality of the endothelium. In this study the authors aimed to evaluate corneal endothelium in mice at different times after transplantation and to correlate endothelial integrity with corneal graft survival.

METHODS

Syngeneic and allogeneic corneal grafts at various times (days 0-60) after engraftment were examined in flat mount preparation by confocal microscopy, by evaluating the hexagonal pattern of the endothelial monolayer using actin staining of the cell cortex. Corneas from untreated mice and from mice, who were grafted after removal of draining lymph nodes served as controls.

RESULTS

In control corneas, more than 90% of the posterior surface was covered by endothelium. Syngeneic grafts were always covered by 54-99% of endothelium. In contrast, the posterior surface of corneal allografts showed great variation in the degree of endothelial cell coverage (0-98%). In addition, clinical opacity grading measure was not a reliable predictor of endothelial coverage.

CONCLUSION

In corneal allografts there is progressive loss of endothelium over time, unlike with syngeneic grafts. However, in the early stages of allograft rejection, the grade of graft opacity does not accurately reflect the degree of endothelial cell coverage. Although corneal opacity grade is considered the main determinant of graft rejection, the data suggest that both the grade of corneal opacity plus a sufficient post-graft time duration (>8 weeks in the mouse) are required for the diagnosis of irreversible graft rejection.

Microfilament disruption in a noncycling organized tissue, the corneal endothelium, initiates mitosis

The adult corneal endothelium represents a noncycling cell population that resides as a monolayer on its basement membrane, Descemet's membrane. Evidence is presented for the first time, showing that mitotic regulation in this organized tissue, residing on its natural basement membrane, is coupled to microfilament integrity. When mitotically quiescent rat corneal endothelia are organ cultured in medium containing serum and cytochalasin B, low levels of mitosis are initiated. Supplementing the culture medium with either insulin or IGF-2 augments this response and results in increased cell density within the tissue monolayer. Fluorescence microscopy of actin using TRITC-conjugated phalloidin revealed that cellular circumferential microfilament bundles appear unaffected by cytochalasin B treatment, whereas the cytoplasmic microfilaments appear to be completely disrupted. These results suggest the possibility that the actin cytoskeleton is involved with the regulation of cell growth in the corneal endothelium.

Cellular changes in transplanted human corneas

PURPOSE

To measure endothelial cell and keratocyte densities in transplanted corneas and the changes in these densities with time.

METHODS

The endothelia of 500 consecutive penetrating corneal transplants were studied longitudinally by specular microscopy for 10 to 20 years. The keratocytes of 36 corneal transplants that varied in postoperative times from 1 month to 20 years were studied cross-sectionally by clinical confocal microscopy. The keratocytes of five transplanted corneas were studied longitudinally by confocal microscopy at 1 day, 1 week, and 1 month postkeratoplasty.

RESULTS

Endothelial cell density decreased progressively at an accelerated rate for 20 years after transplantation, with concurrent increases in the coefficient of variation of cell area and corneal thickness and decreases in the percentage of hexagonal cells. Grafts with insufficient endothelial cells developed late endothelial failure, which was the primary cause of graft failure after the first 5 postoperative years. The grafts with late endothelial failure did not lose endothelial cells faster than grafts that did not fail, but instead had fewer cells immediately after transplantation, diminishing to a critically low cell density earlier. The keratocyte density was also decreased in transplanted corneas. Keratocytes became "activated" during the first week after keratoplasty and in grafts with late endothelial failure.

CONCLUSION

It should be possible to prevent or delay late endothelial failure, the primary cause of graft failure, by increasing the number of endothelial cells on transplanted corneas. The status of the keratocytes appears to affect corneal transparency and, thus, visual quality in the grafted eye.