Electron microscopy of cultured human corneas. Osmotic hydration and the use of a dextran fraction (dextran T 500) in organ culture

Culturing Limbal Epithelial Cells of Long-term Stored Corneal Donors (Organ Culture) In Vitro - A Stepwise Linear Regression Algorithm

PURPOSE

To assess various potential factors on human limbal epithelial cell (LEC) outgrowth in vitro using corneal donor tissue following long-term storage (organ culture) and a stepwise linear regression algorithm.

METHODS

Of 215 donors, 304 corneoscleral rings were used for our experiments. For digestion of the limbal tissue and isolation of the limbal epithelial cells, the tissue pieces were incubated with 4.0 mg/mL collagenase A at 37 °C with 95% relative humidity and a 5% CO2 atmosphere overnight. Thereafter, limbal epithelial cells were separated from limbal keratocytes using a 20-µm CellTricks filter. The separated human LECs were cultured in keratinocyte serum-free medium medium, 1% penicillin/streptomycin (P/S), 0.02% epidermal growth factor (EGF), and 0.3% bovine pituitary extract (BPE). The potential effect of donor age (covariate), postmortem time (covariate), medium time (covariate), size of the used corneoscleral ring (360°, 270°180°, 120°, 90°, less than 90°) (covariate), endothelial cell density (ECD) (covariate), gender (factor), number of culture medium changes during organ culture (factor), and origin of the donor (donating institution and storing institution, factor) on the limbal epithelial cell outgrowth was analyzed with a stepwise linear regression algorithm.

RESULTS

The rate of successful human LEC outgrowth was 37.5%. From the stepwise linear regression algorithm, we found out that the relevant influencing parameters on the LEC growth were intercept (p < 0.001), donor age (p = 0.002), number of culture medium changes during organ culture (p < 0.001), total medium time (p = 0.181), and size of the used corneoscleral ring (p = 0.007), as well as medium time × size of the corneoscleral ring (p = 0.007).

CONCLUSIONS

The success of LEC outgrowth increases with lower donor age, lower number of organ culture medium changes during storage, shorter medium time in organ culture, and smaller corneoscleral ring size. Our stepwise linear regression algorithm may help us in optimizing LEC cultures in vitro.

Thickness and Curvature Changes of Human Corneal Grafts in Dextran-Containing Organ Culture Medium Before Keratoplasty

PURPOSE

To determine the changes of corneal thickness and curvature of human corneal grafts in organ culture medium II, containing dextran T500 6%, before keratoplasty.

METHODS

We examined the tomography of 24 corneas from our eye bank transferred from medium I into medium II. Images were repeated hourly during 24 hours using an anterior segment optical coherence tomography. The central corneal thickness (CCT) was measured with the manual measurement tool of the anterior segment optical coherence tomography. The radii of curvature (anterior flat and steep and posterior flat and steep) were measured with a MATLAB self-programmed software for "sterile donor tomography."

RESULTS

The mean CCT (±SD) at baseline (T0) was 727 ± 156 μm. It reached 581 ± 103, 506 ± 84, 472 ± 79, and 456±7 μm after 6, 12, 18, and 24 hours, respectively. After 12 hours, 83% of the final deswelling was achieved. The radii of curvature (±SD) at baseline (T0) were (posterior flat, posterior steep, anterior flat, and anterior steep) 6.6 ± 0.5, 6.2 ± 0.5, 7.7 ± 0.4, and 7.4 ± 0.4 mm, respectively. After 24 hours, the radii of curvature reached 6.8 ± 0.1, 6.6 ± 0.3, 7.6 ± 0.1, and 7.4 ± 0.2 mm, respectively.

CONCLUSIONS

The kinetics of the deswelling process in medium II follow a hyperbolic curve. Considering a CCT of 506 μm at T12, we assume that a time interval of 12 hours in medium II might be enough for clinical purposes. This result might help to keep storage in medium II as short as possible to escape potential toxic effects of dextran in medium II. The radius of curvature does not seem to change within 24 hours for all measured surfaces.

Dehydration of corneal anterior donor tissue with polyethylene glycol (PEG)-enriched media

Anterior donor grafts (including scleral rim, without Descemet membrane) increase in thickness and become hazy upon storage in organ culture (OC) medium. Transfer of these grafts to standard dehydration media just before transplantation does not reduce their thickness to normal. Therefore, we assessed the efficacy of different media enriched with polyethylene glycol (PEG) as dehydrating agents for organ-cultured anterior donor grafts. Grafts were harvested and stored in the commercial OC medium 'Max' (without dextran) for 1 week, and subsequently dehydrated in the standard commercial dehydration medium 'Jet' (with dextran) supplemented with 4-20% PEG3350, or 'Max' supplemented with 20% PEG6000 and PEG20.000, or 5-20% PEG35.000. Central corneal thickness (CCT), as assessed by anterior segment-optical coherence tomography, and transparency were evaluated before, and at 1, 4 and 7 days of dehydration. Transfer of grafts after 1 week of OC (average 1,200 µm) to 'Jet' supplemented with PEG3350 revealed a concentration-dependent effect of dehydration; CCT was restored to normal (500-600 µm) when 10% PEG3350 was added. However, transparency was only temporarily restored; after 1 day, the grafts turned hazy. In contrast, grafts transferred to 'Max' supplemented with 20% PEG35.000 were transparent throughout the evaluation period, but were dehydrated to beyond normal levels (average 300 µm). 'Max' supplemented with 5% PEG35.000 dehydrated grafts to normal values and restored transparency throughout. Thus, dehydration of anterior donor grafts prior to surgery in dextran-free OC medium supplemented with 5% PEG35.000 reduces graft thickness to normal and may facilitate anterior keratoplasty procedures.

Characteristics of the low density corneal endothelial monolayer

Corneal endothelial cells form a leaky barrier on the posterior surface of the cornea, allowing influx of nutrient-carrying aqueous humor through the paracellular space and efflux of excess fluid. Corneal edema arises when the density of these non-proliferative endothelial cells declines from endothelial disease or intraocular surgery. The cellular changes occurring at low densities are ill-defined. We therefore investigated the paracellular pathway of corneal endothelial cell monolayers of varying density to determine alterations occurring in paracellular permeability and monolayer morphology. Primary cultures of bovine corneal endothelial cells (BCECs) were passaged onto permeable supports under varying culture conditions to obtain confluent monolayers of <1000, 1000-1999 and >2000 cells/mm(2). Culture growth was monitored by transendothelial electrical resistance measurements. Diffusional permeability to sodium fluorescein, FITC-dextran MW 4000 or FITC-dextran MW 20,000 was measured. Confluent cultures were also analyzed by immunofluorescence localization of the tight junction protein ZO-1 and by transmission electron microscopy. For comparison, we evaluated ZO-1 for low and high density human corneal endothelium. Our results showed that all BCEC cultures grew to the same final transendothelial electrical resistance regardless of final density. In the diffusional permeability assay, permeability increased significantly only for the smallest tracer molecule (sodium fluorescein) in the lowest density monolayers (<1000 cells/mm(2)). ZO-1 immunofluorescence distinctly localized to intercellular junctions in high density BCEC cultures but had more diffuse localization at lower densities. Transmission electron microscopy imaging revealed cells with thinner cross-sectional profiles and longer overlapping intercellular processes at low density relative to high density cultures. Low density human corneal endothelium lacked the diffuse ZO-1 distribution seen in BCECs. Our data supports the hypothesis that barrier integrity is the primary function disrupted in low density corneal endothelial monolayers and contradicts the idea of a linear decline in barrier function with decreasing cell density.

Identification and evaluation of a thinning agent compatible with MegaCell DCS, an animal product-free corneal storage medium

Purpose

MegaCell DCS, an animal product-free culture medium formulated for storing corneas, is superior to the traditionally used MEM (Eagle’s) with Earles salts, Hepes, and supplemented with foetal calf serum (2 %), glutamine and an antibiotic cocktail (EB MEM). Because this medium does not prevent corneal swelling, and Dextran T500, which is traditionally used for reversing this process before transplant may have adverse effects on corneas, the purpose of the current investigation was to identify an alternative polymer that is compatible with MegaCell DCS.

Methods

Corneas maintained in MegaCell DCS or EB MEM were transferred to either EB MEM 5 % Dextran T500 or MegaCell DCS containing 5 % Dextran T500, 4 % polyethylene glycol (PEG) 10,000, PEG 35,000 (2 %, 3 %, 4 %) or Poloxamer 188 (4 %). Endothelial cell losses were determined and corneal hydration levels measured. Stromal cell cultures were generated and immunostained with anti α-SMA antibody. Janus Green was used to compare the viability of endothelial cells of corneas maintained in MegaCell DCS and EB MEM and respectively thinned with PEG 35,000 and Dextran T500.

Results

The rates of endothelial cell loss from corneas held in MegaCell DCS and thinned in MegaCell DCS containing 5 % Dextran T500, 4 % PEG 10,000 and 4 % Poloxamer 188 for 6 days were similar. When explants of these corneas were cultured myofibroblasts were generated. Although at concentrations of 4 % (w/v) both PEG 10,000 and Poloxamer 188 caused excessive dehydration, the hydration levels of corneas held in MegaCell DCS containing 3 % PEG 35,000 were similar to those of corneas held in EB MEM 5 % Dextran T500. Endothelial cell losses after 6 days were negligible, explants of the corneas generated uniform fibroblastic stromal cell cultures and the extents of Janus Green staining were similar. Over 20 days the inclusion of 5 % Dextran T500 in EB MEM but not 3 % PEG 35,000 in MegaCell DCS, increased the rate of endothelial cell loss.

Conclusion

PEG 35,000 at a concentration of 3 % w/v does not induce endothelial cell loss and is compatible with MegaCell DCS for thinning corneas prior to transplantation.

Evaluation of interface quality in organ-cultured lamellar corneal transplants

BACKGROUND

With increasing numbers of lamellar keratoplasties, eye banks are challenged to deliver precut lamellar donor tissue. In Europe, the most common technique of corneal storage is organ culture which requires a deswelling process before surgical processing. The aim of this study was to investigate the influence of different deswelling times on the cutting plane quality after microkeratome-assisted lamellar dissection.

METHODS

Eight paired donor corneas (16 specimens) not suitable for transplantation were organ cultured under standard conditions at the Eye Bank of the Ludwig-Maximilians Universität, Munich, Germany. Pairs of corneal buttons were analyzed during the deswelling process in dextrane-containing medium. While one cornea was cut at an early time point during the deswelling process and put back into deswelling medium thereafter, the partner cornea was completely deswollen and dissected after 72 hours. Specimens were then further processed for scanning electron microscopy. Surface quality was assessed both digitally using Scanning Probe Imaging Processing software, and manually by three blinded graders.

RESULTS

The corneal buttons processed at the beginning of the deswelling process had a smoother surface when compared to the partner cornea that was cut at the end of the deswelling process. In our setting, no relevant difference was detectable between manual and automated microkeratome dissection.

CONCLUSION

For lamellar keratoplasty, organ-cultured corneas should be processed at an early stage during the deswelling process. We interpret the smoother dissection plane during early deswelling as a result of mechanical properties in a highly hydrated cornea.

Poloxamines for deswelling of organ-cultured corneas

BACKGROUND

Poloxamines are amphiphilic tetrofunctional block copolymers composed of four polyoxyethylene-polyoxypropylene arms joined to a central ethylene diamine bridge. Their safe profile allows diverse pharmaceutical and biomedical applications.

AIM

To assess their use for corneal deswelling using a porcine model of organ culture (OC).

METHODS

Five poloxamines (T90R4, T904, T908, T1107 and T1307) were dissolved in a standard commercial OC medium (control) to reach 350 mosm kg(-1). In vitro cytotoxicity was tested using MTT assay on human corneal epithelial and endothelial cell (EC) lines and on primary human corneal fibroblasts. Paired porcine corneas stored in OC for 3 days were assigned for 48 h to a poloxamine medium or to a standard deswelling medium containing 5% dextran T500. Corneal EC density, morphometry, mortality, stromal thickness and transparency were evaluated before and after deswelling. Post-deswelling, EC viability/mortality was determined using a fluorescent live/dead assay.

RESULTS

Besides similar corneal thickness reduction and transparency improvement, T908, T1107 and T1307 decreased EC loss (5.4 ± 1.7% vs. 9.9 ± 2.6% in controls (p < 0.001)) and mortality, improved EC morphometry and reduced endothelial lesions compared to dextran.

CONCLUSION

On this porcine model, poloxamines T908, T1107 and T1307 appear as good candidates to replace dextran for the deswelling. Experiments on human corneas are now necessary to confirm their efficiency and safety profile in OC.

Optimizing the deswelling process of organ-cultured corneas

PURPOSE

During deswelling of organ-cultured human corneas, endothelial cell loss occurs. Therefore, it is necessary to minimize the deswelling time and achieving an optimal central corneal thickness (CCT) of approximately 550 microm at the same time. We investigated the minimal deswelling time necessary and analyzed endothelial cell loss.

METHODS

Fifty-eight human corneas were stored between 13 and 81 days in organ culture. CCT was measured by optical coherence tomography. Measurements were performed before preparation, during culturing, before deswelling, and after varying deswelling periods (1-72 hours) using 5% dextran. Additionally, vital staining was performed in 6 human corneas to assess endothelial cell loss between 24 and 30 hours of deswelling. To evaluate absolute cell loss, endothelial cells were counted on human corneal pairs after 24 and 30 hours of deswelling.

RESULTS

After organ culture, mean CCT was 1194 microm. After 24 hours of deswelling in dextran-containing medium, mean CCT was 600 microm, whereas after 30 hours, mean CCT was 510 microm and hardly any corneas showed a CCT of more than 550 microm. Almost no further decrease in CCT was observed thereafter. No factors could be identified predicting the necessary deswelling time; however, paired corneas showed significant correlation of deswelling characteristics. We did not see any differences in endothelial cell loss 24 and 30 hours of deswelling or the ratio of living to dead endothelial cell counts.

CONCLUSIONS

Deswelling for 24 hours does not provide an optimal corneal thickness. Because endothelial cell loss does not increase between 24 and 30 hours of deswelling, a period of 30 hours is more suitable for obtaining sufficient corneal thickness.

The effects of organ-culture on the density of keratocytes and collagen fibers in human corneas

PURPOSE

Keratocytes are important in regaining corneal transparency during wound healing after surgery or trauma. Hitherto, there are still controversies concerning the effects of organ culture on the density and integrity of keratocytes and collagen fibers. The current study aimed at a systematic analysis of the effects of organ-culture on the morphology and density of keratocytes and collagen fibers.

METHODS

Human corneas were organ-cultured in MEM for 7 (n = 17, 3 pairs), 14 (n = 18, 9 pairs) and 21 days (n = 18, 9 pairs). Of the pairs one cornea was processed in swollen condition and the fellow cornea after reversal of swelling in MEM plus Dextran. Eleven post-mortem corneas (PM) and 11 fresh corneas obtained from melanoma patients were used as controls. Stromal thickness, number of keratocyte profiles (corrected for swelling), number and diameter of collagen fibers were measured in light microscopical sections and electron micrographs.

RESULTS

Stromal swelling due to organ-culture resulted in large keratocyte profiles with many vacuoles and large distances between collagen fibers in the posterior stroma. In contrast both keratocytes and distances between collagen fibers were not affected in the anterior stroma. After reversed-swelling the posterior corneal stroma was similar to that in fresh controls, indicating that the swelling process is largely reversible. The initial decrease in keratocyte density (18%) in the early post-mortem period did not progress during 21 days of organ culture.

CONCLUSION

With respect to the morphology and density of keratocytes and collagen fibers it can be concluded that donor corneas remain suitable for transplantation up to at least 21 days after organ-culture.

Modulation of HLA-DR and CD1a expression on human cornea with low-dose UVB irradiation

PURPOSE

To evaluate the effects of low-dose UVB irradiation of HLA and CD1a expression and the toxic effects of UVB on human corneas.

METHODS

24 pairs of human corneas from 24 donors were studied. One cornea from each pair was randomly irradiated with UVB (100 mJ/cm2) after enucleation. All corneas were then organ-cultured for 2, 7, 14 or 21 days. Endothelium was studied after enucleation and organ culture. Following preservation, corneas were evaluated by means of light microscopy, morphometry and TEM. HLA and CD1a staining was performed using an immuno-alkaline-phosphatase technique.

RESULTS

Endothelial cell loss during organ culture averaged 9.1% in the UVB group and 9.2% in the control group (NS). The number of rosette and reformation figures (p = 0.004) and the coefficient of variation (p = 0.014) were higher in the control group. Epithelial sloughing was more accentuated in the UVB group. We observed the same moderate ultrastructural injuries in both groups. In the epithelium, the average number of HLA-DR+ cells per field was 0.12 in the UVB group and 0.42 in the control group (p = 0.035). In the stroma, these figures were respectively 1.04 and 1.34 (p = 0.026). In the epithelium, the average number of CD1a + cells was respectively 0. 025 and 0.078 (p = 0.019). In the preservation mediums, the average percentage of CD1a + cells was 0.07% in the UVB group and 0.27% in the control group (p = 0.014).

CONCLUSIONS

Low-dose UVB (100 mJ/cm2) decreases HLA-DR and CD1a expression of organ-cultured human corneas and induces moderate corneal injuries. Low-dose UVB might be useful for preventing allograft rejection.

Factors affecting bovine corneal endothelial cell density in vitro

AIMS

To examine factors influencing the density and contact inhibition of bovine corneal endothelial cells cultured in vitro.

METHODS

Cell counts were performed on bovine corneal endothelial cells cultured for various times in the presence of 10% fetal calf serum, with or without varying concentrations of growth factors, 5% dextran T-500, or 2% chondroitin sulphate, at 32 degrees C or 37 degrees C, and after treatment with beta galactosidase.

RESULTS

Both basic fibroblast growth factor (FGFb) and retinal crude extract (RCE), but neither epidermal growth factor (EGF) nor acidic fibroblast growth factor (FGFa), increased endothelial cell density in vitro (p < 0.05). Continuous exposure to RCE resulted in a higher cell density than did a 24 hour pulse (p < 0.01), and higher cell densities were achieved at 37 degrees C than at 32 degrees C (p < 0.0001). In the absence of RCE, dextran T-500 increased cell density modestly (p < 0.05); in the presence of RCE, the addition of dextran T-500 had no effect on final cell density, whereas chondroitin sulphate significantly decreased final cell density (p < 0.01). In the absence of exogenous growth factors, beta galactosidase treatment resulted in a 50% increase in final cell density compared with controls (p < 0.0001).

CONCLUSIONS

Bovine corneal endothelial cell growth can be augmented under conditions different from those used in corneal preservation systems. The final cell density in a confluent monolayer can be increased by treatment with beta galactosidase, suggesting that corneal endothelial cells may be contact inhibited through a beta galactosidase sensitive receptor system.

An in vitro and clinical comparison of corneal storage with chondroitin sulfate corneal storage medium with and without dextran

The safety and efficacy of 1% dextran in Chondroitin Sulfate Corneal Storage Medium (CSM) in reducing corneal swelling after 4 degrees C storage was assessed in a corneal endothelial cell culture system. No difference was found in 3H-thymidine incorporation by cells incubated in either CSM-dextran medium or CSM medium alone. Subsequently, 21 pairs of corneas, stored in either CSM or CSM-dextran from 30 to 112 hours, were transplanted into 42 eyes of 42 patients, paired by diagnostic group and procedure. All CSM grafts and 19 of 21 CSM-dextran grafts were clear at 4 months with no primary donor failures in either group. Intraoperative corneal thickness was significantly greater in the CSM group (0.82 +/- 0.07 mm) than the CSM-dextran group (0.76 +/- 0.06 mm); however, the two groups did not differ thereafter. No differences in all endothelial morphometric parameters were noted between the two groups pre- and postoperatively. Average endothelial cell loss by 4 months was 13.0 +/- 16.4% for the CSM group and 16.4 +/- 15.5% for the CSM-dextran group. The addition of dextran to CSM medium results in significant intraoperative corneal thinning without adversely affecting endothelial DNA synthesis in vitro and endothelial survival in vivo.

Swelling tendency of rabbit cornea in organ culture medium. Influence of button size

The swelling tendency of fresh rabbit corneas kept in organ culture was determined by measuring percentage weight increase. Corneal buttons with diameters of 5, 9, 14, and 16 mm had an average weight increase during the first hour of 53%, 35%, 18%, and 8.6%, respectively. Those with diameter 5 and 9 mm had a continuous weight increase up to a maximum of 350% to 450% of start weight after 25 h and 57 h, respectively. Many corneas with 16 mm diameter (including a thin scleral rim) and some with 14 mm, had a temporary stable weight of average 56% (with an individual cornea stabilizing at only 15%) above start weight between day 2 to day 7. Thereafter a phase with approximately linear weight increase ended at a maximum weight of 350% to 450% of start weight after one month. It is concluded that the rate of corneal swelling in cultivation is highly dependent upon the corneal button size.

Organ-culture preservation of human corneas

Human corneas were preserved up to 40 days in a modified tissue culture medium at 31 degrees C. The corneal endothelium was examined by light microscopy before and after culture. After staining with trypan-blue the number of dead cells was counted and by swelling of the intercellular borders in a 1.8 per cent sucrose solution the cellular mosaic was observed. A loss of endothelial cells was found varying from 0-30 per cent. During culture the stroma increased considerably in thickness. Prior to transplantation the cornea was thinned during 24 h in culture medium containing 5 per cent Dextran T500. The combination of the organ culture procedure and the evaluation of the endothelium enables preservation of human corneas for at least 30 days. In addition the quality of the endothelium is guaranteed and the transport of corneas can be carried out at room temperature.