Long-Term Survival of Isotopically Labeled Stromal and Endothelial Cells in Corneal Homografts*

Long-Term Dehydrated Donor Lamella Survival in Anterior Keratoplasty: Keratocyte Migration and Repopulation of Corneal Stroma

PURPOSE

The aim of this study was to investigate the ability of host keratocytes to colonize the donor lamella transplanted without viable cells (dehydrated) in Descemetic (deep anterior lamellar keratoplasty) and in pre-Descemetic keratoplasty (excimer laser-assisted lamellar keratoplasty).

METHOD

A total of 17 eyes (8 deep anterior lamellar keratoplasties and 9 excimer laser-assisted lamellar keratoplasties) were included in this observational retrospective study; patients underwent ophthalmic examinations, and histological staining was performed ex vivo on the graft in cases of failure.

RESULTS

In Descemetic keratoplasty, the long-term survival of the graft is compromised with the central corneal thickness decreasing; corneal pachymetry and in vivo and ex vivo keratocyte densities are significantly reduced (pachymetric reduction of -86 μm in the apex and -87 μm in the thinnest point; density cell reduction of 72% at a depth of 100 μm, 62% at a depth of 250 μm, and -66% at a depth of 400 μm). In pre-Descemetic keratoplasty, clinical complications, reduction of central thickness, or alterations of keratocyte density were not observed.

CONCLUSIONS

In Descemetic keratoplasty, the migration of the host peripheral keratocytes does not seem enough to repopulate the donor graft, whereas in pre-Descemetic keratoplasty, long-term survival of the graft is good. Keratocyte repopulation was observed only by extensive contact between the donor and host parenchyma.

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.

Hydrolase participation in allograft rejection in rat penetrating keratoplasty

BACKGROUND

A rat model of orthotopic corneal graft rejection was used to investigate the alterations in hydrolase activity within the corneal graft or within cellular infiltrates during acute rejection.

METHODS

The distribution of the lysosomal enzymes [acid phosphatase (AP), N-acetyl-beta-D-glucosaminidase (NAG), beta-glucuronidase (beta-Gluc), beta-galactosidase (beta-Gal), dipeptidylpeptidase II (DPPII)] and of the membrane-bound proteases [aminopeptidase M (APM), aminopeptidase A (APA), gamma-glutamyltransferase (GGT), alkaline phosphatase (ALP), dipeptidylpeptidase IV (DP-PIV)] were investigated by histochemical methods in the grafts at 3, 5, 8, 10 and 12 days following allogeneic transplantation. Serial sections of the grafts were also examined for RT1b, CD4, CD4+, CD8, CD11b/c and CD45, in order to determine hydrolase activity within infiltrating cells.

RESULTS

Allogeneic grafts were invaded by macrophages, CD4- and CD8-positive lymphocytes. In contrast, syngeneic grafts, performed as a control, contained occasional lymphocytes and focal aggregations of macrophages around suture sites. The allogeneic cellular infiltrate stained intensely for AP and ALP; moderately for beta-Gluc, NAG and beta-Gal; and mildly for GGT, DPPII and APM in grafts at all postoperative times. Serial sectioning indicated that the majority of the lysosomal hydrolases were located in macrophages; AP, APM and GGT were, however, observed in lymphocytes. Vessel ingrowth could be observed with enzyme staining for AP, beta-Gluc, NAG, ALP, APA and APM. Hydrolase activity in the corneal endothelium served as an indicator of endothelial function during the rejection process.

CONCLUSION

Changes in normal hydrolase activities in corneal grafts in the rat model indicate decreasing corneal function during the rejection process. Hydrolases released from infiltrating cells contribute to the morphological disruption and, possibly, to graft rejection.

The corneal endothelium

The endothelium is a monolayer of cells on the posterior corneal surface that transports water from the stroma into the anterior chamber. This movement of water counters a natural tendency for the stroma to swell and is necessary to maintain a transparent cornea. Embryologic studies, in particular the demonstration of the derivation of the endothelium from the neural crest, have provided insight into the factors that govern the response of this tissue to disease. In some species the endothelium can regenerate after injury, but in man cellular enlargement is the main mechanism of repair after cell loss. A clinical estimate of endothelial cell density and function is provided by specular microscopy, fluorophotometry and pachymetry. In this paper we review the development, structure and function of the corneal endothelium, and then consider the pathological processes that can affect this tissue.

Comparison of collagen degradation and synthesis in the peripheral and central areas of experimental corneal grafts

The distribution of collagen degradation and synthesis (turnover) between the central and peripheral areas of clear corneal allografts was determined, following transplantation of 6-mm [3H]proline-labelled corneal buttons from extensively pre-labelled, young rabbits into 13 nonradioactive mature rabbits. Control, contralateral 6-mm corneal buttons of each radioactive pair were frozen at -70 degrees C after being trephined into a central 3-mm button and a peripheral ring of cornea. The grafts were removed 10-100 days after the operation and trephined similarly to the paired controls. The central and peripheral areas of the graft and control corneal buttons were analysed separately for total hydroxyproline and radioactivity. A significant degradation of old collagen (as seen by the loss of total radioactivity) occurred early in both the central (19.4 +/- 6.1%) and peripheral (12.3 +/- 7.5%) areas of the grafted corneas, compared to the corresponding areas of the contralateral controls. A significant increase in new, non-radioactive collagen occurred in both the central (22.0 +/- 7.6%) and peripheral areas (30.6 +/- 6.1%); however, a significant net increase in total collagen occurred only in the peripheral area (18.3 +/- 11.4). Loss of original (old) collagen was significantly greater in the central area than in the peripheral area; whereas, the increase in new collagen was significantly greater in the peripheral area than the central area, accounting for a significant increase in total collagen. There was no correlation in collagen turnover with time after surgery. Collagen turnover occurs acutely throughout the clear corneal graft and thereafter is stable.

Collagen degradation and synthesis in experimental corneal grafts

Thirty-two weanling New Zealand white rabbits were labelled repeatedly with [2,3-3-H]-proline for 4 weeks. Four weeks after the end of labelling, 32 rabbits underwent bilateral 6 mm trephinations for donor purposes. One control cornea of each pair was frozen at -70 degrees C. The contralateral corneas were used for autografts, allografts, and xenografts. Grafts were observed from 7-200 days, then retrephined out for determination of loss in total radioactivity in hydroxyproline (specific for collagen), increase in newly synthesized collagen, and net change in collagen mass, by comparing with the matched, ungrafted control corneas. For all three transplant groups there was a small, but significant, decrease early in total radioactivity (loss of original collagen) and a significant increase in new collagen. These data also indicated that the loss of original collagen was replaced by an equivalent or greater increase in new collagen in all transplant groups. Significant relationships between graft clarity and collagen turnover were noted in both the auto- and allograft groups. The degradation of old collagen was significantly greater in the cloudy vs. the clear grafts; however, there was no significant difference in the increase in new collagen between these groups. A progressive loss of original collagen over time was noted in the cloudy autografts, but not the allografts. A trend toward a progressive increase in new collagen was noted over time in both the cloudy auto- and allografts. No relation for these variables to time, however, was noted in the clear grafts.

Thymidine incorporation by human corneal endothelium during organ culture

Nine corneas from 5 human adult donors were obtained 11 to 31 h post mortem. In 4 corneas the endothelium was wounded by freezing and in 1 cornea by mechanical means. Care was taken to minimize endothelial damage in the remaining 4 corneas. The corneas were incubated at 31 degrees C for 6 days in a tissue culture medium containing 3H thymidine. Autoradiographs were made of the flat preparations of the endothelium. All corneas contained radioactive endothelial cell nuclei, with the highest concentration of labelled nuclei being in the wound areas. The greatest number of labelled cells was found in the cornea from the youngest donor, 19 years of age, but thymidine uptake also occurred in the oldest cornea, 89-years-old, which additionally had signs of endothelial dystrophy.

Congenital endothelial corneal dystrophy. Clinical, pathological, and genetic study

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Fate of endothelium in corneal homografts. An experimental study

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The pathology of human aortic valve homografts

Beta-propiolactone sterilized, freeze-dried aortic valve homografts offer a dead framework which is accepted by the host and is capable of immediate and full function. The survival time of such grafts as fully functional units may be limited by physical and chemical alterations produced in the tissues by sterilization and freeze-drying. The organizing reaction of the host in covering the grafts or using them as a scaffolding may also be affected by these processes. It is possible that better long-term results may be achieved by using fresh grafts. There is no difference in host cellular response to fresh and sterilized and/or freeze-dried grafts. There is a possibility that heterogeneous reactions to polypeptides in the graft may occur in some individuals. Organization and covering of the graft by host tissue occurs from host tissues contiguous to the graft. Cells circulating in the bloodstream play no part in this by seeding on the surface. Thrombosis, in the absence of infection, is a rare complication. (Anticoagulants were not used in these patients.) Calcification occurs as only a late complication in persisting `dead' tissue. Unsuspected, and often extensive, myocardial ischaemia occurs frequently under bypass conditions with coronary artery perfusion and substantially contributes to immediate post-operative mortality and morbidity.