The effect of ultraviolet-B irradiation on the cell shedding rate of the corneal epithelium

Factors associated with keratopathy in captive pinnipeds

OBJECTIVE To identify factors associated with keratopathy in captive pinnipeds and to provide guidance for preventive measures. ANIMALS 319 captive pinnipeds (229 otariids [sea lions and fur seals], 74 phocids [true seals], and 16 odobenids [walrus]) from 25 facilities. PROCEDURES Descriptive data collected from questionnaires completed by facilities and from medical records and physical examinations of pinnipeds were compiled and evaluated. Variables were assessed with χ² tests of homogeneity to determine potential association with keratopathy, and variables with values of P ≤ 0.25 were inserted into the multivariable logistic regression model. RESULTS Results indicated that variables associated with significantly increased odds of keratopathy in captive pinnipeds included lighter or reflective pool color (OR, 2.11; 95% confidence interval [CI], 1.20 to 3.97), pool water salinity < 29 g/L (OR, 3.48; 95% CI, 1.89 to 6.56), and history of eye disease (OR, 3.30; 95% CI, 1.85 to 5.98), trauma (OR, 3.80; 95% CI, 1.72 to 8.89), and having been tested for leptospirosis (OR, 3.83; 95% CI, 1.54 to 10.26). However, odds of keratopathy decreased with UV index ≤ 6 (OR, 0.39; 95% CI, 0.2 to 0.72) and age < 20 years (OR, 0.32; 95% CI, 0.15 to 0.66). CONCLUSIONS AND CLINICAL RELEVANCE Findings indicated that odds of keratopathy in pinnipeds could be reduced by maintenance of pool water salinity ≥ 29 g/L and reduction of UV radiation exposure (eg, with adequate shade structures and use of darker, natural colors). Because UV radiation exposure is cumulative, even small attempts to reduce lifetime exposure to it could help control keratopathy in pinnipeds.

Damaging Effects of Ultraviolet Radiation on the Cornea

The cornea sits at the anterior aspect of the eye and, like the skin, is highly exposed to ultraviolet radiation (UVR). The cornea blocks a significant proportion of UVB from reaching the posterior structures of the eye. However, UVA can penetrate the full thickness of the cornea, even reaching the anterior portion of the lens. Epidemiological data indicate that UVR is a contributing factor for a multitude of diseases of the cornea including pterygium, photokeratitis, climatic droplet keratopathy and ocular surface squamous neoplasia (OSSN), although the pathogenic mechanisms of each require further elucidation. UVR is a well-known genotoxic agent, and its effects have been well characterized in organs such as the skin. However, we are only beginning to identify its effects on the cornea, such as the UVR signature C → T and CC → TT transversions identified by sequencing and increased proliferative and shedding rates in response to UVR exposure. Alarmingly, a single low-dose exposure of UVR to the cornea is sufficient to elicit genetic, molecular and cellular changes, supporting the consideration of using protective measures, such as wearing sunglasses when outdoors. The aim of this review was to describe the adverse effects of UVR on the cornea.

TNF-R1 and FADD mediate UVB-Induced activation of K+ channels in corneal epithelial cells

The goal of this study was to elucidate the role of Fas, TNF-R1, FADD and cytochrome c in UVB-induced K+ channel activation, an early step in UVB-induced apoptosis, in human corneal limbal epithelial (HCLE) cells. HCLE cells were treated with Fas, TNF-R1 or FADD siRNA and exposed to 80 or 150 mJ/cm2 UVB. K+ channel activation and loss of intracellular K+ were measured using whole-cell patch-clamp recording and ion chromatography, respectively. Cytochrome c was measured with an ELISA kit. Cells in which Fas was knocked down exhibited identical UVB-induced K+ channel activation and loss of intracellular K+ to control cells. Cells in which TNF-R1 or FADD were knocked down demonstrated reduced K+ channel activation and decreased loss of intracellular K+ following UVB, relative to control cells. Application of TNF-α, the natural ligand of TNF-R1, to HCLE cells induced K+ channel activation and loss of intracellular K+. Cytochrome c was translocated to the cytosol by 2 h after exposure to 150 mJ/cm2 UVB. However, there was no release by 10 min post-UVB. The data suggest that UVB activates TNF-R1, which in turn may activate K+ channels via FADD. This conclusion is supported by the observation that TNF-α also causes loss of intracellular K+. This signaling pathway appears to be integral to UVB-induced K+ efflux, since knockdown of TNF-R1 or FADD inhibits the UVB-induced K+ efflux. The lack of rapid cytochrome c translocation indicates cytochrome c does not play a role in UVB-induced K+ channel activation.

Involvement of the extrinsic and intrinsic pathways in ultraviolet B-induced apoptosis of corneal epithelial cells

The goal of this study was to elucidate the pathway by which UVB initiates efflux of K(+) and subsequently apoptosis in human corneal limbal epithelial (HCLE) cells. The initial focus of the study was on the extrinsic pathway involving Fas. HCLE cells transfected with Fas siRNA were exposed to 80-150 mJ/cm(2) UVB and incubated in culture medium with 5.5 mM K(+). Knockdown of Fas resulted in limited reduction in UVB-induced caspase-8 and -3 activity. Patch-clamp recordings showed no difference in UVB-induced normalized K(+) currents between Fas transfected and control cells. Knockdown of caspase-8 had no effect on the activation of caspase-3 following UVB exposure, while a caspase-8 inhibitor completely eliminated UVB activation of caspase-3. This suggests that caspase-8 is a robust enzyme, able to activate caspase-3 via residual caspase-8 present after knockdown, and that caspase-8 is directly involved in the UVB activation of caspase-3. Inhibition of caspase-9 significantly decreased the activation of caspases-8 and -3 in response to UVB. Knockdown of Apaf-1, required for activation of caspase-9, resulted in a significant reduction in UVB-induced activation of caspases-9, -8, and -3. Knockdown of Apaf-1 also inhibited intrinsic and UVB-induced levels of apoptosis, as determined by DNA fragmentation measured by TUNEL assay. In UVB exposed cultures treated with caspase-3 inhibitor, the percentage of apoptotic cells was reduced to control levels, confirming the necessity of caspase-3 activation in DNA fragmentation. The lack of effect of Fas knockdown on K(+) channel activation, as well as the limited effect on activation of caspases-8 and -3, strongly suggest that Fas and the extrinsic pathway is not of primary importance in the initiation of apoptosis in response to UVB in HCLE cells. Inhibition of caspase-8 and -3 activation following inhibition of caspase-9, as well as reduction in activation of caspases-9, -8, and -3 and DNA fragmentation in response to Apaf-1 knockdown support the conclusion that the intrinsic pathway is more important in UVB-induced apoptosis in HCLE cells.

Ultraviolet light and ocular diseases

The objective of this study is to review the association between ultraviolet (UV) light and ocular diseases. The data are sourced from the literature search of Medline up to Nov 2012, and the extracted data from original articles, review papers, and book chapters were reviewed. There is a strong evidence that ultraviolet radiation (UVR) exposure is associated with the formation of eyelid malignancies [basal cell carcinoma (BCC) and squamous cell carcinoma (SCC)], photokeratitis, climatic droplet keratopathy (CDK), pterygium, and cortical cataract. However, the evidence of the association between UV exposure and development of pinguecula, nuclear and posterior subcapsular cataract, ocular surface squamous neoplasia (OSSN), and ocular melanoma remained limited. There is insufficient evidence to determine whether age-related macular degeneration (AMD) is related to UV exposure. It is now suggested that AMD is probably related to visible radiation especially blue light, rather than UV exposure. From the results, it was concluded that eyelid malignancies (BCC and SCC), photokeratitis, CDK, pterygium, and cortical cataract are strongly associated with UVR exposure. Evidence of the association between UV exposure and development of pinguecula, nuclear and posterior subcapsular cataract, OSSN, and ocular melanoma remained limited. There is insufficient evidence to determine whether AMD is related to UV exposure. Simple behaviural changes, appropriate clothing, wearing hats, and UV blocking spectacles, sunglasses or contact lens are effective measures for UV protection.

p53 protein subcellular localization and apoptosis in rodent corneal epithelium cell culture following ultraviolet irradiation

The tumor-suppressor gene p53 encodes a phosphoprotein involved in the control of cell growth. p53 expression and function have been documented in malignancy, apoptosis and the aging processes. Recently, p53 has been mapped and characterized in the normal cornea across different species. In the present study, high levels of cytoplasmic p53 protein were noted in normal primary corneal epithelium cultures by immunohistochemistry and western blot analysis. Following ultraviolet (UV) irradiation, the level of cytoplasmic p53 protein expression was increased beginning from 30 min and lasting until 6 h post-irradiation and then returned close to control levels by 24 h. Cytoplasmic p53 phosphorylation was detected from 30 min following UV treatment until 6 h post-irradiation. p53 protein became apparent in the nucleus in a fraction of these cultured cells beginning 30 min following UV irradiation and was still present 24 h later. We also found that p53 colocalized with mitochondria 2 h following UV irradiation in some of the cells and remained there up to 24 h. As the expression levels of p53 transcription following UV irradiation were not significantly altered, the increase in cytoplasmic p53 protein expression may be conditional only upon post-translational stabilization. We also observed that the apoptotic index increased following UV irradiation in the same time frame as the p53 nuclear transfer and was partially suppressed by pifithrin-α, which is a reversible inhibitor of p53-mediated apoptosis and p53-dependent gene transcription. The present study offers new evidence suggesting that cytoplasmic p53 in rodent corneal epithelium is functionally active.

A retrospective survey of the ocular histopathology of the pinniped eye with emphasis on corneal disease

OBJECTIVE

A retrospective review of globes from 70 pinnipeds submitted to the Comparative Ocular Pathology Laboratory of Wisconsin (COPLOW) describing the type and frequency of ocular disease.

ANIMALS STUDIED

The study included 50 California sea lions, four animals listed only as 'sea lion', nine Northern elephant seals, five harbor seals, 1 Northern fur seal, and 1 Hooded seal.

PROCEDURES

Globes were classified by microscopic findings. Categories were not mutually exclusive.

RESULTS

The largest category was corneal disease (63 globes from 40 pinnipeds). The second largest was cataractous changes (35 globes from 23 pinnipeds). Additional ocular diseases included traumatic ocular injuries (nine globes from eight animals), phthisis bulbi (nine globes from eight pinnipeds), neoplasia (nine globes from six adult California sea lions), amyloid deposition in the corneal stroma, ciliary body, or both locations (five globes from four pinnipeds), and fungal disease (three globes from two pinnipeds). Pinnipeds with corneal disease were further categorized: stromal pathology (39 globes from 27 pinnipeds); epithelial pathology (37 globes from 27 pinnipeds); Descemet's pathology (11 globes from eight pinnipeds); endothelial attenuation or absence (33 globes from 22 pinnipeds); presence of retrocorneal membranes (15 globes from 10 pinnipeds); anterior synechia (eight globes from six animals), and keratitis (seven globes from five pinnipeds).

CONCLUSIONS

This is the first report of ocular amyloid in pinniped eyes. All cases of neoplasia were in a pattern suggesting metastatic disease. In this study, there was a higher prevalence of ocular disease in captive pinnipeds, particularly in the posterior cornea.

Stratified corneal limbal epithelial cells are protected from UVB-induced apoptosis by elevated extracellular K⁺

The goal of this study was to determine whether elevated [K(+)] protects stratified corneal epithelial cells from entering apoptosis following exposure to ambient levels of UVB radiation. Human corneal limbal epithelial (HCLE) cells were stratified to form multilayered constructs in culture. The cells were exposed to UVB doses of 100-250 mJ/cm(2) followed by incubation in medium with 5.5-100 mM K(+). The protective effect of K(+) was determined by measuring the caspase-3 and -8 activity and TUNEL staining of the stratified HCLE constructs. In response to UVB exposure, activation of apoptotic pathways peaked at 24 h. Caspase-8 in stratified cells was activated by exposure to UVB at 100-250 mJ/cm(2), and activity was significantly reduced in response to 50 or 100 mM K(+). Caspase-3 was activated in the stratified cells in response to 100-250 mJ/cm(2) UVB and showed a significant reduction in activity in response to 25, 50 or 100 mM K(+). DNA fragmentation, as indicated by TUNEL staining, was elevated after exposure to 200 mJ/cm(2) UVB, and decreased following incubation with 25-100 mM K(+). These results show that in a culture system that models the intact corneal epithelium, elevated extracellular K(+) can reduce UVB-induced apoptosis which is believed to be initiated by loss of K(+) from cells. This is the basis of damage to the corneal epithelium caused by UVB exposure. Based on these observations it is suggested that the relatively high K(+) concentration in tears (20-25 mM) may play a role in protecting the corneal epithelium from ambient UVB radiation.

Altered expression of matrix metalloproteinases and their tissue inhibitors as possible contributors to corneal droplet formation in climatic droplet keratopathy

PURPOSE

Climatic droplet keratopathy (CDK) is an acquired corneal disease characterized by progressive scarring of the cornea. In several corneal diseases, matrix metalloproteinases (MMPs) are upregulated during the degradation of epithelial and stromal tissues. We investigated the levels, degree of activation and molecular forms of MMP-2, MMP-9, MMP-8 and MMP-13 and their tissue inhibitors TIMP-1 and TIMP-2 in tear fluid of patients with CDK.

METHODS

Seventeen CDK patients and 10 controls living in Argentine Patagonia received a complete eye examination, and MMPs and TIMP-1/2 were determined by immunofluorometric assay (IFMA), gelatin zymography and quantitative Western immunoblot analysis in tear samples.

RESULTS

The MMPs were detected mostly in their latent forms. The levels of MMP-9 and MMP-2 were found to be significantly elevated in CDK patients, whereas latent and active MMP-8 levels were significantly enhanced in controls. There was no significant difference in the level of MMP-13. TIMPs were found as part of complexes, and the TIMP-1 levels were significantly lower in patients than controls.

CONCLUSION

Elevated MMP-2 and MMP-9 levels have been implicated in the failure of corneal re-epithelialization, and enhanced MMP-2 and MMP-9 levels in CDK patients suggest that these MMPs may play a role in corneal scarring in CDK. Elevated levels of MMP-8 suggest a defensive role for this MMP in inflammatory reactions associated with recurring corneal traumas. Decreased expression of TIMP-1 in CDK patients suggest deficient antiproteolytic shield likely to render the corneas of CDK patients vulnerable to enhanced MMPs. Overall, these data suggest a mechanistic link between MMPs and TIMP-1 level in cornea and tears with corneal scarring in CDK.

A class I (Senofilcon A) soft contact lens prevents UVB-induced ocular effects, including cataract, in the rabbit in vivo

PURPOSE

UVB radiation from sunlight is known to be a risk factor for human cataract. The purpose in this study was to investigate the ability of a class I UV-blocking soft contact lens to protect against UVB-induced effects on the ocular tissues of the rabbit in vivo.

METHODS

Eyes of rabbits were exposed to UVB light for 30 minutes (270-360 nm, peak at 310 nm, 1.7 mW/cm(2) on the cornea). Eyes were irradiated in the presence of either a UV-blocking senofilcon A contact lens, a minimally UV-blocking lotrafilcon A contact lens, or no contact lens at all. Effects on the cornea and lens were evaluated at various times after exposure.

RESULTS

Eyes irradiated with no contact lens protection showed corneal epithelial cell loss plus lens epithelial cell swelling, vacuole formation, and DNA single-strand breaks, as well as lens anterior subcapsular opacification. The senofilcon A lens protected nearly completely against the UVB-induced effects, whereas the lotrafilcon A lens showed no protection.

CONCLUSIONS

The results indicate that use of a senofilcon A contact lens is beneficial in protecting ocular tissues of the rabbit against the harmful effects of UVB light, including photokeratitis and cataract.

Elevated extracellular K+ inhibits apoptosis of corneal epithelial cells exposed to UV-B radiation

The goal of this study was to determine if the high [K(+)] in tears, 20-25 mM, serves to protect corneal epithelial cells from going into apoptosis after exposure to ambient UV-B radiation. Human corneal-limbal epithelial (HCLE) cells in culture were exposed to UV-B at doses of 50-200 mJ/cm(2) followed by measurement of K(+) channel activation and activity of apoptotic pathways. Patch-clamp recording showed activation of K(+) channels after UV-B exposure at 80 mJ/cm(2) or 150 mJ/cm(2) and a decrease in UV-induced K(+) efflux with increasing [K(+)](o). The UV-activated current was partially blocked by the specific K(+) channel blocker, BDS-1. DNA fragmentation, as measured by the TUNEL assay, was induced after exposure to UV-B at 100-200 mJ/cm(2). DNA fragmentation was significantly decreased when cells were incubated in 25, 50 or 100mM K(o)(+) after exposure to UV-B. The effector caspase, caspase-3, was activated by exposure to UV-B at 50-200 mJ/cm(2), but there was a significant decrease in activation when the cells were incubated in 25, 50 or 100mM K(o)(+) following exposure to UV-B. A decrease in mitochondrial potential, a possible activator of caspase-3, occurred after exposure to UV-B at 100-200 mJ/cm(2). This decrease in mitochondrial potential was prevented by 100mM K(o)(+); however, 25 or 50mM K(o)(+) provided minimal protection. Caspase-9, which is in the pathway from mitochondrial potential change to caspase-3 activation, showed little activation by UV-B radiation. Caspase-8, an initiator caspase that activates caspase-3, was activated by exposure to UV-B at 50-200 mJ/cm(2), and this UV-activation was significantly reduced by 25-100mM K(o)(+). The data show that the physiologically relevant [K(+)](o) of 25 mM can inhibit UV-B induced activation of apoptotic pathways. This suggests that the relatively high [K(+)] in tears reduces loss of K(+) from corneal epithelial cells in response to UV exposure, thereby contributing to the protection of the ocular surface from ambient UV radiation.

Protective effects of melatonin in experimental free radical-related ocular diseases

Melatonin (N-acetyl-5-methoxytryptamine) is an indoleamine with a range of antioxidative properties. Melatonin is endogenously produced in the eye and in other organs. Current evidence suggests that melatonin may act as a protective agent in ocular conditions such as photo-keratitis, cataract, glaucoma, retinopathy of prematurity and ischemia/reperfusion injury. These diseases are sight-threatening and they currently remain, for the most part, untreatable. The pathogenesis of these conditions is not entirely clear but oxidative stress has been proposed as one of the causative factors. Elevated levels of various reactive oxygen and nitrogen species have been identified in diseased ocular structures. These reactants damage the structure and deplete the eye of natural defense systems, such as the antioxidant, reduced glutathione, and the antioxidant enzyme superoxide dismutase. Oxidative damage in the eye leads to apoptotic degeneration of retinal neurons and fluid accumulation. Retinal degeneration decreases visual sensitivity and even a small change in the fluid content of the cornea and crystalline lens is sufficient to disrupt ocular transparency. In the eye, melatonin is produced in the retina and in the ciliary body. Continuous regeneration of melatonin in the eye offers a frontier antioxidative defense for both the anterior and posterior eye. However, melatonin production is minimal in newborns and its production gradually wanes in aging individuals as indicated by the large drop in circulating blood concentrations of the indoleamine. These individuals are possibly at risk of contracting degenerative eye diseases that are free radical-based. Supplementation with melatonin, a potent antioxidant, in especially the aged population should be considered as a prophylaxis to preserve visual functions. It may benefit many individuals worldwide, especially in countries where access to medical facilities is limited.

Pathogenesis of pterygia: role of cytokines, growth factors, and matrix metalloproteinases

Pterygium is a common ocular surface disease apparently only observed in humans. Chronic UV exposure is a widely accepted aetiological factor in the pathogenesis of this disease and this concept is supported by epidemiological data, ray tracing models and histopathological changes that share common features with UV damaged skin. The mechanism(s) of pterygium formation is incompletely understood. Recent data have provided evidence implicating a genetic component, anti-apoptotic mechanisms, cytokines, growth factors, extracellular matrix remodelling (through the actions of matrix metalloproteinases), immunological mechanisms and viral infections in the pathogenesis of this disease. In this review, the current knowledge on pterygium pathogenesis is summarised, highlighting recent developments. In addition, we provide novel data further demonstrating the complexity of this intriguing disease.

Photokeratitis and other phototoxic effects on the cornea and conjunctiva

Except when sleeping, the cornea and interpalpebral conjunctiva are exposed to the ambient environment, both natural and man-made. Levels of solar ultraviolet irradiance reaching the eye may exceed the damage threshold under a number of circumstances. The consequences of overexposure may be acute after a latent period, sequelae to an acute exposure, or long-term chronic effects. Previously derived action spectra for photokeratitis and photoconjunctivitis due to incoherent ultraviolet are presented. These reveal interspecies similarities for the levels of radiant energy reaching each tissue. The initial in vivo (clinical) signs of photokeratitis are due to lost or damaged epithelial cells with other signs produced by this primary response. The conjunctival signs include injection and chemosis. Chronic exposure to solar ultraviolet is a factor in climatic droplet keratopathy and pterygium. Phototoxic compounds or their by-products potentially can reach the cornea from the air, via the tears or aqueous humor, or from the limbal capillaries. However, the human cornea appears to be much less susceptible to the influence of phototoxic agents than the skin.

Pilot study on the time course of apoptosis in the regenerating corneal epithelium

PURPOSE

To determine whether apoptosis contributes to regeneration of the corneal epithelium following erosion and following ultraviolet irradiation.

METHODS

Central corneal erosions were made on one eye of 16 rats. One eye of another set of 16 rats was exposed to UVB irradiation. The rats were killed at time intervals varying from 12 hours to 7 days after treatment. Enucleated eyes were fixed in buffered formaldehyde and evaluated by terminal deoxynucleotidyl transferase-mediated deoxyuridinetriphosphate (dUTP) nick-end labelling (TUNEL) and morphology. The total number of cells and the number of TUNEL positive cells were counted in perpendicular sections using light microscopy.

RESULTS

Following central erosion the total epithelial cell number was restored by day 3. During the first 5 days, TUNEL positive cells were observed only in small numbers, but an increase occurred by days 6 and 7. After UVB, an increase in TUNEL positive cells was noted for at least 3 days, and by day 7 there was a small increase of TUNEL positive cells. This differed from the results seen in control animals.

CONCLUSIONS

The present study indicates that after injury, apoptosis occurs in two distinct phases. There is an initial early phase of apoptosis which subsides at about the time the cell mass is being restored and after damaged cells have been removed. A later phase of apoptosis occurs suggests it has a homeostatic role which contributes to the regulation of the cell population.

Lactoferrin protects against UV-B irradiation-induced corneal epithelial damage in rats

PURPOSE

Lactoferrin supplementation suppresses ultraviolet light B (UV-B)-induced oxidation of cultures of human corneal epithelial cells. To investigate the protective effect of lactoferrin containing eyedrops against UV-B-induced corneal damage in vivo, we examined lactoferrin efficacy in a rat UV-B keratitis model.

METHODS

Sprague-Dawley rats were irradiated with >10 kJ/m2 after anesthetization, and then corneal epithelial defect was observed at 24 h postirradiation. The pre- or postapplication of vehicle or lactoferrin-containing eyedrops was performed, and then corneal epithelial damage was scored based on fluorescein staining.

RESULTS

Posttreatment with lactoferrin did not inhibit the extent of corneal damage and did not affect wound healing. However, pretreatment by topical application of lactoferrin suppressed development of a corneal epithelial defect induced by UV-B irradiation in rats.

CONCLUSION

These results suggest that the presence of lactoferrin in human tear fluid may inhibit UV-induced corneal epithelial damage.

The effect of a shear force on the cell shedding rate of the corneal epithelium

PURPOSE

During blinking the lids apply a shear force to the corneal epithelium. The aim of this study was to determine if a shear force applied to the epithelial surface increases the rate at which cells shed.

METHODS

The shedding rate was studied in perfused whole rabbit eyes, and the effect of a shear force examined by exposing the corneas to a stirred solution. Control corneas were exposed to a static solution. The shedding rate and size of shed cells were measured, and the number of terminally differentiated cells on the corneal surface determined after 6 h of perfusion using ethidium bromide.

RESULTS

Compared with controls, the shear force increased the cell shedding rate from the corneal surface significantly (p < 0.01, paired t-test). The increase was due to small cells with a longest dimension less than 25 microm. The number of terminally differentiated cells on the epithelial surface did not increase.

CONCLUSION

Because of the decrease in size, and the change in appearance of shedding cells, it is proposed that the increase in cell shedding rate was due to an increase in the number of apoptotic cells, and not to an increase in terminally differentiated cells. It is suggested that in the human eye, under adverse conditions, shear forces due to blinking may play a role in creating apoptotic cells.

The cell shedding rate of the corneal epithelium--a comparison of collection methods

PURPOSE

To determine the spontaneous shedding rate of cells from the rabbit corneal epithelium using different harvesting methods.

METHODS

Cells were collected from the rabbit corneal epithelium using two in vitro methods and three in vivo methods. Cells were counted and the shedding rate calculated after adjustment for the collection time.

RESULTS

The shedding rates obtained from the in vitro methods were (cells/min/cornea, mean +/- SE): 78.0 +/- 9.4 (corneal superfusion), and 5.4 +/- 1.3 (whole-eye perfusion). For the in vivo corneas, the shedding rates were: 10.0 +/- 2.3 (corneal superfusion), 8.1 +/- 0.4 (corneal immersion), and 14.5 +/- 1.5 (corneal irrigation). In vitro corneal superfusion was significantly different from the other four methods (P < 0.01).

CONCLUSIONS

The results suggest that the spontaneous cell shedding rate of the in vivo rabbit corneal epithelium is 5 to 15 cells/min/cornea. This is much lower than estimates of about 100 cells/min/cornea based upon in vitro corneal superfusion. One explanation of this slow shedding rate is that factors which were absent during our collection methods (such as blinking) would normally increase shedding. Another possibility is that cells in the corneal epithelium may have a much longer life span than previously reported; rather than a few days, the epithelium could take several months to completely replace all cells. Whatever the explanation, the measured spontaneous shedding rate does not complement the reported production rate of new cells. It is necessary to revise our understanding of the kinetics of epithelial homeostasis.