The alkali burned cornea I. Epithelial and stromal repair

The Role of Sensory Innervation in Homeostatic and Injury-Induced Corneal Epithelial Renewal

The cornea is the window through which we see the world. Corneal clarity is required for vision, and blindness occurs when the cornea becomes opaque. The cornea is covered by unique transparent epithelial cells that serve as an outermost cellular barrier bordering between the cornea and the external environment. Corneal sensory nerves protect the cornea from injury by triggering tearing and blink reflexes, and are also thought to regulate corneal epithelial renewal via unknown mechanism(s). When protective corneal sensory innervation is absent due to infection, trauma, intracranial tumors, surgery, or congenital causes, permanent blindness results from repetitive epithelial microtraumas and failure to heal. The condition is termed neurotrophic keratopathy (NK), with an incidence of 5:10,000 people worldwide. In this report, we review the currently available therapeutic solutions for NK and discuss the progress in our understanding of how the sensory nerves induce corneal epithelial renewal.

Re-epithelialization of adult skin wounds: Cellular mechanisms and therapeutic strategies

Cutaneous wound healing in adult mammals is a complex multi-step process involving overlapping stages of blood clot formation, inflammation, re-epithelialization, granulation tissue formation, neovascularization, and remodelling. Re-epithelialization describes the resurfacing of a wound with new epithelium. The cellular and molecular processes involved in the initiation, maintenance, and completion of epithelialization are essential for successful wound closure. A variety of modulators are involved, including growth factors, cytokines, matrix metalloproteinases, cellular receptors, and extracellular matrix components. Here, we focus on cellular mechanisms underlying keratinocyte migration and proliferation during epidermal closure. Inability to re-epithelialize is a clear indicator of chronic non-healing wounds, which fail to proceed through the normal phases of wound healing in an orderly and timely manner. This review summarizes the current knowledge regarding the management and treatment of acute and chronic wounds, with a focus on re-epithelialization, offering some insights into novel future therapies.

Fundamental pharmacological expressions on ocular exposure to capsaicin, the principal constituent in pepper sprays

Eye irritation assessment is compulsory to anticipate health risks in military personnel exposed to riot control agents such as capsaicin, the principal constituent of oleoresin capsicum, or pepper sprays. The present work investigates certain fundamental yet unaddressed pharmacological manifestations on ocular exposure to capsaicin. Ocular pharmacology of capsaicin was studied using acute eye irritation (AEI), bovine corneal opacity and permeability (BCOP) assay, corneal fluorescein staining and indirect ophthalmoscopy studies, transcorneal permeation, Schirmer tear secretion test, nerve conduction velocity study and enzyme-linked immunosorbent assay (ELISA). Additionally, histopathology and scanning electron microscopy (SEM) of bovine corneas and rat optic nerves were done to further estimate capsaicin induced morphological variations. Our findings demonstrated that AEI, BCOP, corneal fluorescein staining and indirect ophthalmoscopy were useful in assessing capsaicin induced ocular irritation; AEI and BCOP also contributed towards indicating the eye irritation potential of capsaicin as per the United Nations Globally Harmonized System of Classification and Labelling of Chemicals categorization. Additional experimental observations include considerable transcorneal permeation of capsaicin, capsaicin induced reduction in tear secretions and nerve conduction velocity and increased expression of proinflammatory cytokines by ELISA. Histopathology and SEM were favourable techniques for the detection of capsaicin induced ocular physiological modifications.

Treatment of corneal chemical alkali burns with a crosslinked thiolated hyaluronic acid film

PURPOSE

The study objective was to test the utilization of a crosslinked, thiolated hyaluronic acid (CMHA-S) film for treating corneal chemical burns.

METHODS

Burns 5.5mm in diameter were created on 10 anesthetized, male New Zealand white rabbits by placing a 1N NaOH soaked circular filter paper onto the cornea for 30s. Wounds were immediately rinsed with balanced salt solution (BSS). CMHA-S films were placed in the left inferior fornix of five injured and five uninjured animals. Five animals received no treatment. At 0h, 48h, 96h, and on day 14 post chemical burn creation, eyes were evaluated by white light imaging, fluorescein staining, and optical coherence tomography (OCT). Corneal histology was performed using H&E and Masson's Trichrome stains.

RESULTS

Image analysis indicated biocompatible CMHA-S treatment resulted in significant decreases in the areas of corneal opacity at 48h, 96h, and on day 14 postoperatively. A significant increase in re-epithelialization was seen 14days post injury. CMHA-S treated corneas showed significantly less edema than untreated burns. No pathological differences were observed in corneal histological samples as a result of CMHA-S treatment.

CONCLUSIONS

CMHA-S films facilitate re-epithelialization and decrease the area of corneal opacity in our corneal alkali burn rabbit model.

In vivo confocal microscopy of early corneal epithelial recovery in patients with chemical injury

PURPOSE

To investigate the early recovery of corneal epithelium in patients with chemical injuries by the application of in vivo laser scanning confocal microscopy (LSCM).

METHODS

LSCM was performed on 26 eyes of 18 consecutive patients at 1, 2, and 3 months post injury. The morphology of central corneal epithelium and limbus was evaluated. Analysis was performed to compare the densities of corneal apical surface cells (ASCs) and basal epithelial cells (BECs) among different injury severity and time points after injury. The comparisons were also made on inflammatory cells (ICs) and dendritic cells (DCs) infiltrating at the limbus. Moreover, the presence rate of palisades of Vogt (POV) was analyzed at 3 months post injury.

RESULTS

Corneal re-epithelialization was found within 3 months post injury in all eyes, except that persistent epithelium defect was identified in grade IV injured eyes even at 3 months after injury. The injury severity had a significantly negative correlation with the densities of ASCs. However, it had no relationship with the densities of BECs. The density of ICs at the limbus decreased significantly with the prolongation of follow-up, except in the grade I injured eyes. The presence rate of POV also had a significantly negative correlation with injury severity.

CONCLUSION

Epithelial recovery after chemical injury varied in cellular morphology and the densities of ASCs among eyes with different injury severity. The presence rates of POV decreased with the aggravation of injury. LSCM is a feasible method for observing the early recovery of corneal epithelium in patients with chemical injuries.

Limbal stem cells: Central concepts of corneal epithelial homeostasis

A strong cohort of evidence exists that supports the localisation of corneal stem cells at the limbus. The distinguishing characteristics of limbal cells as stem cells include slow cycling properties, high proliferative potential when required, clonogenicity, absence of differentiation marker expression coupled with positive expression of progenitor markers, multipotency, centripetal migration, requirement for a distinct niche environment and the ability of transplanted limbal cells to regenerate the entire corneal epithelium. The existence of limbal stem cells supports the prevailing theory of corneal homeostasis, known as the XYZ hypothesis where X represents proliferation and stratification of limbal basal cells, Y centripetal migration of basal cells and Z desquamation of superficial cells. To maintain the mass of cornea, the sum of X and Y must equal Z and very elegant cell tracking experiments provide strong evidence in support of this theory. However, several recent studies have suggested the existence of oligopotent stem cells capable of corneal maintenance outside of the limbus. This review presents a summary of data which led to the current concepts of corneal epithelial homeostasis and discusses areas of controversy surrounding the existence of a secondary stem cell reservoir on the corneal surface

Effects of conditioned media from human amniotic epithelial cells on corneal alkali injuries in rabbits

This study was performed to evaluate the effects of conditioned media (CM) from human amniotic epithelial cells (HAECs) on the corneal wound healing process. Eighteen rabbits (36 eyes) were used and randomly assigned to three groups according treatment: CM from HAECs (group 1), vehicle alone (group 2), and saline (group 3). Corneal alkali injuries were induced with 1 N sodium hydroxide. Each reagent used for treatment evaluation was injected into the dorsal bulbar subconjunctiva and the area of the corneal epithelial defect was measured every other day. Two animals from each group were euthanized at a time on days 3, 7, and 15, and the cornea was removed for histological examination. The sum of the epithelial defect areas measured on day 0 to day 6 as well as day 0 to day 14 in group 1 was significantly smaller than those of other groups. Histological examination revealed that the group 1 corneas had less inflammatory cell infiltration and showed more intact epithelial features compared to the other groups. These results suggest that CM from HAECs promote corneal wound healing in rabbits.

Comparison of the therapeutic effects of extracts from Spirulina platensis and amnion membrane on inflammation-associated corneal neovascularization

AIM

To compare the therapeutic effects of polysaccharide extract from Spirulina platensis (PSP) and extract from amnion membrane (AME) on alkali burn-induced corneal neovascularization (CorNV).

METHODS

PSP and AME were extracted from dry powder of Spirulina platensis and human aminion membrane respectively. Murine CorNV was induced by applying 1N sodiumhydroxide (NaOH) solution directly on the mice corneas. PSP and AME extracts were administered topically on the corneas 4 times daily for 7 days. The therapy effects of PSP and AME extracts were evaluated daily using slit-lamp. At the end of the therapy, corneas were harvested for H&E staining, masson trichrome staining, immunohistochemical study, and semi-quantification reverse transcriptive PCR (RT-PCR) was utilized for measurement of inflammation-related molecules.

RESULTS

Topical application of PSP extract had significant therapeutic effects on CorNV that could be shown in various assays of the corneas. Compared with AME extract, PSP extract treatment was more effective in suppressing CorNV in terms of vessel length and levels of cluster of differentiation 31 (CD31) proteins or the angiogenesis related genes like vascular endothelial growth factor (VEGF), matrix metalloproteinase-2 (MMP2) and matrix metalloproteinase-9 (MMP9). PSP also inhibited inflammation more markedly by more effectively inhibiting mononuclear and polymorphonuclear cells infiltration into the corneal stroma and reducing levels of stromal cell-derived factor-1 (SDF1), tumor necrosis factor-alpha (TNFα) and macrophage inflammatory protein-3 (MIP3a). In additon, corneas of PSP group had a more regular and compact architecture of collagen with thinner corneal thickness than in the AME group.

CONCLUSION

Polysaccharide extract from Spirulina platensis inhibited alkali burn-induced inflammation and CorNV more effectively than AME extract at the studied doses, thus may be used for the therapy of corneal diseases involving neovascularization and inflammation.

Role of platelet-activating factor in cell death signaling in the cornea: A review

Platelet-activating factor (PAF) is a potent bioactive lipid generated in the cornea after injury whose actions are mediated through specific receptors. Studies from our laboratory have shown that PAF interactions with its receptors activate several transmembrane signals involved in apoptosis. Continuous exposure to PAF during prolonged inflammation increases keratocyte apoptosis and inhibition of epithelial adhesion to the basement membrane. As a consequence, there is a marked delay in wound healing, which is not countered by the action of growth factors. While apoptosis of stroma cells is rapid and potent, epithelial cells as well as myofibroblasts, which appear in the stroma during the repair phase, are resistant to apoptosis. However, PAF accelerates apoptosis of corneal epithelial cells exposed to oxidative stress by stimulating phospholipase A2, producing an early release of cytochrome C from mitochondria and activating caspase-3. In myofibroblasts, PAF has a synergistic action with tumor necrosis factor-alpha (TNF-alpha), increasing apoptosis of the cells to 85%. PAF antagonists block the effects of PAF and could have a therapeutic role in maintaining a healthy and transparent cornea.

Reactive formation of hyaluronic acid in the rabbit corneal alkali burn

The presence and distribution of reactively formed hyaluronic acid (HA) was assessed in the rabbit cornea following a penetrating alkali burn. The injury was inflicted by applying a round, 5.5 mm, filter paper soaked in 1 N NaOH centrally on the cornea for 60 seconds. Biochemical analysis revealed a significant increase of HA two weeks after injury, a peak concentration after 1 month, and a decrease again at three months. Histochemical analysis revealed the presence of hyaluronic acid in the healing epithelium, in the repopulating keratocytes/fibroblasts, and in the cells forming the retrocorneal membrane. Extensive amounts filled lacunae in the stroma as well as the spaces between collagen lamellae. A slow restoration of normal appearing corneal stroma took place at the periphery. Significant staining for HA in lacunae was present centrally in the wound after three months.

Clinical measures to promote corneal epithelial healing

It is necessary to know the specific pathobiology of a persistent epithelial defect to determine the strategy to be employed to assist in its repair. Lid position and function must be normal and any deficiency in the quantity and quality of the tears enhanced by tear preparations and closure of the lacrimal canaliculi. Adverse drug effects must be eliminated. Multiple corneal punctures and excision of reduplicated basal lamina have virtually eliminated the problem of recurrent corneal erosions. Control of any inflammatory process also speeds healing. Vitamin supplements, especially A, reverse defects associated with xerophthalmia. In any of these diseases, mechanical treatments consisting of soft contact lenses for persistent epithelial defects and collagen shields for the delivery of antibiotics or steroids to the eye may be employed. Tarsorrhaphy relieves the problem of persistent epithelial defects in neurotrophic keratitis and a variety of other conditions characterized by persistent surface breakdown. Preliminary data from open label studies of epidermal growth factors and fibronectin are encouraging but not yet conclusive.

Amniotic membrane patching promotes healing and inhibits proteinase activity on wound healing following acute corneal alkali burn

Amniotic membrane (AM) contains basement membrane components and various proteinase inhibitors. Furthermore, when used as a graft, the basement membrane of AM could block inflammatory insults to a damaged corneal surface. Thus, we evaluated whether amniotic membrane patching could promote the healing process by inhibiting proteolytic damage. Alkali wounds were inflicted on the central corneas of rabbits by applying a round filter paper, 6.0 mm in diameter, soaked in 1 N NaOH for 30 sec. Amniotic membrane patching was performed over the perilimbal sclera immediately after wounding. A total of 115 rabbits were divided into four groups: (1) immediately covered by AM with the amnion cell side down up to the perilimbal sclera (n =26); (2) covered by AM with the stromal side down up to the perilimbal sclera (n =19); (3) anchored to the fornix (n =29); and (4) uncovered as a control (n =41). AM was removed 3 days postoperatively. During follow-ups, epithelial defects, corneal thickness and its opacity of each eye were measured. Some corneas were removed for histopathologic studies and for proteinase activity assay and zymography. The epithelial healing was faster and the corneal thickness was thicker in all three AM-covered groups than in the control (P<0.05). No significant difference was found between covered and anchored groups (P>0.05). Corneal opacity was least in the amnion cell side down group. Infiltration of polymorphonuclear leukocytes (PMNs) was much less in AM-covered groups than in the control. Pathological results were associated with zymographic findings, which revealed much higher proteinase activity in uncovered group than AM-covered groups. Immediate intervention for acute alkali burns with AM as a temporary patch promotes wound healing by inhibiting proteinase activity and PMNs infiltration.

Chemical injuries of the eye: current concepts in pathophysiology and therapy

Chemical injuries of the eye may produce extensive damage to the ocular surface epithelium, cornea, and anterior segment, resulting in permanent unilateral or bilateral visual impairment. Pathophysiological events which may influence the final visual prognosis and which are amenable to therapeutic modulation include 1) ocular surface injury, repair, and differentiation, 2) corneal stromal matrix injury, repair and/or ulceration, and 3) corneal and stromal inflammation. Immediately following chemical injury, it is important to estimate and clinically grade the severity of limbal stem cell injury (by assessing the degree of limbal, conjunctival, and scleral ischemia and necrosis) and intraocular penetration of the noxious agent (by assessing clarity of the corneal stroma and anterior segment abnormalities). Immediate therapy is directed toward prompt irrigation and removal of any remaining reservoir of chemical contact with the eye. Initial medical therapy is directed promoting re-epithelialization and transdifferentiation of the ocular surface, augmenting corneal repair by supporting keratocyte collagen production and minimizing ulceration related to collagenase activity, and controlling inflammation. Early surgical therapy if indicated, is directed toward removal of necrotic corneal epithelium and conjunctiva, prompt re-establishment of an adequate limbal vascularity, and re-establishment of limbal stem cell population early in the clinical course, if sufficient evidence exists of complete limbal stem cell loss. Re-establishment of limbal stem cells by limbal autograft or allograft transplantation, or by transfer in conjunction with large diameter penetrating keratoplasty, may facilitate development of an intact, phenotypically correct corneal epithelium. Limbal stem cell transplantation may prevent the development of fibrovascular pannus or sterile corneal corneal ulceration, simplify visual rehabilitation, and improve the visual prognosis. Advances in ocular surface transplantation techniques which allow late attempts at visual rehabilitation of a scarred and vascularized cornea include limbal stem cell transplantation for incomplete transdifferentiation and persistent corneal epithelial dysfunction, and conjunctival and/or mucosal membrane transplantation for ocular surface mechanical dysfunction. Rehabilitation of the ocular surface may be followed, if necessary, by standard penetrating keratoplasty if all aspects of ocular surface rehabilitation are complete, or by large diameter penetrating keratoplasty if successful limbal stem cell transplantation cannot be achieved but other ocular surface rehabilitation is complete.

Ascorbic acid phosphate ester and wound healing in rabbit corneal alkali burns: epithelial basement membrane and stroma

We examined the effect of L-ascorbic acid 2-phosphate (P-Asc) on the healing of alkali-burned corneas in rabbits. Round filter paper containing 1 N NaOH was applied to the central cornea for 60 or 120 s to produce the alkali burn. Animals were treated with topical saline, 10% ascorbate, or 6.5% P-Asc applied on the cornea. The corneas were then examined histologically. Burned stroma showed no toluidine blue staining, indicating a loss of glycosaminoglycan. In the 60-s burn group, P-Asc reduced the size of the unstained area as compared with the control. Transmission electron microscopy showed basal lamina under new epithelia in the corneas treated with ascorbate or P-Asc, but not in controls. These observations support the theory that P-Asc may have a therapeutic role in the repair of corneal alkali burns.

Corneal alkali burn in the rabbit. Waterbalance, healing and transparency

Healing following a standardized central corneal alkali wound was studied morphologically in New Zealand white rabbits up to one year after the initial wound. Clinical examination, light and transmission electron microscopy was performed. The study was focused on how permanent scar tissue formed. Following the penetrating alkali injury, all cells (epithelium, keratocytes and endothelium) in the wound area disappeared. The fibroblasts/keratocytes repopulated an extensively swollen central corneal stroma. Cells and extracellular matrix filled stromal lacunae in an irregular fashion and upon deswelling the lacunae remained as irregularities in the stroma, reducing the transparency. In the periphery of the wound repopulation occurred in a less swollen stroma and normal cytoarchitecture and transparency resumed. It appears that the degree of swelling decides the degree of scar tissue formation in the corneal stroma following alkali wound healing.

Platelet-activating factor (PAF) accumulation correlates with injury in the cornea

This study reports the accumulation of platelet-activating factor (PAF) in corneas injured with either 0.1 N NaOH or 1 N NaOH. The degree of injury in corneas exposed to alkali for 5, 10, 20, or 60 sec was assessed by light microscopy and scanning electron microscopy. PAF accumulation in vivo increased with time (up to 24 hr) after injury and also according to the severity of the alkali injury. PAF was isolated by high-performance liquid chromatography (HPLC) and assayed by platelet aggregation of the HPLC fraction containing PAF. The specificity of the aggregating bioactivity was ascertained from inhibition of platelet aggregation by selective PAF antagonists. BN 50726, a new synthetic PAF antagonist, applied in vivo topically or subconjunctivally, was effective in inhibiting PAF formation. Because PAF is accumulated in vivo as soon as 30 min after corneal injury, this lipid mediator seems to be synthesized by corneal cells and not be recruited inflammatory cells, since they arrive later. Moreover, if the injury causes stromal edema and endothelial damage, the amount of PAF accumulated is even greater. Results from isolated corneas stimulated in vitro with calcium ionophore A23187 suggest that PAF synthesis is the result of stimulation of phospholipase A2 to form lyso-PAF and subsequent activation of an acetyltransferase.

Kinematics of epithelial wound closure in the rabbit cornea

The kinematics of wound closure in the healing rabbit corneal epithelium are modelled. The analysis is able to model the movement of the wound edge on the corneal surface. A unified formalism is described which embeds the surface velocity of the wound margin within the computational algorithm to enable calculation of the surface velocity of the wound margin from the time course of (1) planar wound area, or (2) wound diameter. The procedure can be implemented with the standard least-squares estimation procedure. A polynomial velocity function is discussed, but the model allows for any appropriate velocity function to be used to suit particular conditions of wound closure. Corneal curvature effects are incorporated to avoid the errors associated with a planar representation of wound closure on a curved corneal surface. The approach offers greater flexibility by avoiding the theoretical flaws of previous models which are often limited in scope and nonbiological in nature.

Hyaluronate in healing of corneal alkali wound in the rabbit

The effect of topically applied 1% and 2% sodium hyaluronate was studied on the healing of a standardized rabbit corneal alkali wound. The healing of the epithelium, the stroma, and the endothelium was evaluated separately by applying quantitative methods. The healing was followed for 8 weeks. Both 1% and 2% sodium hyaluronate have a statistically significant positive influence on the epithelial re-surfacing, especially during the late healing phase. Furthermore, sodium hyaluronate has a positive healing influence on the stromal healing at a certain time interval during the observation period. This was reflected as a better maintained stromal dry mass level compared to control. The stromal healing was parallel to a faster re-surfacing of the endothelium.

Dry mass and water content in the corneal epithelium and superficial stroma during healing of corneal alkali wounds

The dry mass contents of the rabbit corneal epithelium and of the superficial stroma were determined during healing of a standardized corneal alkali wound. Quantitative microradiography was applied to freeze-sectioned corneal specimens. This method allows a morphological resolution of the order of 1 micron and quantitative dry mass determinations in volumes down to about 100 microns 3. For comparison, the epithelial morphology was studied using Toluidine Blue stained sections. Flat epithelial cells covered the wound surface during the first day, the dry mass content being equivalent to that of the normal wing cells. The dry mass content in the epithelium and, indirectly, the water content, followed the hydration state of the superficial stroma. A progressive thickening of the epithelium and a varying morphological picture were found during the first 2 months following initial damage. Normal thickness and cytoarchitecture were found after 6 months. Generally, the dry mass content of the epithelium was reduced compared with physiological values for 2 months. Later, the cells close to the stroma regained normal values. Cells in the middle zone and the superficial zone reached normal values after 6 months. The significance of the endothelial damage for the hydration state of the epithelium was discussed.

Increased cytochrome oxidase activity in alkali-burned corneas

Cytochrome oxidase (CO) activity on alkali-burned rabbit corneas was investigated histochemically to determine the metabolic change in inflamed corneas during wound healing. Cryostat sections of chemically burned and mechanically scraped corneas were stained for CO activity, which is regarded as an index of metabolic activity. Following chemical injury, positive CO activity was detected initially in the vascular endothelial cells of limbal blood vessels. Numerous active polymorphonuclear leukocytes (PMNs) and monocytes were found intravascularly and perivascularly. Fibroblasts that appeared at the wound site exhibited marked CO activity around the limbus. Over a period of 13 days, PMNs gradually invaded the central cornea, followed by fibroblasts of high metabolic activity. The areas of PMN infiltration were the same areas in which fibroblasts showed intense staining, suggesting that a PMN-derived mediator or secondary products might affect the activation of fibroblasts. Epithelial resurfacing was delayed in the chemically burned corneas, although reepithelialization was completed within two to three days in the scraped corneas. Limbal epithelial cells, which recently were suggested as the source of epithelial renewal, showed a remarkable increase of metabolic activity in response to chemical inflammatory stimulation, whereas those in the scraped model did not. This suggests that epithelial cell renewal at the limbus was accelerated in the presence of disturbed reepithelialization.

Fibronectin in corneal wound healing

Fibronectin is an extracellular structural protein with the unique ability to bind to both cells and collagen. It plays a major role in the development of the cornea. The universal appearance of fibronectin within 8 hours of corneal wounding has promoted major interest in its wound healing properties. The early clinical evidence for fibronectin treatment of recurrent corneal erosion and certain forms of keratitis sicca is strong. The current major problem preventing commercial use is the antigenic nature of fibronectin derived from separate species. Human pooled fibronectin has been suggested as a commercial source of this ubiquitously occurring protein.

Clinical patterns of corneal epithelial wound healing

We studied the reepithelialization of corneal abrasions in 21 patients. All abrasions, irrespective of the nature of injury, followed a consistent pattern during reepithelialization. Three to six convex leading fronts of migrating epithelial sheets developed along the circumference of the defect and progressed toward the center. Neighboring fronts met along their sides, resulting in the formation of various geometric shapes. In the final stage of the healing process a contact line shaped like a "Y" or two "Y's" placed with their long axes end to end was seen where the advancing fronts of migrating epithelial sheets met. The rate of healing of the abrasions was determined by measuring the area of the abrasions at daily intervals from serial photographs. The area of the epithelial defects decreased exponentially with time, indicating a constant rate of epithelial cell migration.

The behaviour of corneal epithelium following a standardized alkali wound

The short and long-term healing of the rabbit corneal epithelium was studied after a standardized alkali wound. The wound was inflicted by applying a round filter paper, 5.5 mm in diameter, soaked in 1 N NaOH, for 60 seconds on the central cornea. The wound size and intensity was chosen not to cause melting and perforation, and not to cause vascular ingrowth. n-Heptanol corneal wounds of the same size were used as control. The eyes were followed for 8 weeks. Two phases of epithelial healing were discerned. The initial healing phase lasted 48 h during which the would was completely resurfaced. In spite of the more extensive tissue damage caused by alkali, the initial epithelial healing rate was faster than in n-heptanol wounds. The late healing phase consisted of recurrent epithelial break down, sometimes seen preceded by epithelial blister formation. Four weeks after trauma the state of epithelial healing was at its worst.

Stromal reaction and repair after corneal alkali wound in the rabbit: a quantitative microradiographic study

The corneal stromal reaction and repair were studied after alkali wound in the rabbit. The wound was followed for 6 months. The size and opacity of the wound were followed in the slit lamp and photographically. Morphology and dry mass or water content were studied using quantitative microradiography. The size of the wound opacity diminished throughout the observation period. The density of the opacity varied with the stromal water content. An increased water content as well as large variation in water content caused increased opacity. The corneal edema, determined already after 1 hr, was not detectable by 1 week using quantitative microradiography. Subsequently, an extensive edema developed, water accumulation being most marked in the posterior part of the cornea. After 8 weeks, an increase in interlamellar dry mass content could be determined as a sign of a stromal healing process. Stromal thickness and dry mass content had returned to normal values by 6 months.

Growth factor effects on corneal wound healing

Corneal wound healing and the many factors which may promote or prevent it are of concern to both clinicians and researchers. Ophthalmologists are often confronted with corneal dystrophies and problematic injuries resulting in persistent and recurrent epithelial erosion and with permanent endothelial loss. Scientists have long noted that corneal epithelial and endothelial cell layers are among the most simple and accessible for experimental manipulation and non-invasive study in both animal and human models. Healing epithelium provides a classical model of cell locomotion over a surface, while endothelial cell viability and function is of fundamental importance to maintenance of corneal clarity and vision during the aging process as well as after disease, surgery, or laser treatment. Growth factors can now be produced homogeneously by genetic engineering, increasing the potential of commercial development for purposes such as corneal wound healing. This article reviews some known properties of growth factors whose effects have been at least partially characterized in corneal tissues.

Epithelial healing: quantitative monitoring of the cornea following alkali burn

A micropolarographic system was used as a quantitative means of monitoring the healing course of corneal epithelium following a 10 second exposure to 0.20 N sodium hydroxide solution. Concentrations of less than that strength produced inconsistent flux baselines due to incomplete damage to the epithelium, while higher concentrations commonly involved the stroma as well. Virtually complete epithelial destruction (down to the basement membrane) and reproducible flux baselines were found, however, with the 0.20 N induced lesion studied in detail here. The healing course following those exposures consisted of two well defined phases: an initial period of hypoflux lasting some 48 h before rising back up to the pre-lesion baseline, followed then by a period of hyperflux lasting about 7 days before decreasing once again down to the pre-lesion baseline. This oxygen flux sequence closely parallels certain sliding and mitotic phases of epithelial healing already established in the literature.

The effects of chemical injury on the ocular surface

Chemical burns of the eye may destroy all of the corneal epithelium and large portions of conjunctival epithelium into the fornices. Restoration of the ocular surface after a chemical burn depends on the centritedal movement of conjunctival epithelial cells and their adherence to the altered corneal stroma. Epithelial movement after a corneal burn is normal for 72 hours after a burn, but persistent epithelial defects thereafter commonly are not resolved until total corneal vascularization occurs. Although this fresh epithelium may be protected by a bandage soft contact lens, more promising and far-reaching approaches may follow epithelial supplementation and even replacement. The link between the health and integrity of the epithelial layer as it relates to the corneal substratum and its cellular constituents remains to be forged.

Indications for conjunctival transplantation

Long-term follow-up (two to five years) of 12 unilateral chemical burn patients treated by conjunctival transplantation shows permanent stabilization of the ocular surface. The procedure was used in another group of five patients with unilateral recalcitrant epithelial defects. Regardless of the etiology of the epithelial defect, prompt healing of the surface occurs after conjunctival transplantation with no further stromal loss and long-term stabilization of the surface. Such results suggest that epithelial replacement may be a valuable therapeutic approach to a variety of ocular surface disorders.

Enzymatic activity of beta-N-acetylglucosaminidase in the alkali-burned rabbit cornea

Rabbit eyes burned with 1 n NaOH were studied for enzymatic activity by biochemical methods. The activity of beta-N-acetylglucosaminidase (E.C. 3.2.1.50), one of the hydrolases, was found to be significantly increased in corneal epithelium and stroma 1, 2, 3, and 4 weeks after alkali burns. The increase in enzyme activity seemed to be in correlation with the polymorphonuclear leukocytes (PMNs) infiltration of the stroma, as found by histopathologic studies. Therefore the PMNs might be the source of the increasing activity of this enzyme after alkali burns.

Regulation of corneal collagenase production: stimulation of serially passaged stromal cells by blood mononuclear cells

After three or more passages, stromal cell outgrowths from explants of normal rabbit corneas and from corneas previously burned with alkali did not produce collagenase until they were stimulated by the addition of rabbit blood-derived mononuclear cells or media conditioned by them. Normal stromal cells required stimulation by lymphocytes or their products from alkali-burned animals, whereas those from alkali-burned corneas were stimulated by lymphocytes from either normal or alkali-burned rabbits.