Ординатура / Офтальмология / Английские материалы / Ocular Periphery and Disorders_Dartt, Bex, Amore_2011

Figure 4 Diffuse damage to the corneal epithelium following the accumulation of environmental debris beneath the upper eyelid. Fluorescein was applied topically to show the epithelial defect (arrow).

Table 1 Examples of staining patterns associated with corneal pathology

iron is believed to be related to the uneven distribution of the tear film over the corneal surface in these areas (Table 2).

Band keratopathy

Deposition of calcium in the epithelium and calcification of Bowman’s membrane is termed band keratopathy. Of unknown etiology, this process tends to occur in eyes with significant chronic disease. It has also been seen in patients with elevated serum calcium levels, such as occurs with hyperparathyroidism.

Chemical/thermal burns

Significant chemical or thermal burns of the corneal epithelium can result in not only diffuse epithelial damage, but also damage to limbal stem cells, perhaps limiting the capacity of the epithelium to heal. These cases may require further surgical intervention, such as limbal stem cell transplant.

Medication toxicity

Topical and systemic medications and preservatives used in topical medications are an important cause of induced epithelial changes.

Table 2 Iron deposition in the corneal epithelium

The commonly used eyedrop preservative benzalkonium (BAK), non-steroidal anti-inflammatory eyedrops, trifluridine, proparacaine, and tetracaine are a few examples of medications that can result in nonhealing epithelial defects. Usually, if identified early enough and if without infection, the induced punctate epithelial defects will rapidly heal. However, if left unchecked and continued use of such medications occurs, corneal melt and even perforation can occur.

Deposition of material in the epithelium can occur with certain eyedrops and systemic medication. For example, the use of amiodarone can result in a whorl-like deposition of material. This is not an indication for stopping the medication, as it generally does not affect vision in any manner. Ciprofloxacin, when given topically, can result in a deposition of white crystals in epithelial defects. This can have visual symptoms, and, therefore, the medication should be stopped and replaced with an alternative antibiotic.

Thygeson’s punctate keratitis

The presence of round central punctate white opacities without corneal edema is the hallmark presentation of Thygeson’s punctate keratitis. The etiology of this disease is unknown. Patients generally present complaining of a significant foreign-body sensation and may have decreased vision if subepithelial haze is present. While usually controlled by steroid or cyclosporin topical treatment, this entity commonly recurs.

Subepithelial Disease

Epithelial basement membrane dystrophy

Also known as Cogan’s dystrophy and map-dot- fingerprint dystrophy, this disease is characterized by areas of thickened epithelial basement membrane appearing as grey epithelial patches or lines along with microcystic epithelial changes appearing as epithelial dots. Although usually asymptomatic, the disease can result in recurrent corneal erosion which is frequently difficult to treat and requires medical or possibly even surgical intervention.

Subepithelial infiltrates

Following infection with adenovirus resulting in epidemic keratoconjunctivitis, patients may experience persistent haze in their vision. This may be related to persistent subepithelial infiltrates in the visual axis. These infiltrates

may improve with time. Although treatment with topical steroid may clear the infiltrates rapidly, they routinely reappear once steroid treatment is discontinued. A very slow taper is advocated in these situations. Overall, avoidance of topical steroids as primary treatment is recommended due to this issue of steroid dependence.

Stromal Disease

Infection

A variety of infections can result in significant damage to the corneal tissue. The pathogens include bacterial, viral, fungal, and parasitic microbes.

Differentiating between bacterial corneal ulcers on examination can be quite difficult. A broad-spectrum antibiotic is generally initially prescribed until further information is obtained from corneal cultures. The more common bacterial pathogens include staphylococcus, streptococcus, and pseudomonas, but cultures must be used to rule out the more rare bacterial organisms such as Bacillus, Corynebacterium, Actinomyces, and Neisseria. While most bacteria require an epithelial defect of some degree to allow bacteria to enter the corneal tissue, it is important to note Neisseria can penetrate through intact epithelium (Figure 5).

Herpes simplex virus is of significant importance to consider when evaluating a patient with infectious keratitis. While the characteristic dendritic appearance of the herpes virus moving along corneal nerves makes it fairly obvious to diagnose in these situations, the sometimes atypical appearing stromal infiltrates may require a culture to identify the causative organism.

Fungal corneal ulcers, though more rare than bacterial ulcers, recently had a resurgence possibly related to contaminated contact lens solution. The corneal infiltrates tend to have a feathery appearance along their edges and do not respond to antibiotics. Sending fungal cultures at the same time as bacterial cultures is key to avoid late diagnosis of these infections.

Figure 5 Infectious keratitis with diffuse conjunctival injection (dilated blood vessels), corneal edema, and a dense corneal infiltrate (arrow).

Even more rare are parasitic corneal ulcers, such as acanthameoba. Acanthameoba ulcers are most often seen in patients with a history of contact lens wear who frequently have a recent history of swimming. Generally these ulcers are more painful than they appear they should be. Initial appearance on slit-lamp examination can mimic Herpes keratitis, and these ulcers are often initially misdiagnosed as such. Later in the course of the disease a ring infiltrate can present, sometimes mimicking the corneal pattern seen in topical anesthetic abuse. The acanthameoba parasites can be difficult to culture and may require corneal biopsy for diagnosis. Sometimes, it is also possible to visualize the parasitic cysts on confocal microscopy.

Dystrophies

Stromal corneal dystrophies are rare, yet important, inherited corneal diseases to identify. There are three main types of stromal corneal dystrophies, including granular, macular, and lattice dystrophy. Table 3 reviews the inheritance patterns, material deposited, and appropriate stain to visualize the material on pathologic specimen.

Degenerations

Corneal degenerations encompass a large category of corneal disease, which includes such more common processes as keratoconus, pellucid marginal degeneration, and arcus senilis.

Keratoconus has an unclear etiology, but pathological specimens reveal degeneration of the corneal stroma, Descemet’s membrane breaks, and damage to Bowman’s membrane. Clinically, progressive steepening and thinning of the central cornea can be seen, which eventually can create a cornea too irregular for even hard contact lens wear. Other corneal changes that may be visualized include an iron line around the area of corneal steepening (Fleischer’s ring), stress lines on the posterior cornea (Vogt’s striae), a scissoring reflex on retinoscopy, or areas of corneal opacification and edema from ruptures of Descemet’s membrane (acute hydrops). These changes may be seen on slit-lamp examination; however, it may be necessary to employ computerized tomography or topography to diagnose early keratoconic (forme fruste) changes. Patients with advanced degeneration can consider either intrastromal ring segments or corneal transplantation (Figure 6).

Pellucid marginal degeneration has a similar disease process as keratoconus; however, while the cornea in keratoconus is thinnest at the steepest location, the cornea in pellucid marginal degeneration is thinnest just below the steepest location, which is often near the inferior limbus. Similar pathologic changes can be seen and similar surgical interventions can be considered.

Arcus senilis is a peripheral corneal degeneration that is quite common in those 50 years of age or older. When examined pathologically, the white ring seen peripherally consists of lipid deposits. This can be a sign of systemic hyperlipidemia in those younger than 50 years of age or can be a sign of carotid stenosis in those with asymmetric arcus senilis.

Descemet’s copper deposition

The deposition of copper in Decesmet’s membrane can sometimes be seen in Wilson’s disease. This often difficult to diagnose disease of copper transport, which leads to copper deposition in the liver and brain, can sometimes be diagnosed quickly at the slit-lamp examination with visualization of the green–brown deposition of copper in the posterior cornea (Kayser–Fleischer ring).

Endothelial Disease

Fuchs’ dystrophy

Fuchs’ dystrophy is characterized by dysfunction of corneal endothelial cells. Wart-like excrescences are deposited by the endothelium into Descemet’s membrane, which can

be visualized on slit-lamp examination and on confocal microscopy. Confocal microscopy can also reveal the change in size (polymegathism) and change in shape (pleomorphism) of the dysfunctional endothelial cells. As the endothelium becomes further damaged, corneal edema worsens and may require medical treatment with hypertonic eyedrops and, eventually, even surgical intervention with cornea transplantation to improve visual acuity.

Pseudophakic bullous keratopathy

During phacoemulsification of the lens, significant stress can be placed on the corneal endothelium, either mechanically with intraocular instruments, or through transfer of energy in the form of ultrasound. These processes can result in loss of endothelial cells, with resultant increased pleomorphism and polymegathism in an attempt to maintain corneal deturgescence. When too much endothelial damage has occurred, persistent corneal edema may present postoperatively and may worsen with time. These patients may require medical intervention with hypertonic eyedrops or, eventually, may require cornea transplantation.

Surgery

Once a thorough corneal examination is completed, a proposed medical or surgical intervention can be planned. To understand which surgical options are available, slitlamp biomicroscopy must be used to accurately identify the layers of the cornea affected by the disease process.

The answer to this question will guide the physician to determine which of a vast array of surgical procedures could be undertaken.

The grand scope of the surgical treatment of corneal disease cannot be covered, in full, in this article. A brief overview of some of the more frequently used interventions will be reviewed.

Surgical Intervention of Epithelial Disease

Amniotic membrane graft

Amniotic membranes have many applications in ocular surgery. For nonhealing or slowly healing epithelial defects, this tissue can be sutured in place over the anterior surface of the cornea, where it helps protect against further corneal degradation and promotes epithelial healing. The amniotic membrane provides a matrix on which new cells can grow, helps prevent excessive inflammation, and aids in preventing corneal scarring and neovascularization.

Conjunctival flap

For poorly healing or nonhealing damage to the corneal surface, a conjunctival flap is an option. A partial or complete (Gunderson) flap can be dissected to cover the affected area, depending on the peripheral or central location of the corneal damage. Any patients with an active corneal process, such as infectious keratitis, are not good candidates for a conjunctival flap as it will not only interfere with appropriate treatment measures, but will also obscure the surgeon’s view when attempting to examine the affected tissue.

Corneal glue

Cyanoacrylate tissue adhesive can be used to adhere small corneal perforations or severely thinned corneal tissue. Areas of perforated cornea from infection, trauma, postoperative wound leaks, or other etiologies can be treated with cynanoacrylate adhesive, which can at least temporarily renew the integrity of the globe.

Disodium ethylenediaminetetraacetic acid chelation

Band keratopathy can present as a dense deposition of calcium hydroxyapetite in the visual axis. Following removal of the corneal epithelium, the anterior corneal calcium deposition can be removed by soaking the affected tissue in disodium EDTA, and scraping away the residual calcium with an ophthalmic blade. Patching or bandage contact lens must be placed until the epithelium heals adequately (Figure 7).

Limbal stem cell transplant

Poorly healing epithelial defects may be a result of damaged epithelial limbal stem cells. When enough damage

Figure 7 Pre- (top) and postoperative (bottom) band keratopathy following disodium-EDTA chelation. Notice the white calcium hydroxypatite in the top photograph.

has occurred to these stem cells, vascularization of the corneal surface ensues. This can result from a variety of conditions such as infections, chemical burns, or post operatively. If the disease is unilateral, a stem cell autograft can be attempted from the fellow eye. If both eyes are affected, an allograft from donor tissue must be used, leading to a poorer prognosis and the necessity to systemically immunosuppress the patient postoperatively.

Phototherapeutic keratectomy

Phototherapeutic keratectomy (excimer laser) can be used to remove diseased tissue in the anterior cornea, such as deposits associated with anterior corneal dystrophies or anterior corneal scars. Although typically no significant refractive change is induced, with the removal of more tissue, the risk of corneal flattening inducing hyperopia increases. Postoperative scarring can occur, and many advocate the use of topical mitomycin C intraoperatively to decrease the risk of scarring.

Pterygium excision

Pterygium refers to a benign growth of the conjunctiva. A pterygium commonly grows from the nasal side of the sclera and is associated with ultraviolet-light exposure.

The most successful technique for pterygium excision has been under much debate for many years. The options of leaving behind a bare sclera, using a conjunctival autograft, rotational graft, or amniotic membrane graft are all under current use. The debate centers around the question of which technique results in the lowest frequency of recurrence. The gold standard seems to be pterygium excision with conjunctival autograft, despite several ongoing studies reviewing the newer option of using the amniotic membrane graft.

Stromal puncture

The epithelial disease of anterior basement membrane dystrophy often leads to chronic recurrent corneal erosions. In an effort to create better adherence of the epithelium to its basement membrane and stroma, stromal puncture is attempted, usually with a 25-gauge needle inserted into the anterior stroma. Most commonly, this procedure is undertaken in affected areas outside of the visual axis and only in those patients that have failed medical therapy with lubrication and hypertonic solutions such as sodium chloride, which are also used to try to create a more firm adherence between the anterior cornea layers.

Tarsorrhaphy

Tarsorrhaphy is a surgical procedure in which the eyelids are sutured together to protect the cornea. A temporary or permanent tarsorrhaphy can be one of the most important options in the corneal surgeons armamentarium. Particularly helpful in creating a more hospitable environment for epithelial healing, this process can be used to promote epithelial healing in instances such as post operatively following penetrating keratoplasty (PK), neurotrophic epithelial defects, Bells palsy, or chemical burns. A newer option – using botulinum toxin A to create a temporary tarsorrhaphy – is currently being studied as an alternative, which perhaps affords similar protection to a sutured tarsorrhapy while allowing for easier slit-lamp examination.

Surgical Intervention of Stromal Disease

Anterior lamellar keratoplasty

Anterior corneal scars or deposits can be treated with an anterior lamellar keratoplasty. Rather than replacing a full-thickness corneal button as is accomplished in a PK, a partial-thickness trephination or dissection is accomplished to remove only the anterior diseased tissue without performing any manipulation of the posterior endothelial layer. This is an especially important option in young patients with excellent endothelial cell counts and morphology, but whose vision is obstructed by stromal opacities.

Penetrating keratoplasty

Unlike lamellar keratoplasty, a PK involves the fullthickness removal of diseased corneal tissue from the host and replacement of all layers of corneal tissue with a donor cadaveric corneal button. The procedural technique has evolved over the past 100 years, resulting in the high success rate that is now enjoyed by the thousands of patients that undergo full-thickness corneal transplantations each year (Figure 8). Topical immunosuppression, usually with prednisolone acetate 1%, is generally a long-term necessity to prevent rejection. Irregular astigmatism generally requires suture manipulation or hard contact lens placement post operatively to attain the best-corrected visual acuity.

Corneal biopsy

For those patients with unusual corneal processes without clear diagnosis despite corneal culture or scrapings, corneal biopsy is the next option. Removing a small anterior portion of corneal tissue from the periphery affords a tissue sample for further pathological examination.

Corneal laceration repair

With the advent of the 10-0 nylon suture, the ability of the corneal surgeon to repair the integrity of the globe following penetrating corneal injury improved significantly. Ensuring the absence of vitreous or iris incarceration in the wound, that the wound edges are well apposed, and that the wound is watertight are of vital importance. Fulfilling these objectives as soon as possible after the penetrating injury occurs will hopefully help to promote healing and lower the risk of endophthalmitis (visionthreatening inflammation of the internal ocular tissues).

Refractive surgery

Since its introduction, the field of corneal refractive surgery has grown exponentially. Earlier procedures, such as

Figure 8 One-year status post penetrating keratoplasty. A single 10-0 prolene running suture remains.

radial keratotomy (RK) and laser thermokeratoplasty (LTK) – which have since fallen out of favor – provided important knowledge and insight into the manipulation of corneal tissue. The current options include procedures such as laser-assisted in situ keratomileusis (LASIK), photorefractive keratectomy (PRK), laser-assisted subepithelial keratomileusis (LASEK), conductive keratoplasty (CK), and intrastromal ring segments. The goal of these procedures is usually to remove the need for corrective lenses in all, or most, distance and/or near-daily situations. A thorough review of the importance of appropriate patient selection, preoperative evaluations, individual surgical options, the risks and benefits of each procedure, and the appropriate postoperative care is beyond the scope of this article (Figure 9).

Prosthetic keratoplasty

For those patients with extensive corneal neovascularization, severe damage to limbal stem cells, or who have failed prior PKs, the prosthetic keratoplasty is available (i.e., Boston type 1 keratoprosthesis). Although the technology has evolved substantially over the last 10 years, the keratoprosthesis generally still requires glaucoma surgical management concomitantly and is not appropriate for those patients requiring straightforward PK without a history of such issues as significant corneal vascularization or repeat PK failures.

Surgical Intervention of Endothelial Disease

The surgical manipulation and replacement of diseased corneal endothelium has taken substantial strides forward in recent years. After going through several generations of endothelial surgery with disappointing results, the ophthalmology community has arrived at the more promising current technique of Descement’s Stripping Automated Endothelial Keratoplasty (DASEK).

Descemets Stripping Automated Endothelial Keratoplasty

The most common diseases resulting in endothelial dysfunction are Fuchs’ dystrophy and pseudophakic bullous keratopathy. Although it has been well understood for quite some time that corneal edema in these situations was a result of dysfunctional corneal endothelium, until recently, a full-thickness (penetrating) keratoplasty was the sole viable option. The well-documented time-consuming process of helping these patients obtain their best corrected visual acuity following PK could not be avoided. Now, however, with the advent of the DSAEK procedure, the corneal surgeon can carefully dissect off the Descement’s basement membrane and endothelium of the host, and replace a thin posterior donor cornea lenticule in its place (Figure 10). Early studies suggest that, when successful, this process results in faster healing, less induced postoperative astigmatism, and patients reach their best-corrected visual acuity faster when compared to PK.

See also: Adaptive Immune System and the Eye: Mucosal Immunity; Artificial Cornea; Contact Lenses; Corneal Dystrophies; Corneal Endothelium: Overview; Corneal Epithelium: Cell Biology and Basic Science; Corneal Epithelium: Response to Infection; Corneal Epithelium: Transport and Permeability; Corneal Imaging: Clinical; Corneal Nerves: Anatomy; Corneal Nerves: Function; Corneal Scars; The Corneal Stroma; Drug Delivery to Cornea and Conjunctiva: Esteraseand Protease-Directed Prodrug Design; Gene Therapy for the Cornea, Conjunctiva, and Lacrimal Gland; Imaging of the Cornea; Knock-Out Mice Models: Cornea, Conjunctiva, Eyelids and Lacrimal Gland; Lids: Anatomy, Pathophysiology, Mucocutaneous Junction; Refractive Surgery and Inlays; Regulation of Corneal Endothelial Cell Proliferation; Regulation of Corneal Endothelial Function; Stem Cells of the Ocular Surface; Tear Film; The Surgical

Treatment for Corneal Epithelial Stem Cell Deficiency, Corneal Epithelial Defect, and Peripheral Corneal Ulcer.

Further Reading

Ang, L., Chua, K., and Tan, D. (2007). Current concepts and techniques in pterygium treatment. Current Opinion in Ophthalmology 18: 308–313.

Bahar, I., Kaiserman, I., McAllum, P., et al. (2008). Comparison of posterior lamellar keratoplasty techniques to penetrating keratoplasty. Ophthalmology 115: 1525–1533.

Beebe, D. (2008). Maintaining transparency: A review of the developmental physiology and pathophysiology of two avascular tissues. Seminars in Cell and Developmental Biology 19: 125–133.

Cauchi, P., Ang, G., Azuara-Blanco, A., et al. (2008). A systematic literature review of surgical interventions for limbal stem cell deficiency in humans. American Journal of Ophthalmology 146: 251–259.

Dua, H. and Azuara-Blanco, A. (2000). Limbal stem cells of the corneal epithelium. Survey of Ophthalmology 44: 415–425.

Gomes, J., Romano, A., Santos, M., et al. (2005). Amniotic membrane use in ophthalmology. Current Opinion in Ophthalmology

16: 233–240.

Hersh, P., Brint, S., Maloney, R., et al. (1998). Photorefractive keratectomy versus laser in situ keratomileusis for moderate to high myopia. Ophthalmology 105: 1512–1523.

Ma, J., Graney, J., and Dohlman, C. (2005). Repeat penetrating keratoplasty versus the Boston keratoprosthesis in graft failure.

International Ophthalmology Clinics 45: 49–59.

Meek, K., Dennis, S., and Khan, S. (2003). Changes in the refractive index of the stroma and its extrafibrillar matrix when the cornea swells. Biophysical Journal 85: 2205–2212.

Meek, K., Leonard, D. W., Connon, C. J., et al. (2003). Transparency, swelling and scarring in the corneal stroma. Eye 17: 927–936.

McCarey, B., Edelhauser, H., and Lynn, M. (2008). Review of corneal endothelial specular microscopy for FDA clinical trials of refractive procedures, surgical devices and new intraocular drugs and solutions. Cornea 27: 1–16.

Suh, L., Yoo, S., Deobhakta, B. S., et al. (2008). Complications of descemet’s stripping with automated endothelial keratoplasty. Ophthalmology 115: 1517–1524.

Sutphin, J. (ed.) (2007). Basic and Clinical Science Course. External Disease and Cornea. San Francisco, CA: American Academy of Ophthalmology.

Trattler, W. and Barnes, S. (2008). Current trends in advanced surface ablation. Current Opinion in Ophthalmology 19: 330–334.

Yanoff, M. and Duker, J. (2004). Ophthalmology. St. Louis, MO: Mosby.

Glossary

Adhesion complex – The term used to refer to the different components of the cell: substrate junction that is present at the basal aspect of the stratified squamous epithelial tissues of the body, including the skin and the cornea. The adhesion complex includes not only the hemidesmosomes that contain a6b4 integrin and collagen XVII, but also the inner hemidesmosomal plaque that contains plectin and BPA230, the anchoring filaments that are made of laminin 332, the anchoring fibers and the adhesions plaques that both contain collagen type VII. This structure forms during development and must partially or completely disassemble during wound healing and tissue regeneration and then reassemble after healing is complete. If the adhesion complex fails to form due to mutations in one of its components, blistering of the epithelial sheets covering the skin and cornea occurs.

Basement membrane zone – Sheets of epithelial cells that form the outer surfaces of the body are separated from their underlying mesechymal cells by basement membranes. Basement membranes are composed of the network-forming collagen type IV, the heparan sulfate proteoglycan perlecan, and several different types of laminins. The basement membrane zone consists of the basal plasma membrane surface of the basal epithelial cells, the hemidesmosomes, and the basement membrane itself. At the EM level, it consists of the inner and outer plaques of the hemidesmosomes, the lamina lucida, and the lamina densa.

Defensins – These are small cysteine-rich cationic proteins first characterized in leukocytes in 1985. The name was chosen because these proteins have antibacterial, antifungi, and antiviral activity. They consist of 18–45 amino acids including six (in vertebrates) to eight conserved cysteine residues. They bind to proteins on the surfaces of pathogens and form much of the basis of what immunologists call the innate defense system. Human tears have at least 4 different defensins and their presence in the tears is regulated during the healing of corneal wounds. Glycocalyx – This term means sugar coat and in the cornea it refers to the thin film of sugar-containing material at the apical surface of the apical cells on the cornea. The primary molecules that make up the

glycocalyx in the cornea are called mucins. There are several different types of mucins, some secreted and some cell-surface bound, on the healthy, wet ocular surface. The mucins of the glycocalyx help the tear film to spread over the ocular surface.

Integrins – A family of glycoproteins that function as heterodimers to mediate both cell:matrix interaction and cellular signaling. Integrins are integral membrane single pass proteins whose cytoplasmic domains bind to elements of the cytoskeleton including actin for most of the integrins and intermediate filaments for a6b4. Their extracellular domains bind to extracellular matrix proteins including collagens, fibronectin, and laminins. Integrins are considered mechanotransducers of forces from outside cells and tissues to the cytoskeleton to allow cells to change shape during development and wound healing.

Niche – From the old French word nichier meaning to nest, this term is usually used to refer to the site where adult stem cells reside. In the cornea, the niche is believed to be located at the palisades of Vogt at the corneoscleral junction. Stem cell niches consist of both specialized extracellular matrix molecules that the stem cells adhere to as well as other cell types that play supportive roles by expressing cell:cell adhesion proteins and/or growth factors that act to maintain the stem cells within the niche and inhibit their differentiation.

Refractive surgery – This term refers to surgical procedures developed to improve the refractive state of the eye and decrease dependence of the patient on glasses or contact lenses. The major forms of refractive surgery are laser-assisted in situ keratomileusis and photorefractive keratectomy. According to the American Society of Cataract and Refractive Surgery, more than 900 000 refractive procedures were performed in the US in 2005.

The Corneal Epithelium Has Vital

Functions in Vision

The corneal epithelium is a transparent covering that allows light entry into the eye. It is the outermost

layer of the cornea and is comprised of three to five anatomically distinct layers of stratified squamous nonkeratinizing epithelial cells (Figure 1(a)). It protects the inner neuronal tissues of the retina from microbial invasion and prevents water loss from the corneal stroma.

When light waves hit the curved corneal surface and pass from the air to an aqueous medium, they are bent. This refraction of light waves focuses the light on the fovea of the retina. The crystalline lens is less powerful than the cornea at bending light waves because the light waves are already in an aqueous environment when they pass from the cornea to the lens. Although the lens does refract light, the differences in the refractive indices of the corneal stroma, the aqueous humor, and the lens are small compared to the difference in refractive index between air and the tear film at the corneal surface. Thus the cornea is the major refractive surface of the eye. Because of its significant impact on the refraction of light, changes in the curvature of the cornea alter the position where light focuses on the retina. This property of the cornea has lead to the development of procedures to alter corneal curvature to reduce or eliminate refractive errors such as myopia and astigmatism. One of the most popular of these procedures worldwide is refractive surgery using laserassisted in situ keratomileusis (LASIK) which involves cutting a flap of tissue at the front of the cornea through the epithelium and stroma, removing or ablating tissue from the stroma using an excimer laser, and repositioning the flap of tissue. LASIK patients will experience an immediate improvement in their sight; as a result, millions of these procedures are performed. It remains to be determined whether or not there will be long term consequences for people who have had LASIK or other corneal refractive procedures as their corneas age and

for this reason basic researchers in the field of corneal wound healing are concerned over the popularity of this procedure.

The Apical Squames Possess Specializations to Promote Tear Film Spreading

The apical-most epithelial surface of the cornea has flattened cells called squames. The apical surfaces of these cells have protrusions called microplicae (Figure 1(b)). These are specialized structures that facilitate the spreading of the tear film. The microplicae have bound to them the glycocalyx that contains the membrane-associated mucins and other glycoproteins whose functions include facilitating tear film spread. The outermost layer of the tear film is a layer of lipid molecules that increases the stability of the tear film and prevents tear evaporation. The tear film itself has several functions. It prevents the eyelids from adhering to the corneal epithelium. It is the medium through which carbon dioxide, a product of corneal metabolism, is exchanged for oxygen from the air. In addition, it contains small molecules called defensins that inhibit the growth of microbial agents. When the corneal epithelium is injured, the concentration of these defensins in tears increases transiently and then, after the wound is closed, decreases back to the levels before wounding. These data suggest that defensins play important roles in protecting the cornea from infection especially after the epithelial barrier is disrupted during wound healing. Underneath the superficial apical layer are two to three layers of cells often referred to as wing cells by corneal biologists. The final layer that sits on the basement membrane is called the basal cell layer.

The Epithelial Basal Cells Adhere to the Underlying Basement Membrane via tight Adhesive Junctions called Hemidesmosomes

The corneal basal cells maintain the tight association of the epithelium with the basement membrane through specialized adhesion junctions called hemidesmosomes. A schematic representation of the adhesion complex, including the hemidesmosomes, is shown in Figure 2(a) and a transmission electron micrograph is shown in Figure 2(b). The hemidesomosomes form a tight rivet holding the epithelial cells on to the basement membrane and anterior stroma. Unlike adherens junctions and focal adhesions, hemidesmosomes utilize intermediate filaments rather than actin to stabilize cell adhesion and maintain cell:matrix adhesions. Hemidesmosomes must disassemble when the corneal epithelium is injured to permit the sheet of epithelial cells to move. The failure of the hemidesmosomes to disassemble during wound healing can lead to delayed wound closure, and failure to reassemble after migration is complete can lead to corneal epithelial erosions. The molecules that are present within hemidesmosomes that mediate these events are beginning to be characterized. a6b4 integrin acts as the primary mechanotransducer of forces from the basal cells to the extracellular matrix molecule laminin 332 in the basement membrane. In addition, a membrane-associated collagen, collagen XVII/BPA180, is also important for the stability of these structures. The function of these integral membrane molecules within the hemidesmosome is to associate with laminin 332, which makes up the anchoring filaments and type VII collagen that makes up the anchoring fibers. The anchoring filaments are found in the basement

membrane and the anchoring fibers in the anterior corneal stroma. Thus, the hemidesomosomes are an essential part of an adhesion complex that distributes sheering forces from the cell surface to deep within the stroma. This adhesion complex includes the intraand extracellular aspects of the hemidesmosomal plaque at the plasma membrane as well as the anchoring filaments and anchoring fibers.

The need for rapid mobilization of the corneal epithelial cells after a corneal wound combined with the sheerness of the corneal epithelial basement membrane forced a compromise during evolution: the basal surfaces of the corneal basal cells are more exposed to sheering forces from the environment compared to the basal surfaces of the basal cells in skin. The enfolding and ridges in the epidermis result in a basement membrane surface area that is increased in the basal keratinocytes relative to their lateral and apical surfaces. This provides increased protection from debridement of the epidermis. Because of the sheerness of the corneal epithelial basement membrane, it is more prone to recurrent epithelial erosions compared to the skin. However, this sheer surface also permits the corneal epithelial sheet to migrate rapidly and thereby minimizes dessication of the cornea and allows wounds to close quickly to minimize infections.

While the cells that make up the corneal epithelial basal cell layer are the most proliferative of all the corneal epithelial layers, research has shown that the cornea epithelial basal cell layer does not contain the corneal epithelial stem cells (CESCs). If the CESCs were primarily located in the central cornea, they would be exposed to the DNAdamaging affects of ultraviolet light, and, lacking pigment to absorb light energy, would sustain irreversible mutations. To maintain the corneal epithelium throughout life,