Comparison of protein kinase C subtype expression between normal and aniridic human ocular surfaces: implications for limbal stem cell dysfunction in aniridia

Dysfunction of the limbal epithelial stem cell niche in aniridia-associated keratopathy

PURPOSE

Abnormalities in the limbal niche microenvironment have been suggested to be causally involved in aniridia-associated keratopathy (AAK), but histological analyses on the limbal structure and composition in AAK are lacking. Here, we investigated morphologic and molecular alterations of the limbal epithelial stem cell niche in human congenital aniridia.

METHODS

The blind, buphthalmic and painful left eye of a 16-year old girl with congenital aniridia and juvenile glaucoma had to be enucleated because of uncontrolled intraocular pressure. The diagnosis of AAK was based on classical clinical features and partial limbal stem cell deficiency in the superior half. Genetic analysis identified a large heterozygous PAX6 gene deletion encompassing exons 11-15 as well as exon 9 of the neighboring ELP4 gene. Three limbal biopsies were taken from the superior, nasal and temporal regions to isolate and cultivate limbal epithelial progenitor cells and subject them to mRNA expression analyses. The globe was vertically bisected and processed for light and transmission electron microscopy and immunohistochemistry.

RESULTS

Comparative analysis of the superior and inferior limbal zones showed a gradual degradation of palisade structures associated with the transition from a hyperplastic to an attenuated corneal epithelium, inflammatory cell infiltrations and basement membrane irregularities. The clinically unaffected inferior part revealed no distinct stem cell clusters in the preserved palisade region, but a uniform population of hyperproliferative, undifferentiated progenitor cells in the basal/suprabasal layers of limbal and corneal epithelia, which gave rise to maldifferentiated epithelial cells exhibiting a conjunctival/epidermal phenotype and nuclear-to-cytoplasmic translocation of Pax6. The structure of the limbal niche was fundamentally perturbed, showing marked alterations in extracellular matrix composition, dislocation of atypical melanocytes lacking melanosomes and melanin, aberrant Wnt/β-catenin and retinoic acid signaling, and massive immune cell infiltration.

CONCLUSIONS

Considering the limitations of a single Case study, the findings suggest that ocular surface alterations in AAK are caused by a primary dysfunction and gradual breakdown of the limbal stem cell niche through Pax6-related effects on both melanogenesis and epithelial differentiation.

Investigation of Visual System Involvement in Spinocerebellar Ataxia Type 14

Spinocerebellar ataxia type 14 (SCA-PRKCG, formerly SCA14) is a rare, slowly progressive disorder caused by conventional mutations in protein kinase Cγ (PKCγ). The disease usually manifests with ataxia, but previous reports suggested PRKCG variants in retinal pathology. To systematically investigate for the first time visual function and retinal morphology in patients with SCA-PRKCG. Seventeen patients with PRKCG variants and 17 healthy controls were prospectively recruited, of which 12 genetically confirmed SCA-PRKCG patients and 14 matched controls were analyzed. We enquired a structured history for visual symptoms. Vision-related quality of life was obtained with the National Eye Institute Visual Function Questionnaire (NEI-VFQ) including the Neuro-Ophthalmic Supplement (NOS). Participants underwent testing of visual acuity, contrast sensitivity, visual fields, and retinal morphology with optical coherence tomography (OCT). Measurements of the SCA-PRKCG group were analyzed for their association with clinical parameters (ataxia rating and disease duration). SCA-PRKCG patients rate their vision-related quality of life in NEI-VFQ significantly worse than controls. Furthermore, binocular visual acuity and contrast sensitivity were worse in SCA-PRKCG patients compared with controls. Despite this, none of the OCT measurements differed between groups. NEI-VFQ and NOS composite scores were related to ataxia severity. Additionally, we describe one patient with a genetic variant of uncertain significance in the catalytic domain of PKCγ who, unlike all confirmed SCA-PRKCG, presented with a clinically silent epitheliopathy. SCA-PRKCG patients had reduced binocular vision and vision-related quality of life. Since no structural retinal damage was found, the pathomechanism of these findings remains unclear.

Does oral spirulina protect the cornea from formaldehyde exposure? Application to anatomy laboratories

In this study, the protective effect of spirulina on corneal injury after formaldehyde (FA) exposure was assessed. Thirty adult male albino rats were divided into four groups. Group I: 12 rats were divided into two subgroups: I-a (negative control) and I-b (positive control). Group II (spirulina group): six rats received spirulina via an oral gavage feeding needle at a daily dose of 400 mg/kg b.w. Group III (FA exposure group): six rats were subjected to 10% FA inhalation for 2 h per day for 2 weeks (5 days per week). Group IV (FA exposure group treated with spirulina): six rats were exposed to 10% FA as in group III, with co-administration of spirulina as in group II. After 2 weeks, all the rats were sacrificed; the corneas were dissected and processed for paraffin sections. The sections were stained with hematoxylin and eosin (H&E), Masson's trichrome (MTC), or avidin-biotin peroxidase, and examined by light microscopy. The sections of rat cornea exposed to FA (Group III) showed disorganized and compressed epithelium with erosions. Subepithelial mononuclear cell infiltration and invasion of blood vessels were also evident. Stromal collagen fibers were disorganized and widely separated. All these changes were ameliorated by administration of spirulina (Group IV). Corneal thickness was nearly normal in Group IV, statistically significantly less than in Group III. It was concluded that spirulina protects against FA-induced corneal injury in rats. Clin. Anat. 31:830-837, 2018. © 2018 Wiley Periodicals, Inc.

Limbal Stem Cell Deficiency in the Context of Autoimmune Polyendocrinopathy

PURPOSE

To report two sisters with bilateral progressive visual loss and photophobia secondary to stem cell deficiency due to multiple endocrine deficiency.

METHODS

Case reports and review of medical literature.

RESULTS

The younger sister had severe photophobia and decreased visual acuity since May 2000. Despite multiple outpatient visits no definite cause was found and conservative treatments failed. On slit lamp examination severe meibomian gland dysfunction, loss of eyelashes, decreased tear meniscus, diffuse corneal vascularization, and delayed punctate fluorescein staining of corneal epithelium were detected. She also had episodes of hypotension, oral candidiasis, and seizures. Her systemic workup revealed multiple endocrine deficiency (Addison's disease and hypoparathyroidism). Hormone replacement therapy with fludrocortisone and oral calcium accompanied by punctual occlusion led to significant clinical recovery . The older sister showed a similar pattern but interestingly the onset was later and the signs and symptoms were milder.

CONCLUSIONS

In the pediatric age group with diffuse corneal vascularization and no definite cause, systemic workup should be done to rule out multiple endocrine deficiencies. The therapy consists of hormone replacement therapy and management of dry eye.

Comparison of Two Rabbit Models with Deficiency of Corneal Epithelium and Limbal Stem Cells Established by Different Methods

Limbal stem cell defect model is an important animal model that provides a basis for the study of ocular surface diseases. The rabbit cornea is of moderate size and is widely used in such studies as an experimental animal model. At present, the main modeling methods are alkali burns, and corneal limbus girdling and corneal epithelium doctoring. Each method has its own characteristics. In this study, we observed rabbit models with severe ocular surface defect established by the two methods and changes after amniotic membrane transplantation. In the first, second, third, and fourth week after operation, the clinical manifestations, corneal transparency, and new vessels were observed according to the standard rating scale of ocular surface, compared between the two methods, and then statistically analyzed. In the fourth week after operation, the rabbits were sacrificed and their corneas and corneal limbus were extracted from sclera, embedded by optimum cutting temperature compound, frozen, and sliced for hematoxylin and eosin staining and pathological examination. There were two groups in this study. Group 1 (alkali burns) had more severe complications, such as, conjunctiva, nubecula, new vessel hyperplasia, and so on, compared to group 2 (corneal limbus girdling and corneal epithelium doctoring). In addition, there were striking differences in corneal transparency and new vessels between the two groups (p < 0.05). Corneal transparency in group 1 was lower than in group 2. New vessels in group 1 were less in the first 2 weeks, but obviously increased compared to group 2 in the subsequent weeks. Alkaline burn could be used to study new vessel hyperplasia, while corneal limbus girdling and corneal epithelium doctoring are more suitable for studying stem cell transdifferentiation, interactive roles of stem cells and microenvironment, and so on.

Diadenosine polyphosphates in the tears of aniridia patients

PURPOSE

To quantify diadenosine polyphosphate levels in tears of congenital aniridia patients to estimate the ocular surface changes associated with congenital aniridia compared to normal individuals.

METHODS

Fifteen patients diagnosed with congenital aniridia and a control group of forty volunteers were studied. Tears were collected to quantify the levels of diadenosine polyphosphates Ap4 A and Ap5 A by high-performance liquid chromatography (H.P.L.C). Break-up time (BUT), corneal staining, McMonnies questionnaire and the Schirmer I test were applied to both groups.

RESULTS

Dinucleotides in congenital aniridia patients were higher than in control subjects. For the congenital aniridia group, under 15 years old, the values were 0.77 ± 0.01 μm and 0.17 ± 0.02 μm for Ap4 A and Ap5 A, respectively. The group aged from 15 to 40 years old provided concentrations of 4.37 ± 0.97 μm and 0.46 ± 0.05 μm for Ap4 A and Ap5 A, the group over 40 gave concentrations of 11.17 ± 5.53 μm and 0.68 ± 0.17 μm for Ap4 A and Ap5 A. Dinucleotide concentrations increased with age, being statistically significant different among the three age groups (p < 0.05). Congenital aniridia patients showed a normal tear secretion and no dry eye McMonnies scores, except for the group over 40 years old. BUT values decreased and corneal staining increased with age and correlated with the levels of diadenosine polyphosphates (p < 0.05).

CONCLUSIONS

The levels of dinucleotides in tears increase in aniridia patients compared with healthy subjects, and they seem to be related with the progression of corneal disorders in aniridia patients, both of which increase with ageing.

Treatment of congenital aniridia associated with subluxated infantile cataract

A 5 year old boy was presented at Eye clinic University clinical center Tuzla with congenital aniridia in both eyes. Clinical examination revealed visual acuity of 0,08 without correction in right and 0.7 with -5.0 Dsph and -1.0 Dcyl Axx 109° in left eye. Opthalmologic examination showed bilateral aniridia associated with moderate cataract in the right and incipient cataract in the left eye. In the right eye, zonular weakness with incipient capsular displacement and esotropia of Δ6º, were noted. The patient underwent phacoemulsification, implantation of capsular tension ring and Artificial Iris implant in the capsular bag. Phacoemulsification went uneventful and early postoperative recovery was successful with no signs of aniridia-associated keratopathy development and normal values of intra ocular pressure. Patient was not motivated for operation of the left eye and it was corrected with soft contact lens. Six month after the operation visual acuity in the right eye improved to 0.9 with +1.25Dsph and maintained stable in left eye, with complete elimination of esotropia and signs of binocular vision restoration. Small incision cataract extraction with IOL and Artificial Iris implantation in one procedure can be used to correct congenital aniridia and cataract with significant visual function improvement.

Identification of human fibroblast cell lines as a feeder layer for human corneal epithelial regeneration

There is a great interest in using epithelium generated in vitro for tissue bioengineering. Mouse 3T3 fibroblasts have been used as a feeder layer to cultivate human epithelia including corneal epithelial cells for more than 3 decades. To avoid the use of xeno-components, we evaluated human fibroblasts as an alternative feeder supporting human corneal epithelial regeneration. Five human fibroblast cell lines were used for evaluation with mouse 3T3 fibroblasts as a control. Human epithelial cells isolated from fresh corneal limbal tissue were seeded on these feeders. Colony forming efficiency (CFE) and cell growth capacity were evaluated on days 5–14. The phenotype of the regenerated epithelia was evaluated by morphology and immunostaining with epithelial markers. cDNA microarray was used to analyze the gene expression profile of the supportive human fibroblasts. Among 5 strains of human fibroblasts evaluated, two newborn foreskin fibroblast cell lines, Hs68 and CCD1112Sk, were identified to strongly support human corneal epithelial growth. Tested for 10 passages, these fibroblasts continually showed a comparative efficiency to the 3T3 feeder layer for CFE and growth capacity of human corneal epithelial cells. Limbal epithelial cells seeded at 1×10⁴ in a 35-mm dish (9.6 cm²) grew to confluence (about 1.87–2.41×10⁶ cells) in 12–14 days, representing 187–241 fold expansion with over 7–8 doublings on these human feeders. The regenerated epithelia expressed K3, K12, connexin 43, p63, EGFR and integrin β1, resembling the phenotype of human corneal epithelium. DNA microarray revealed 3 up-regulated and 10 down-regulated genes, which may be involved in the functions of human fibroblast feeders. These findings demonstrate that commercial human fibroblast cell lines support human corneal epithelial regeneration, and have potential use in tissue bioengineering for corneal reconstruction.

Ocular and systemic findings in a survey of aniridia subjects

PURPOSE

To determine the prevalence of ocular and systemic abnormalities in a group of subjects with aniridia.

METHODS

Survey forms developed by Aniridia Foundation International were sent to all members prior to the 2010 AFI member conference. An additional form was provided for completion by physicians caring for patients. Forms were then collected from all members who attended the meeting.

RESULTS

A total of 155 surveys were distributed, of which 83 (53%) were completed. The mean age was 25.4 ± 18.4 years, with 65% sporadic and 35% familial cases, and 2.4% with WAGR (Wilms' tumor, aniridia, genitourinary anomalies, and mental retardation) syndrome. Ocular abnormalities included nystagmus (83%), cataract (71%), dry eye (53%), glaucoma (46%), keratopathy (45%), foveal hypoplasia (41%), strabismus (31%), and retinal disease (5%). The mean age at diagnosis of aniridia was 22.1 months (median, 1.5 months) and glaucoma was 13.6 years (median, 8.5 years). Of 38 subjects with aniridia and glaucoma, 76% were treated medically, and 58% had been treated surgically. In subjects with glaucoma, the mean number (± SD) of glaucoma medications was 1.8 ± 1.3, and number of surgical procedures was 1.7 ± 2.0. Developmental delay was reported in 17%. The mean body mass index and the prevalence of obesity in subjects with aniridia was significantly greater (P = 0.003) than in siblings without aniridia.

CONCLUSIONS

In this study, aniridia was associated with nystagmus and other motility problems, cataract, glaucoma, and keratopathy. Systemic abnormalities included increased average body mass index and obesity, which appeared to occur not only in WAGR syndrome but more broadly in aniridia.

Immunohistochemical expression of epithelial cell markers in corneas with congenital aniridia and ocular cicatrizing pemphigoid

PURPOSE

We investigated the immunohistochemical characteristics of corneal specimens in congenital aniridia and pemphigoid using various corneal markers to determine the status of the corneal epithelium.

METHODS

Conjunctivalization was clinically suspected in all corneas. Ten aniridia and seven pemphigoid paraffin-embedded corneal specimens were stained with periodic Schiff reagent (PAS) and antibodies against CK3/12, CK12, CK19, breast cancer resistance protein 1 (BCRP) and p63.

RESULTS

Aniridia: six cases contained goblet cells, four were negative. Both groups had cases with (three of six; one of four) and without CK19 positivity and cases with (two of six; three of four) and without p63 positivity. All aniridia cases except two in the goblet cell group were CK3/12- and CK12-positive and BCRP-negative. Pemphigoid: only one of the seven cases contained goblet cells. This case stained positively for CK19, 3/12, 12 and p63 and negatively for BCRP. The other six cases were positive for CK3/12, five of which were positive for CK12; only one case was CK19-positive. Three cases were p63-positive and two BCRP-positive. The CK12 staining was heterogenous in most cases and was often found in the superficial layer.

CONCLUSION

Three different stages of epithelial characteristics were found in congenital aniridia and pemphigoid: (i) CK19-negative and inhomogenous CK12-positive cases indicating epithelium mainly from (partly) CK12-deficient limbal stem cells; (ii) CK19- and/or goblet cell-positive and CK12-positive cases with their epithelia originating from CK12-deficient limbal stem cells and from incursing conjunctival cells; and (iii) CK19-positive and CK12-negative cases consisting of conjunctival cells alone.

Complications and visual prognosis in children with aniridia

PURPOSE

To characterize the ophthalmological findings, assess surgical outcomes, and review visual outcomes in aniridia.

METHODS

A retrospective case review was performed and data were collected, including patient demographics, incidence of aniridia-associated keratopathy, glaucoma, cataract, retinal breaks or detachments, optic nerve hypoplasia, macular hypoplasia, poor vision, and nystagmus. All outcomes from surgery, including penetrating keratoplasty, trabeculectomy, Ahmed valve insertion, and cataract extraction, were recorded.

RESULTS

Six children (12 eyes) had corneal abnormalities, 4 had optic nerve hypoplasia, 9 had nystagmus, and 2 had retinal detachments. Four patients (7 eyes) required penetrating keratoplasty. Five patients (9 eyes) developed glaucoma and only 1 of the 4 trabeculectomies performed succeeded. Of the 6 Ahmed valve procedures performed, all succeeded in maintaining a satisfactory intraocular pressure but some required needling and 5-fluorouracil. Eight patients developed cataract and 7 required surgery. Visual outcomes were poor despite treatment. Nine patients had Snellen acuity of 6/60 or less and required low visual aids to function.

CONCLUSION

Aniridia is a disorder that requires multiple surgeries. It has a poor visual prognosis despite early diagnosis and aggressive management. Newer techniques such as Ahmed valves and Boston keratoprostheses offer hope, but its proliferative nature makes treatment difficult.

Lycopersicon esculentum lectin is a marker of transient amplifying cells in in vitro cultures of isolated limbal stem cells

The maintenance of a healthy corneal epithelium under both normal and wound healing conditions is achieved by a population of stem cells (SCs) located in the basal epithelium at the corneoscleral limbus. In the light of the development of strategies for reconstruction of the ocular surface in patients with limbal stem cell deficiency, a major challenge in corneal SCs biology remains the ability to identify stem cells in situ and in vitro. To date, not so much markers exist for the identification of different phenotypes. CESCs (corneal epithelial stem cells) isolated from limbal biopsies were maintained in primary culture for 14 days and stained with Hoechst and a panel of FITC-conjugated lectins. All lectins, with the exception of Lycopersicon esculentum, labelled CESCs irrespective of the degree of differentiation. Lycopersicon esculentum, that binds N-acetylglucosamine oligomers, labelled intensely only the surface of TACs (single corneal epithelial stem cells better than colonial cells). These results suggest that Lycopersicon esculentum lectin is a useful and easy-to-use marker for the in vitro identification of TACs (transient amplifying cells) in cultures of isolated CESCs.

Aniridia: current pathology and management

Aniridia is a rare panocular disorder affecting the cornea, anterior chamber, iris, lens, retina, macula and optic nerve. It occurs because of mutations in PAX6 on band p13 of chromosome 11. It is associated with a number of syndromes, including Wilm's tumour, bilateral sporadic aniridia, genitourinary abnormalities and mental retardation (WAGR) syndrome. PAX6 mutations result in alterations in corneal cytokeratin expression, cell adhesion and glycoconjugate expression. This, in addition to stem-cell deficiency, results in a fragile cornea and aniridia-associated keratopathy (AAK). It also results in abnormalities in the differentiation of the angle, resulting in glaucoma. Glaucoma may also develop as a result of progressive angle closure from synechiae. There is cataract development, and this is associated with a fragile lens capsule. The iris is deficient. The optic nerve and fovea are hypoplastic, and the retina may be prone to detachment. Aniridia is a profibrotic disorder, and as a result many interventions--including penetrating keratoplasty and filtration surgery--fail. The Boston keratoprosthesis may provide a more effective approach in the management of AAK. Guarded filtration surgery appears to be effective in glaucoma. Despite our increasing understanding of the genetics and pathology of this condition, effective treatment remains elusive.

Corneal epithelial stem cells: deficiency and regulation

The corneal epithelium is continuously renewed by a population of stem cells that reside in the corneoscleral junction, otherwise known as the limbus. These limbal epithelial stem cells (LESC) are imperative for corneal maintenance with deficiencies leading to in-growth of conjunctival cells, neovascularisation of the corneal stroma and eventual corneal opacity and visual loss. One such disease that has traditionally been thought to be due to LESC deficiency is aniridia, a pan-ocular congenital eye disease due to mutations in the PAX6 gene. Corneal changes or aniridia related keratopathy (ARK) seen in aniridia are typical of LESC deficiency. However, the pathophysiology behind ARK is still ill defined, with current theories suggesting it may be caused by a deficiency in the stem cell niche and adjacent corneal stroma, with altered wound healing responses also playing a role (Ramaesh et al, International Journal of Biochemistry & Cell Biology 37:547-557, 2005) or abnormal epidermal differentiation of LESC (Li et al., The Journal of Pathology 214:9, 2008). PAX6 is considered the master control gene for the eye and is required for normal eye development with expression continuing in the adult cornea, thus inferring a role for corneal repair and regeneration (Sivak et al., Developments in Biologicals 222:41-54, 2000). Studies of models of Pax6 deficiency, such as the small eyed (sey) mouse, should help to reveal the intrinsic and extrinsic mechanisms involved in normal LESC function.

Autologous Serum Eyedrops in the Treatment of Aniridic Keratopathy

OBJECTIVE

To study the effect of autologous serum eyedrop application in aniridic keratopathy.

DESIGN

Prospective, consecutive, comparative, interventional case series.

PARTICIPANTS

Twenty-six eyes from 13 patients (7 males and 6 females) with aniridic keratopathy treated with autologous serum eyedrops.

METHODS

All patients underwent a complete ophthalmic examination. The ocular surface examinations included corneal impression cytologic analysis and tear film evaluation. The eyes were divided into 4 groups according to the Mackman classification. Ocular surface photography was used to evaluate the corneal surface and tear film before treatment and every 2 or 3 days until serum drops were stopped. Tear films were evaluated by tear film break-up time (BUT) (normal, 10 seconds or more), Schirmer's test with anesthesia (normal, 10 mm/5 minutes or more), tear meniscus level (normal, 0.5 mm or more), and rose bengal and fluorescein staining pattern of the cornea. Impression cytologic analysis was carried out both before starting the serum eyedrops treatment and a few days after its finalization.

MAIN OUTCOMES MEASURES

Tear film production and stability, corneal epithelialization, and corneal epithelium squamous metaplasia.

RESULTS

There were no local side effects from autologous serum treatment. Clinical manifestations and slit-lamp findings were in relation to the severity of keratopathy. All patients showed a subjective improvement of keratopathy symptoms after the autologous serum applications. The corneal epithelialization, corneal epithelial cell squamous metaplasia, and tear stability improved significantly with the treatment, but visual acuity, regression of vascular pannus, and subepithelial scarring showed only slight improvement with treatment.

CONCLUSIONS

Autologous serum eyedrops improved the aniridic keratopathy in all patients, particularly in patients with light or moderate severity. In these patients, use of autologous serum eyedrops was superior to conventional therapy with substitute tears for improving the ocular surface and subjective comfort.

[Corneal epithelial diseases related to limbal stem cell deficiency]

Treatment of corneal epithelial diseases induced by limbal stem cell deficiency is an important challenge in ocular surface reconstruction. Since the 1990s, corneal stem cells have been localized in the limbus. This new concept completely changed the way we consider ocular surface reconstruction, with new diseases now found to be isolated in the ocular surface. Limbus insufficiency syndromes are specific depending on their origin (congenital or acquired), their expression (unilateral or bilateral, partial or total), their progression (acute or chronic), and the mechanism involved (burn, infection, chronic inflammation, etc.). Some of these diseases are local diseases and others are systemic diseases. Clinically, limbus insufficiency is a switch of the normal corneal epithelial phenotype (expression of a specific keratin, avascularity, and transparency of the corneal matrix) in an opaque and fibrovascularized cornea. In terms of cellular biology, a phenotype is a terminal expression of a cell differentiation process. This process is the outcome of the interaction between the genome of a cell or a group of cells with their microenvironment. In limbus insufficiency, epithelial cells and corneal matrix are destroyed, and it is the destruction of these two components that leads to limbus insufficiency syndrome.

Keratitis Associated With Multiple Endocrine Deficiency

PURPOSE

To report an 8-year-old girl with bilateral progressive visual loss and photophobia secondary to stem cell deficiency as a result of multiple endocrine deficiency.

METHODS

A case report and review of medical literature.

RESULTS

The patient suffered from severe photophobia and decreased visual acuity since May 2000. Despite multiple outpatient visits, no definite cause was found, and conservative treatments failed. On slit-lamp examination severe meibomian gland dysfunction, loss of eyelashes, decreased tear meniscus, diffuse corneal vascularization, and delayed punctate fluorescein staining of corneal epithelium were detected. She had also episodes of hypotension, oral candidiasis, and seizures. Her systemic work up revealed multiple endocrine deficiency (Addison disease and hypoparathyroidism). Hormone replacement therapy with fludrocortisone and oral calcium accompanied by punctual occlusion led to significant clinical recovery.

CONCLUSION

In a pediatric patient with diffuse corneal vascularization and no definite cause, systemic workup should be done to rule out multiple endocrine deficiencies. Treatment consists of hormone replacement therapy and management of the dry eye.

Identification and characterization of limbal stem cells

The maintenance of a healthy corneal epithelium under both normal and wound healing conditions is achieved by a population of stem cells (SC) located in the basal epithelium at the corneoscleral limbus. In the light of the development of strategies for reconstruction of the ocular surface in patients with limbal stem cell deficiency, a major challenge in corneal SC biology remains the ability to identify stem cells in situ and in vitro. Until recently, the identification of limbal stem cells mainly has been based on general properties of stem cells, e.g. lack of differentiation, prolonged label-retaining, indefinite capacity of proliferation exemplified by the clonogenic assay as well as their special role in corneal wound healing. During the last years, a number of molecular markers for the limbal SC compartment has been proposed, however, their role in distinguishing limbal SC from their early progeny is still under debate. Data reported from the literature combined with our own recent observations suggest, that the basal epithelial cells of the human limbus contain ABCG2, K19, vimentin, KGF-R, metallothionein, and integrin alpha9, but do not stain for K3/K12, Cx43, involucrin, P-cadherin, integrins alpha2, alpha6, and beta4, and nestin, when compared to the basal cells of the corneal epithelium. A relatively higher expression level in basal limbal cells was observed for p63, alpha-enolase, K5/14, and HGF-R, whereas there were no significant differences in staining intensity for beta-catenin, integrins alphav, beta1, beta2, and beta5, CD71, EGF-R, TGF-beta-RI, TGF-beta-RII, and TrkA between limbal and corneal basal epithelial cells. Therefore, a combination of differentiation-associated markers (e.g. K3/K12, Cx43, or involucrin) and putative SC-associated markers (e.g. ABCG2, K19, vimentin, or integrin alpha9) may provide a suitable tool for identification of human limbal SC. While most putative SC markers label the majority of limbal basal cells and, therefore, may not distinguish SC from progenitor cells, only ABCG2 was strictly confined to small clusters of basal cells in the limbal epithelium. At present, ABCG2 therefore appears to be the most useful cell surface marker for the identification and isolation of corneal epithelial SC. Moreover, the characteristics of the specific microenvironment of corneal SC, as provided by growth factor activity and basement membrane heterogeneity in the limbal area, could serve as additional tools for their selective enrichment and in vitro expansion for the purpose of ocular surface reconstruction.

Evolving concepts on the pathogenic mechanisms of aniridia related keratopathy

Heterozygosity for PAX6 deficiency (PAX6+/-) results in aniridia. Corneal changes in aniridia-related keratopathy (ARK) include corneal vascular pannus formation, conjunctival invasion of the corneal surface, corneal epithelial erosions and epithelial abnormalities, which eventually result in corneal opacity and contribute to visual loss. Corneal changes in aniridia have been attributed to congenital deficiency of corneal limbal stem cells. The aim of this paper is to review the potential mechanisms that may underlie the pathogenesis of aniridia related keratopathy. Current evidence, based on clinical observations and an animal model of aniridia suggest that the proliferative potential of the corneal limbal stem cells may not primarily be impaired. The corneal changes in aniridia may be related to an abnormality within the limbal stem cell niche. The mechanisms underlying progressive corneal pathology in aniridia appear multi-factorial and include: (1) abnormal corneal healing responses secondary to anomalous extracellular matrix metabolism; (2) abnormal corneal epithelial differentiation leading to fragility of epithelial cells; (3) reduction in cell adhesion molecules in the PAX6 heterozygous state, rendering the cells susceptible to natural shearing forces; and (4) conjunctival and corneal changes leading to the presence of cells derived from conjunctiva on the corneal surface.

Idiopathic limbal stem cell deficiency

PURPOSE

To describe and to characterize the clinical findings and prognosis of patients with idiopathic limbal stem cell deficiency (LSCD).

DESIGN

Retrospective case series.

PARTICIPANTS/METHODS

We reviewed records from seven patients whose LSCD had been diagnosed clinically and confirmed by impression cytology and in whom the cause of LSCD was never identified. A detailed history, clinical records, and results of slit-lamp biomicroscopy, photography, vital staining, and impression cytology were evaluated.

RESULTS

Six of seven patients (86%) were women, indicating a female predominance. Two patients were from the same family, whereas one other had a positive family history. Severe photophobia was noted in all patients and reduced vision in three patients. The main clinical findings included superficial vascularization, worse in the superior followed by the inferior and nasal cornea. The limbal regions showed a loss of limbal palisades of Vogt, and the adjacent peripheral cornea revealed an irregular and hazy epithelium with positive late fluorescein staining and the presence of conjunctival goblet cells by impression cytology. LSCD was bilateral in all patients but asymmetric in four. During a mean follow-up of 6.1+/- 3.8 years, the visual acuity decreased in both eyes of one patient after cataract extraction and in both eyes of two other patients without surgery. The process of conjunctivalization advanced in four patients (57%) and remained stable in three (43%) without surgical intervention.

CONCLUSIONS

Idiopathic LSCD is a rare and as yet poorly recognized clinical entity, and the findings reported herein may help explore how progressive loss of limbal stem cells occurs. Correct diagnosis of idiopathic LSCD is important so that the patient will not be subjected to unnecessary surgeries, which may actually severely worsen the clinical course.

Characteristics of the human ocular surface epithelium

An appreciation of the biological characteristics of the human ocular surface epithelium affords us a great insight into the physiology of the human ocular surface in health and disease. Here, we review five important aspects of the human ocular surface epithelium. First, we recognize the discovery of corneal epithelial stem cells, and note how the palisades of Vogt have been suggested as a clinical marker of their presence. Second, we introduce the concept of the gene expression profile of the ocular surface epithelium as arrived at using a new strategy for the systematic analysis of active genes. We also provide a summary of several genes abundantly or uniquely expressed in the human corneal epithelium, namely clusterin, keratin 3, keratin 12, aldehyde dehydrogenase 3 (ALDH3), troponin-I fast-twitch isoform, ssig-h3, cathepsin L2 (cathepsin V), uroplakin Ib, and Ca(2+)-activated chloride channel. Genes related to limbal and conjunctival epithelia are also described. Third, we touch upon the genetic abnormalities thought to be involved with epithelial dysfunction in Meesmann's dystrophy, gelatinous drop-like corneal dystrophy, and the ssig-h3-mutated corneal dystrophies. Fourth, we provide an update regarding the current state of knowledge of the role of cytokines, growth factors and apoptosis in relation to ocular surface homeostasis and tissue reconstruction; the main factors being epidermal growth factor (EGF), keratinocyte growth factor (KGF), hepatocyte growth factor (HGF), transforming growth factor-ss (TGF-ss), and some inflammatory cytokines. Fifth, corneal epithelial barrier function and dysfunction as measured by fluorophotometry is remarked upon, with an explanation of the FL-500 fluorophotometer and its ability to detect corneal epithelial dysfunction at a subclinical level. The research described in this review has undoubtedly generated a complete understanding of corneal epithelial pathophysiology-an understanding that, directly or indirectly, has helped advance the development of new therapeutic modalities for ocular surface reconstruction.

Limbal stem cells of the corneal epithelium

Stem cells have certain unique characteristics, which include longevity, high capacity of self-renewal with a long cell cycle time and a short S-phase duration, increased potential for error-free proliferation, and poor differentiation. The ocular surface is made up of two distinct types of epithelial cells, constituting the conjunctival and the corneal epithelia. Although anatomically continuous with each other at the corneoscleral limbus, the two cell phenotypes represent quite distinct subpopulations. Stem cells for the cornea reside at the corneoscleral limbus. The limbal palisades of Vogt and the interpalisade rete ridges are believed to be repositories of stem cells. The microenvironment of the limbus is considered to be important in maintaining the stemness of stem cells. Limbal stem cells also act as a "barrier" to conjunctival epithelial cells and normally prevent them from migrating on to the corneal surface. Under certain conditions, however, the limbal stem cells may be partially or totally depleted, resulting in varying degrees of stem cell deficiency with resulting abnormalities in the corneal surface. Such deficiency of limbal stem cells leads to "conjunctivalization" of the cornea with vascularization, appearance of goblet cells, and an irregular and unstable epithelium. This results in ocular discomfort and reduced vision. Partial stem cell deficiency can be managed by removing the abnormal epithelium and allowing the denuded cornea, especially the visual axis, to resurface with cells derived from the remaining intact limbal epithelium. In total stem cell deficiency, autologous limbus from the opposite normal eye or homologous limbus from living related or cadaveric donors can be transplanted on to the affected eye. With the latter option, systemic immunosuppression is required. Amniotic membrane transplantation is a useful adjunct to the above procedures in some instances.

Selective changes in protein kinase C (PKC) isoform expression in rabbit corneal epithelium during wound healing. Inhibition of corneal epithelial repair by PKCalpha antisense

Protein kinase C (PKC) isoforms display different sensitivities to modulators, tissue specificities and subcellular localizations. PKCalpha increases during rabbit corneal epithelial wound healing. Here we report differential expression of PKC isoforms in the cornea of rabbits at 1, 2, 4 and 8 days during re-epithelization. Cytosolic, membrane and detergent-insoluble fractions from epithelium were analysed by Western blot using monoclonal antibodies against the different PKC isoforms. We have identified PKCalpha, gamma, epsilon, mu and iota. PKCalpha and gamma were expressed only in the cytosolic fraction, with the expression of PKCalpha markedly increasing 4 days after injury. Corneas cultured in the presence of rabbit-specific PKCalpha antisense showed a greater than 50% inhibition of wound closure, compared to controls. The PKCepsilon and mu were expressed in the soluble, as well as in the membrane fraction. Additionally, 12% of PKCmu was found attached to the detergent insoluble fraction. The expression of the membrane-bound PKCepsilon and mu isoforms decreased between 1 and 2 days following injury. Only 10% of the PKCiota expressed in corneal epithelium was membrane bound, but between 4 and 8 days after de-epithelization, the expression in this fraction increased three-fold. Our results suggest that changes in the expression and distribution within the various fractions of selective isoforms of PKC after injury could be involved in events leading to wound healing and that PKCalpha is a key modulator in rabbit corneal wound repair.