Immunohistochemical characterization of extracellular matrix in the developing human cornea

Thickness Measurement of Endothelium-Descemet Membrane in Descemt Membrane Detachment Patients Using High-Definition Optical Coherence Tomography

Purpose: (1) To measure the corneal endothelium-Descemet membrane (EDM) layer thickness in Descemet membrane detachment (DMD) patients in vivo using high-definition optical coherence tomography (HD-OCT), and to investigate its correlation with age. (2) To explore whether the detachment time will affect the EDM thickness. (3) To explore whether the EDM thickness of cornea with DMD was different from that without DMD. Participants: Patients with DMD were divided into three groups. Group 1 included twenty-three patients whose Descemet membrane (DM) was partial or complete detached from the corneal stroma after various ocular surgeries. Group 2 included eight patients from group 1 who underwent twice HD-OCT examination on different days before the DM reattached to the stroma. Group 3 included nine patients from group 1 who had clear grayscale boundary between the DM and stroma in HD-OCT images after DM reattachment. Methods: All patients underwent HD-OCT and EDM thickness was measured using Image -Pro Plus 6.0. In Group 1, regression analyses were used to evaluate the correlation between EDM thickness and age, and the thickness difference between the ≤50-year-old group and the >50-year-old group was analyzed by independent sample t-test. In Group 2, paired samples t-test was used to check whether detachment time would affect EDM thickness. In Group 3, paired samples t-test was used to check whether the EDM thickness of cornea with DMD was different from that without DMD. p < 0.05 was considered significant. Results: In Group 1, the EDM thickness measured on the first post-operative day was 27.8 ± 3.6 μm, and a positive correlation was found between EDM thickness and age (r = 0.619, p < 0.05). The EDM thickness of ≤50-year-old group and >50-year-old group were 23.9 ± 3.2 and 29.2 ± 2.6 μm, and there was a significant difference between the two groups (p = 0.001). In Group 2, the first measurement of EDM thickness was 27.5 ± 4.0 μm, the second measurement was 27.6 ± 4.2 μm, the interval between the two measurements was 2.1 ± 1.6 days, and there was no significant difference between the two measurements (p = 0.328). In Group 3, the EDM thickness with DM detachment was 28.3 ± 3.5 μm, with DM reattachment was 23.4 ± 2.4 μm, there was a significant difference between the two measurements (p = 0.002). Conclusions: The EDM thickness in the state of DMD is thicker than its actual thickness in normal cornea, and EDM thickness of the >50-year-old group is much thicker than that of the ≤50-year-old group.

Challenges and strategies for the delivery of biologics to the cornea

Biologics, like peptides, proteins and nucleic acids, have proven to be promising drugs for the treatment of numerous diseases. However, besides the off label use of the monoclonal antibody bevacizumab for the treatment of corneal neovascularization, to date no other biologics for corneal diseases have reached the market. Indeed, delivering biologics in the eye remains a challenge, especially at the level of the cornea. While it appears to be a rather accessible tissue for the administration of drugs, the cornea in fact presents several anatomical barriers to delivery. In addition, also intracellular delivery barriers need to be overcome to achieve a promising therapeutic outcome with biologics. This review outlines efforts that have been reported to successfully deliver biologics into the cornea. Biochemical and physical methods for achieving delivery of biologics in the cornea are discussed, with a critical view on their efficacy in overcoming corneal barriers.

Cell sheet technology: Influence of culture conditions on in vitro-cultivated corneal stromal tissue for regenerative therapies of the ocular surface

The in vitro reconstruction of stromal tissue by long-term cultivation of corneal fibroblasts is a smart approach for regenerative therapies of ocular surface diseases. However, systematic investigations evaluating optimized cultivation protocols for the realization of a biomaterial are lacking. This study investigated the influence of supplements to the culture media of human corneal fibroblasts on the formation of a cell sheet consisting of cells and extracellular matrix. Among the supplements studied are vitamin C, fetal bovine serum, L-glutamine, components of collagen such as L-proline, L-4-hydroxyproline and glycine, and TGF-β1, bFGF, IGF-2, PDGF-BB and insulin. After long-term cultivation, the proliferation, collagen and glycosaminoglycan content and light transmission of the cell sheets were examined. Biomechanical properties were investigated by tensile tests and the ultrastructure was characterized by electron microscopy, small-angle X-ray scattering, antibody staining and ELISA. The synthesis of extracellular matrix was significantly increased by cultivation with insulin or TGF-β1, each with vitamin C. The sheets exhibited a high transparency and suitable material properties. The production of a transparent, scaffold-free, potentially autologous, in vitro-generated construct by culturing fibroblasts with extracellular matrix synthesis-stimulating supplements represents a promising approach for a biomaterial that can be used for ocular surface reconstruction in slowly progressing diseases.

A Review of Structural and Biomechanical Changes in the Cornea in Aging, Disease, and Photochemical Crosslinking

The study of corneal biomechanics is motivated by the tight relationship between biomechanical properties and visual function within the ocular system. For instance, variation in collagen fibril alignment and non-enzymatic crosslinks rank high among structural factors which give rise to the cornea's particular shape and ability to properly focus light. Gradation in these and other factors engender biomechanical changes which can be quantified by a wide variety of techniques. This review summarizes what is known about both the changes in corneal structure and associated changes in corneal biomechanical properties in aging, keratoconic, and photochemically crosslinked corneas. In addition, methods for measuring corneal biomechanics are discussed and the topics are related to both clinical studies and biomechanical modeling simulations.

Basement membranes in the cornea and other organs that commonly develop fibrosis

Basement membranes are thin connective tissue structures composed of organ-specific assemblages of collagens, laminins, proteoglycan-like perlecan, nidogens, and other components. Traditionally, basement membranes are thought of as structures which primarily function to anchor epithelial, endothelial, or parenchymal cells to underlying connective tissues. While this role is important, other functions such as the modulation of growth factors and cytokines that regulate cell proliferation, migration, differentiation, and fibrosis are equally important. An example of this is the critical role of both the epithelial basement membrane and Descemet's basement membrane in the cornea in modulating myofibroblast development and fibrosis, as well as myofibroblast apoptosis and the resolution of fibrosis. This article compares the ultrastructure and functions of key basement membranes in several organs to illustrate the variability and importance of these structures in organs that commonly develop fibrosis.

Thermo-responsive cell culture carriers based on poly(vinyl methyl ether)-the effect of biomolecular ligands to balance cell adhesion and stimulated detachment

Two established material systems for thermally stimulated detachment of adherent cells were combined in a cross-linked polymer blend to merge favorable properties. Through this approach poly(N-isopropylacrylamide) (PNiPAAm) with its superior switching characteristic was paired with a poly(vinyl methyl ether)-based composition that allows adjusting physico-chemical and biomolecular properties in a wide range. Beyond pure PNiPAAm, the proposed thermo-responsive coating provides thickness, stiffness and swelling behavior, as well as an apposite density of reactive sites for biomolecular functionalization, as effective tuning parameters to meet specific requirements of a particular cell type regarding initial adhesion and ease of detachment. To illustrate the strength of this approach, the novel cell culture carrier was applied to generate transplantable sheets of human corneal endothelial cells (HCEC). Sheets were grown, detached, and transferred onto planar targets. Cell morphology, viability and functionality were analyzed by immunocytochemistry and determination of transepithelial electrical resistance (TEER) before and after sheet detachment and transfer. HCEC layers showed regular morphology with appropriate TEER. Cells were positive for function-associated marker proteins ZO-1, Na+/K+-ATPase, and paxillin, and extracellular matrix proteins fibronectin, laminin and collagen type IV before and after transfer. Sheet detachment and transfer did not impair cell viability. Subsequently, a potential application in ophthalmology was demonstrated by transplantation onto de-endothelialized porcine corneas in vitro. The novel thermo-responsive cell culture carrier facilitates the generation and transfer of functional HCEC sheets. This paves the way to generate tissue engineered human corneal endothelium as an alternative transplant source for endothelial keratoplasty.

Ultrastructure of the posterior corneal stroma

PURPOSE

To reinvestigate the ultrastructure of the posterior stroma of the human cornea and to correlate the findings with the stromal behavior after big-bubble creation.

DESIGN

Observational consecutive 3-center case series.

SPECIMENS

Fresh corneoscleral buttons from human donors (n = 19) and organ-cultured corneoscleral buttons (n = 10) obtained after Descemet's membrane endothelial keratoplasty.

METHODS

Corneal specimens were divided into central (3 mm), mid peripheral (8 mm), and peripheral parts by trephination and processed for transmission electron microscopic and immunohistochemical analyses. A big bubble was created by air injection into the stroma of organ-cultured corneas before fixation.

MAIN OUTCOME MEASURES

The distance of keratocytes to Descemet's membrane, number of collagen lamellae between keratocytes and Descemet's membrane, diameter and arrangement of collagen fibrils, thickness of stromal lamella created by air injection, and immunopositivity for collagen types III, IV, and VI.

RESULTS

Stromal keratocytes were observed at variable distances from Descemet's membrane, increasing from 1.5 to 12 μm (mean, 4.97±2.19 μm) in the central, 3.5 to 14 μm (mean, 8.03±2.47 μm) in the midperipheral, and 4.5 to 18 μm (mean, 9.77±2.90 μm) in the peripheral regions. The differences in mean distances were significant (P < 0.0001). The number of collagen lamellae between Descemet's membrane and most posterior keratocytes varied from 2 to 10 and the diameter of collagen fibrils averaged 23.5±1.8 nm and corresponded with that of the remaining stroma. A thin layer (0.5-1.0 μm thick) of randomly arranged, unaligned collagen fibers, which was positive for collagen types III and VI, was observed at the Descemet-stroma interface. The residual stromal sheet separated by air injection in 8 of 10 donor corneas varied in thickness from 4.5 to 27.5 μm, even within individual corneas (≤3-fold), and was composed of 5 to 11 collagen lamellae that revealed keratocytes on their anterior surface and in between.

CONCLUSIONS

Barring an anchoring zone of interwoven collagen fibers at the Descemet-stroma interface, the findings did not provide any evidence for the existence of a distinctive acellular pre-Descemet's stromal layer in the human cornea. The intrastromal cleavage plane after pneumodissection seems to be nonreproducibly determined by the intraindividually and interindividually variable distances of keratocytes to Descemet's membrane.

Fibronectin antibody labels corneal stromal collagen fibrils in situ along their length and circumference and demonstrates distinct staining along the cell and stromal interfaces of Descemet's membrane

PURPOSE/AIM OF THE STUDY

An immunoperoxidase cytochemical study of fibronectin localization in the rat corneal stroma and Descemet's membrane was conducted following organ culture to determine whether stromal swelling allowed better primary antibody penetration into the normally tough fibrous corneal stroma.

MATERIALS AND METHODS

Following 24 h organ culture, corneas were fixed in 4% paraformaldehyde, washed and stained overnight at 4 °C in anti-fibronectin followed by washing and incubation in an appropriate secondary antibody and exposure to protein A-HRP. Cytochemical processing was carried out in a DAB-containing medium followed by dehydration and Epon embedding.

RESULTS

Observations of the stromal lamellae revealed the presence of a novel punctate staining pattern along the length of the collagen fibrils that extended around the fibril's circumference. Measurements on the peroxidase reaction product spacing indicated a periodicity of approximately 20.69 ± 3.57 nm along the fibril's length. Light microscopic immunocytochemistry revealed the presence of fibronectin staining occurred within the endothelial cell layer but only along the DM/stromal interface. Electron microscopic observations however, revealed that fibronectin staining occurred in distinct linear patterns along the length of both the endothelial and stromal DM interfaces.

DISCUSSION

Results indicate that organ culture mediated swelling helps facilitate the penetration of primary antibody into the corneal stroma. Observations suggest a novel association exists between fibronectin and stromal collagen fibrils that helps to mediate the arrangement and organization of the stromal extracellular matrix. Results also definitively indicate that fibronectin is deposited along both DM interfaces suggesting that it plays a role in the adhesion of both the endothelial cell layer and stroma to Descemet's membrane to help maintain the tissue architecture within this region of the cornea.

Extracellular Matrix is an Important Component of Limbal Stem Cell Niche

Extracellular matrix plays an important role in stem cell niche which maintains the undifferentiated stem cell phenotype. Human corneal epithelial stem cells are presumed to reside mainly at the limbal basal epithelium. Efforts have been made to characterize different components of the extracellular matrix that are preferentially expressed at the limbus. Mounting evidence from experimental data suggest that these components are part of the stem cell niche and play a role in the homeostasis of limbal stem cells. The extracellular matrix provides a mechanical and structural support as well as regulates cellular functions such as adhesion, migration, proliferation, self-renewal and differentiation. Optimization of the extracellular matrix components might be able to recreate an ex vivo stem cell niche to expand limbal stem cells.

Asymmetrical cell division and differentiation are not dependent upon stratification in a corneal epithelial cell line

To determine whether asymmetrical cell division takes place during growth and differentiation of corneal epithelial cells, we analyzed the expression of some proteins required for the correct execution of the asymmetric division in cultured RCE1-(5T5) cells, which mimic the differentiation of corneal epithelial cells. RT-PCR and immunostaining showed that Par-3, LGN (GPSM2), NuMA, and the mammalian homolog of inscuteable (Insc) are expressed by the cultured cells. Semi-quantitative RT-PCR demonstrated that Insc mRNA levels were stable throughout the experiment. Conversely, LGN and NuMA mRNAs increased slightly and steadily in proliferative cells, reaching a peak of about 20% above basal levels when cells were confluent. At later times, LGN and NuMA mRNAs decreased to become barely detectable when cells organized into a four-layered epithelium and expressed terminal phenotype as indicated by the highest expression of LDH-H mRNA. Cultivation under low Ca2+ conditions (0.09 mM) reduced about 50% Insc mRNA expression both in proliferating and confluent cultures, but did not affect the levels of LGN and NuMA mRNAs. Hence, asymmetric cell division seems to take place with a lower frequency in cells grown with low Ca2+ concentrations, in spite of the absence of stratification. Immunostaining experiments raise the possibility of an interaction between k3/K12 keratin cytoskeleton and Par-3. The results show for the first time the coordination between the expression of corneal epithelial cell differentiation and the expression of cell polarity machinery. They also suggest that asymmetric division does not depend on stratification; instead, it seems to be part of the differentiation program.

Challenges to the study of asymmetric cell division in corneal and limbal epithelia

Asymmetrical cell division in mammalian corneal epithelia is not widely accepted and it is not well characterized. Although some data led to propose that asymmetrical division occurs along the entire corneal epithelium, evidence from different laboratories implies that asymmetrical cell division in adult individuals could be confined to limbal epithelium, as suggested by the location of the corneal epithelial stem cells and the distribution of some molecular markers involved in regulation of stratification and cell differentiation. Nevertheless, most evidence sustaining the participation of asymmetric mitosis in corneal development and differentiation is merely an inference. Recent results based in cell culture experiments suggest that asymmetric division is part of the differentiation program; in such case, mitotic spindle orientation would be regulated by the structure, composition and active signaling pathways at the stem cell niche. Together, the results support the view that in adults, asymmetric division might be confined to limbus, and hence, the process takes place both in apico-basal oriented cells and in cells in which the mitotic spindle is horizontally oriented. In contrast, during development, asymmetrical divisions would be determined by intrinsic mechanisms involving cell polarity, predominantly occurring in apico-basal oriented cells. Future studies should be oriented to understand the regulation of the asymmetrical/symmetrical division, and the control of cell fate by the niche.

Changes in the localization of collagens IV and VIII in corneas obtained from patients with posterior polymorphous corneal dystrophy

Posterior polymorphous corneal dystrophy (PPCD) is a hereditary bilateral disorder affecting primarily the endothelium and Descemet's membrane (DM). The aim of this study was to determine the changes in the presence and localization of the alpha1-alpha6 collagen IV chains and alpha1, alpha2 collagen VIII chains in Czech patients with PPCD. Twelve corneal buttons from ten PPCD patients who underwent corneal grafting, as well as eight unaffected corneas, were used. Enzymatic indirect immunohistochemistry was performed on cryosections using antibodies against the alpha1-alpha6 collagen IV chains and alpha1, alpha2 collagen VIII chains. The intensity of the signal was examined separately in the basal membrane of the epithelium (BME), stroma and DM. More than 50% of PPCD specimens exhibited positivity for alpha1 and alpha2 collagen IV chains in the BME and in the posterior stroma, while no staining was detected in these areas in control specimens. The signal for the alpha1 and alpha2 collagen IV chains was more intense in DM of PPCD corneas compared to controls and it was shifted from the stromal side (in control tissue) to the endothelial side of DM (in the patients). A less intensive signal in PPCD corneas for the alpha3 and alpha5 chains in DM and an accumulation of alpha3-alpha5 in the posterior stroma in diseased corneas were the only differences in staining for the alpha3-alpha6 collagen IV chains. The alpha1 collagen VIII chain was detected on both the endothelial and the stromal sides of DM in 90% of patients with PPCD, compared with the prevailing localization on the stromal side of DM in control corneas. A change in the localization of the alpha2 collagen VIII chain in DM from vertically striated features in control specimens to double line positivity in the DM of PPCD corneas and positive staining in the posterior collagenous layer of four patients were also detected. In three PPCD patients a fibrous pannus located under the BME, positive for alpha1-alpha3, alpha5 collagen IV chains and alpha1 collagen VIII chain, was observed. The increased expression of the alpha1, alpha2 collagen IV and alpha1 collagen VIII chains and the change in their localization in DM may contribute to the increased endothelial proliferative capacity observed in PPCD patients.

Two clonal cell lines of immortalized human corneal endothelial cells show either differentiated or precursor cell characteristics

Access to primary human corneal endothelial cells (HCEC) is limited and donor-derived differences between cultures exacerbate the issue of data reproducibility, whereas cell lines can provide sufficient numbers of homogenous cells for multiple experiments. An immortalized HCEC population was adapted to serum-free culture medium and repeated cloning was performed. Clonally grown cells were propagated under serum-free conditions and growth curves were recorded. Cells were characterized immunocytochemically for junctional proteins, collagens, Na,K-ATPase and HCEC-specific 9.3.E-antigen. Ultrastructure was monitored by scanning and transmission electron microscopy. Two clonal cell lines, HCEC-B4G12 and HCEC-H9C1, could be isolated and expanded, which differed morphologically: B4G12 cells were polygonal, strongly adherent and formed a strict monolayer, H9C1 cells were less adherent and formed floating spheres. The generation time of B4G12 cells was 62.26 +/- 14.5 h and that of H9C1 cells 44.05 +/- 5.05 h. Scanning electron microscopy revealed that B4G12 cells had a smooth cell surface, while H9C1 cells had numerous thin filopodia. Both cell lines expressed ZO-1 and occludin adequately, and little but well detectable amounts of connexin-43. Expression of HCEC-specific 9.3.E-antigen was found commensurately in both cell lines, while expression of Na,K-ATPase alpha1 was higher in H9C1 cells than in B4G12 cells. B4G12 cells expressed collagen IV abundantly and almost no collagen III, while H9C1 cells expressed both collagens at reasonable amounts. It is concluded that the clonal cell line B4G12 represents an ideal model of differentiated HCEC, while H9C1 may reflect features of developing or transitional HCEC.

Significant ocular findings are a feature of heritable bone dysplasias resulting from defects in type II collagen

BACKGROUND/AIMS

The type II collagenopathies are a phenotypically diverse group of genetic skeletal disorders caused by a mutation in the gene coding for type II collagen. Reports published before the causative mutations were discovered suggest heritable bone dysplasias with skeletal malformations may be associated with a vitreoretinopathy.

METHODS

A retrospective notes search of patients with a molecularly characterised type II collagenopathy chondrodysplasia who had been examined in the ophthalmology clinic was conducted.

RESULTS

13 of 14 patients had a highly abnormal vitreous appearance. One patient aged 11 presented with a total retinal detachment. Two other children aged 2 and 4 had bilateral flat multiple retinal tears on presentation. 10 of 12 patients refracted were myopic. Two patients had asymptomatic lens opacities: one associated with bilateral inferiorly subluxed lenses and the other with a zonule and lens coloboma.

CONCLUSION

Heritable skeletal disorders resulting from a mutation in the gene coding for type II collagen are associated with abnormal vitreous, myopia and peripheral cataract with lens subluxation. In bone dysplasias resulting from a defect of type II collagen there is likely to be a high risk of retinal detachment with a propensity to retinal tears at a young age.

Structural characterization of bioengineered human corneal endothelial cell sheets fabricated on temperature-responsive culture dishes

For the purpose of corneal regenerative medicine, we fabricated human corneal endothelial cell sheets on temperature-responsive dishes, which could be non-invasively harvested as intact, transplantable sheets by simply reducing the culture temperature. Cells demonstrated hexagonal cell shape with numerous microvilli and cilia, and also exhibited abundant cytoplasmic organelles similar to these cells in vivo. Immunofluorescence for type IV collagen and fibronectin revealed that abundant extracellular matrix (ECM) was deposited on the basal surface throughout culture, and the deposited ECM was harvested along with the cell sheets by reducing culture temperature to 20 degrees C. Faint ECM remnants were observed on the dish surfaces after cell sheet detachment. Immunofluorescence for ZO-1 showed that tight junctions were established between cells, and immunoblotting indicated that intact ZO-1 was maintained during cell sheet harvest, while conventional proteolytic cell harvest methods resulted in the degradation of ZO-1. These results suggest that these transplantable corneal endothelial cell sheets can be applied to treat patients with damaged corneas.

Laser in situ keratomileusis in human corneas

PURPOSE

To establish an in vitro model of laser in situ keratomileusis (LASIK) in human donor eyes and to test its validity in comparison with animal models.

SETTING

Department of Anatomy, Friedrich-Alexander Unviersity, Erlangen, Germany.

METHODS

Laser in situ keratomileusis was performed on 20 organ-cultured human corneal buttons. The excimer laser ablations ranged from 0 to 12.0 diopters. The corneas were maintained in culture for up to 6 months and then evaluated with light microscopy and transmission electron microscopy. In addition, corneal sections were immunohistochemically stained for collagen type III, laminin, and fibronectin. The main outcome measures were the ultrastructural and immunohistochemical features of the stromal incision interface.

RESULTS

Ultrastructural investigations in the peripheral cornea revealed a disarrangement of collagen fibers, indicating scar formation. These findings were not observed in the central area. Immunohistochemical staining for fibronectin and collagen type III was detected over the entire stromal incision interface, whereas laminin staining was related to the ingrowth of epithelial cells.

CONCLUSIONS

The morphological changes after LASIK in an organ culture model can simulate the in vivo situation. Therefore, this model appears appropriate to use in further study of corneal wound-healing changes after LASIK.

Disruption of anterior segment development by TGF-β1 overexpression in the eyes of transgenic mice

Previous experiments showed that transgenic mice expressing a secreted self-activating transforming growth factor (TGF) -beta1 did not show a phenotype in the lens and cornea until postnatal day 21, when anterior subcapsular cataracts, sporadic thickening of the corneal stroma, and thinning of the corneal epithelium were noted (Srinivasan et al., 1998). To examine the effects of higher concentrations of TGF-beta1 on the lens and cornea, we constructed transgenic mice harboring the strong, lens-specific chicken betaB1-crystallin promoter driving an activated porcine TGF-beta1 gene. In contrast to the earlier study, the transgenic mice had microphthalmic eyes with closed eyelids. Already at embryonic day (E) 13.5, the future cornea of the transgenic mice was threefold thicker than that of wild-type littermates due to increased proliferation of corneal stromal mesenchyme cells. Staining of fibronectin and thrombospondin-1 was increased in periocular mesenchyme. At E17.5, the thickened transgenic corneal stroma was vascularized and densely populated by abundant star-shaped, neural cell adhesion molecule-positive cells of mesenchymal appearance surrounded by irregular swirls of collagen and extracellular matrix. The corneal endothelium, anterior chamber, and stroma of iris/ciliary body did not develop, and the transgenic cornea was opaque. Fibronectin, perlecan, and thrombospondin-1 were elevated, whereas type VI collagen decreased in the transgenic corneal stroma. Stromal mesenchyme cells expressed alpha-smooth muscle actin as did lens epithelial cells and cells of the retinal pigmented epithelium. By E17.5, lens fiber cells underwent apoptotic cell death that was followed by apoptosis of the entire anterior lens epithelium between E18.5 and birth. Posteriorly, the vitreous humor was essentially absent; however, the retina appeared relatively normal. Thus, excess TGF-beta1, a mitogen for embryonic corneal mesenchyme, severely disrupts corneal and lens differentiation. Our findings profoundly contrast with the mild eye phenotype observed with presumably lower levels of ectopic TGF-beta and illustrate the complexity of TGF-beta utilization and the importance of dose when assessing the effects of this growth factor.

Polymer surface chemistry and a novel attachment mechanism in corneal epithelial cells

The initial attachment reaction of most cultured cell types to polymers is based on a linkage of integrin receptors to serum-derived fibronectin and vitronectin that adsorb onto the polymer surface. Recently isolated corneal epithelial cells have an additional attachment mechanism, known to operate on tissue culture polystyrene, which involves endogenous protein synthesis and an intact system of microtubules. Here, we determine if this novel attachment mechanism is operative on polymers of different surface chemistries. The attachment, growth, and deposition of basement membrane proteins by corneal epithelial cells was compared on two hydrophilic surfaces (tissue culture polystyrene and Primaria) and one relatively hydrophobic surface (unmodified polystyrene). Superior levels of cell attachment were found on the hydrophilic polymers, but cells also attached effectively to the hydrophobic surface. Growth rates showed that the cells were able to overcome the differential effects of polymer surface chemistry during a 7-day time period. Polymer surface chemistry had subtle effects on the temporal pattern of biosynthesis of extracellular matrix proteins likely to be involved in cell adhesion. These results show that effective attachment and growth can occur on a hydrophobic polymer when corneal epithelial cells use the endogenous attachment mechanism.

Organization of fibrillar collagen in the human and bovine cornea: collagen types V and III

The localization and fibrillar organization of collagen types V and III in the human and bovine corneal stromas were studied. In the chicken cornea, type V co-assembles with type I collagen as heterotypic fibrils and this interaction is involved in the regulation of fibril diameter necessary for corneal transparency. To determine whether this is a regulatory mechanism common to the corneas of different species the human and bovine corneal stroma were studied. Collagen type V was found in the epithelium and Bowman's membrane in the untreated adult human and bovine cornea using immunofluorescence microscopy. In the absence of any treatment, there was no type V reactivity within the stroma. However, type V collagen was detected homogeneously throughout the corneal stroma after treatments that partially disrupt fibril structure. The reactivity was strongest in the cornea, weaker in the limbus and weakest in the sclera. Fetal corneas showed similar reactivity for type V collagen, but unlike the adult, the stroma was slightly reactive. Immunoelectron microscopy demonstrated that type V collagen was associated with disrupted, but not with intact, fibrils in both human and bovine corneal stroma. Type III collagen reactivity was not detected in the cornea, but was present subepithelially in the limbus and in the scleral stroma. These data indicate that type V collagen is a component of striated collagen fibrils throughout the human and bovine corneal stromas. The interaction of type I and V collagen as heterotypic fibrils masks the helical epitope recognized by the monoclonal antibody against type V collagen. The heterotypic interactions of collagen type V indicate a role in the regulation of fibril diameter analogous to that described in the avian cornea.

Localization of basement membrane-associated protein isoforms during development of the ocular surface of mouse eye

The developmental localization patterns of collagen type IV alpha1-5 chains, laminin-1, laminin-5, and laminin alpha2 chain were analyzed in the embryonic mouse eye using isoform specific antibodies and immunofluorescence microscopy. Laminin-1 isoform and alpha1-2(IV) were ubiquitously expressed along the ocular surface basement membranes at a very early stage of eye development. Alpha3-5(IV) were first detected at later stages of development, and exhibited a variable distribution pattern along the ocular surface basement membrane. In contrast, expression of the laminin alpha2 chain was restricted to the conjunctival basement membrane, and was first detected during the same developmental period in which keratin K4-positive, differentiated conjunctival epithelial cells were observed. Although laminin-5 was uniformly expressed along the adult ocular surface basement membrane, during embryogenesis it was first incorporated into the conjunctival basement membrane structure. These data suggest that some of the laminin isoforms, including laminin alpha2 and laminin-5, may play a role in the formation of a conjunctival-type basement membrane. The temporal relationship between the localization of these molecules to the conjunctival basement membrane and the appearance of differentiated conjunctival epithelial cells suggests a role for external influence on the differentiation pathways of ocular surface epithelium.

Production of neocollagen by cells invading hydrogel sponges implanted in the rabbit cornea

BACKGROUND

Poly(2-hydroxyethyl methacrylate) sponges are artificial tissue-equivalent matrices with potential value as materials for the peripheral zone of artificial corneas. A keratoprosthetic device was developed incorporating a poly(HEMA) spongy skirt which allowed cellular invasion. The present in vivo study investigated the biosynthetic activity of stromal fibroblasts growing within a poly(HEMA) sponge implanted into the rabbit cornea.

METHODS

A porous poly(HEMA) hydrogel was synthesized by polymerization in a large excess of water. Specimens with a pore size larger than 10 microns were impregnated with collagen type I and then implanted into the limbal region of cornea in four rabbits. The animals were followed clinically for 28 days, when they were anaesthetized and new sponge specimens were implanted in their second eye. After 2 h, both eyes were enucleated. The 28-day and 2-h explants were subjected to autoradiographic analysis following labelling with tritiated proline and to an immunostaining technique using antibodies to collagen types I-VI.

RESULTS

The autoradiographic analysis showed that the fibroblasts within the 28-day explants continued to be synthetically active and deposited proteins. Using the immunostaining technique, the deposition was most clearly demonstrated by the localization of collagen type III in the tissue invading the sponge. Both techniques failed to indicate any cellular activity in the short-time implants.

CONCLUSIONS

The presence of collagen type III is consistent with a normal healing response of the stromal fibroblasts and indicates that poly(-HEMA) sponges are able to function as tissue-equivalent matrices.

The composition of wide-spaced collagen in normal and diseased Descemet's membrane

Descemet's membrane, the specialised basement membrane of the corneal endothelium, contains a form of extracellular matrix described as wide-spaced collagen. In healthy human Descemet's membrane, wide-spaced collagen forms a highly ordered array in a region called the anterior banded zone. However, in corneal endotheliopathies such as Fuchs' endothelial dystrophy and the iridocorneal-endothelial syndrome large amounts of wide-spaced collagen are deposited posterior to Descemet's membrane in a grotesque parody of the anterior banded zone termed a posterior collagenous layer. The purpose of this study was to identify the composition of the wide-spaced collagen found in the Descemet's membrane of normal and diseased human corneas. Tissue from three normal human corneas, three from patients with Fuchs' endothelial dystrophy and five from patients with the iridocorneal-endothelial syndrome was prepared for immuno-electron microscopy by freezing or embedding in Lowicryl K4M resin. Immunocytochemistry on ultrathin sections was performed with antibodies to collagen Types I, III, V, VI and VIII, fibronectin, laminin, P component and tenascin. Ultrastructural labelling of the wide-spaced collagen in the anterior banded zone of normal and diseased corneas and also of the wide-spaced collagen in the posterior collagenous layer of all the diseased corneas was demonstrated with antibody to collagen Type VIII. Wide-spaced collagen was not labelled by any of the other antibodies used. Large amounts of Type VIII collagen are present in discrete regions of healthy and diseased Descemet's membrane. The deposition of Type VIII collagen may significantly influence the pathobiology of the corneal endotheliopathies.

Descemet's membrane in the iridocorneal-endothelial syndrome: morphology and composition

The iridocorneal-endothelial syndrome is a disease of the ocular anterior segment characterized by corneal failure, glaucoma and iris destruction. Specular photomicroscopical and histological studies suggest the disorder is caused by a population of abnormal corneal endothelial cells. In other corneal endotheliopathies Descemet's membrane, the basement membrane underlying the endothelial cells, is disfigured by the presence of an abnormal region of extracellular matrix termed a posterior collagenous layer, which is laid down by the diseased endothelial cells. In this study we sought to establish the typical morphology and composition of Descemet's membrane in the iridocorneal-endothelial syndrome. Ultrastructural examination of Descemet's membrane in 27 keratoplasty specimens identified three morphologic patterns. In the majority there was a posterior collagenous layer which in all cases consisted of an anterior layer of wide-spaced collagen and a posterior layer of microfibrils embedded in an amorphous matrix. In four specimens which did not possess a posterior collagenous layer the anterior banded zone of Descemet's membrane was absent. In five corneas Descemet's membrane was normal. The composition of the posterior collagenous layer was examined by immunoelectron microscopy (five corneas) and histochemistry (six corneas). Collagen Types I, III, V, VI and VIII, fibronectin, tenascin and oxytalan were microfibrillar components, collagen Type VIII formed wide-spaced collagen whilst laminin was present in the amorphous matrix. The stereotyped derangements of structure and composition identified in the endothelial basement membrane may significantly influence the pathobiology of this disorder.

The distribution of fibronectin and P component in Descemet's membrane: an immunoelectron microscopic study

Descemet's membrane consists of two zones, the 'anterior banded zone' which contains wide-spaced collagen and the amorphous 'posterior non-banded zone'. It is attached anteriorly to the corneal stroma by a narrow transitional zone termed the 'interfacial matrix'. The distribution of fibronectin and P component within the different layers of Descemet's membrane was investigated using an ultrastructural immunogold technique. Seven normal human corneas from an eye bank and one specimen from an orbital exenteration were examined. Fibronectin was predominantly present in the posterior part of the posterior non-banded zone and in the anterior banded zone. The anterior part of the posterior non-banded zone contained less fibronectin. P component was present throughout the anterior banded and posterior non-banded zones. There was a sharp demarcation at the interfacial matrix since neither substance was observed in the corneal stroma. The differences shown in the distribution of fibronectin and P component within Descemet's membrane may have resulted from their binding to other substances or alternatively from differences in the quantities laid down during the evolution of this basement membrane.

Quantitative analysis of immunogold labellings of collagen types I, III, IV and VI in healthy and pathological human corneas

BACKGROUND

We studied the distribution of collagen types I, III, IV and VI in one healthy human cornea and in seven pathological human corneas, in which the disorders were three cases of pseudophakic bullous keratopathy (two severe, one moderate) and one case each of stage IV keratoconus, chronic ulcer, vascularized cornea and disciform keratitis.

METHODS

Transmission electron microscopy examinations were performed on post-embedding immunogold-labelled sections. The staining was evaluated by gold particle count in the different tissues. The presence or absence of a given antigen was determined by statistical analysis, using a d-value test.

RESULTS

Our results on healthy corneal tissues corroborate the data available from previous studies, except for collagen type VI, which we found to be absent in Bowman's layer. In pathological corneas with a collagenous layer posterior to Descemet's membrane, collagen types I, III and especially IV were detected in this collagenous layer. Collagen types I, III and VI were detected in the anterior healed stroma of other pathological corneas, except for the keratoconus cornea, in which intense collagen III staining was observed.

CONCLUSION

The presence of collagen types I and III in the posterior collagenous layer of our pseudophakic bullous keratopathy corneas suggests that this layer corresponds to scar tissue secreted by stimulated endothelial cells.

Cytological and immunocytochemical approaches to the study of corneal endothelial wound repair

The vertebrate corneal endothelium represents a unique model system for investigating many cellular aspects of wound repair within an organized tissue in situ. The tissue exists as a cell monolayer that resides upon its own natural basement membrane that can be prepared as a flat mount to observe the entire cell population. Thus, it readily avails itself to many cytological and immunocytochemical methods at both the light microscopic and ultrastructural levels. In addition, the tissue is easily explanted into organ culture where further investigations can be carried out. These techniques have enabled investigators to use many approaches to explore function and changes in response to injury. In vivo, the endothelium acts as a transport tissue to actively pump Na+ and bicarbonate ions from the corneal stroma into the aqueous humor to control corneal transparency. Physiological findings indicate that fluid diffuses back into the stroma, across the endothelium, and thus hydration is said to be controlled by a pump-leak mechanism. Ultrastructural investigations, some employing horseradish peroxidase and lanthanum, have established the morphological basis for this mechanism as apical focal junctions that are not the classical tight junctions and do not constitute a complete zona occludens. Along with these apical focal junctions are gap junctions that appear identical to their counterparts in other cell types. Cytochemical studies localized both Na+K(+)-ATPase and carbonic anhydrase, the main pump enzymes associated with corneal hydration, to the lateral plasma membranes. Corneal endothelial cells of noninjured tissue do not traverse the cell cycle and are considered to be in the "Go" phase of the cell cycle as determined by microfluorometric analysis with DNA binding dyes such as auramin O and pararosaniline-Feulgen. However, injury can initiate cell cycle transverse and histochemical and cytological methods have been used to understand the tissue's response. Classical histochemical studies revealed that increased staining was observed for metabolic (NADase and NADPase) and lysosomal enzymes in cells bordering the wound area. The use of radiolabelled agents has further lead to an understanding of the endothelial wound response. Autoradiographic analyses of 3H-actinomycin D incorporation indicated that injury initiates changes in chromatin leading to increased binding levels of the drug in cells surrounding the wound. This change suggests that those cells undergo heightened macromolecular synthesis and this was confirmed by examining 3H-uridine and 3H-thymidine incorporation. The major mechanism involved in corneal endothelial repair is cell migration. Cytochemical and immunocytochemical investigations have allowed investigators an opportunity to gain some insight into changes that occur during this cellular process.(ABSTRACT TRUNCATED AT 400 WORDS)

Wound healing of the ocular surface

Wound healing is a complex, long-lasting regulatory sequence that involves expression of a number of genes, which are active during the individual's development. Some of the phenomena differ from normal tissue turnover and growth only quantitatively. This article reviews the current data on corneal wound healing, with particular reference to mesenchymal matrix proteins and their integrin receptors, to growth factors and to proteolytic enzymes. Some inflammatory mediators are also discussed. The theoretical basis for therapeutic interventions is also discussed briefly, in the light of present knowledge.

The expression of the genes for entactin, laminin A, laminin B1 and laminin B2 in murine lens morphogenesis and eye development

The temporal expression of the genes for the excellular matrix proteins entactin and the A, B1, and B2 chains of laminin was examined in the eye of the developing mouse embryo by in situ hybridization of their messenger RNAs. Entactin messenger RNA was found in abundance in specific cells. In the 25 somite embryo entactin message was synthesized by mesenchymal cells and, at later stages, by hyalocytes and lens cells in addition. The message was not detectable in corneal epithelium at embryonic stages E15 and E18.5 and at birth but was present in adjacent stromal cells. At the 28 and 38 somite stages, before pigment granules interfered with the detection of silver grains, no entactin message was detected in pigmented epithelial cells, in contrast to the messages for laminin B1 and B2. Entactin was not found in the neural epithelium at any time during development. The distribution of the laminin B1, B2 and A chain messenger RNAs was distinctly different from that of entactin. In particular, during the early stages of development B1 and B2 messages were synthesized by ectodermal, lens, corneal, pigment epithelial and hyaloid cells. In the older embryos cells in the ganglion layer of the retina synthesized B1 and B2 messages but undetectable amounts of entactin or the A chain messages. In general the A chain message was in lower abundance throughout development. The distribution of laminin and entactin messages suggested that the extracellular matrices, which contained both proteins, can be derived either from a single cell type or from the contributions of multiple cell types. The data demonstrate the complexity of extracellular matrix synthesis and assembly in the diverse structures of the developing eye where the temporal expression of specific molecules are tailored to the specific developmental requirements of particular structures.

Immunogold fine structural localization of extracellular matrix components in aged human cornea. I. Types I-IV collagen and laminin

Using the immunogold technique combined with cryoultramicrotomy and London Resin white (LR white) embedding, we studied the fine structural distribution of types I-IV collagen and laminin in corneal tissue from seven enucleated human eyes (age range, 63-78 years). Type II collagen was not identified in any corneal layer. Type I and type III collagen were distributed in a similar fashion in striated collagen fibrils in Bowman's layer and in the stroma. Type IV collagen was located only in the posterior non-banded region of Descemet's membrane. Laminin was identified in subepithelial anchoring plaques and the sub-endothelial region of Descemet's membrane in accordance with its recognized adhesive function.

Immunogold localisation of laminin in normal and exfoliative iris

Immunoelectron microscopic studies of exfoliative iris tissue (seven specimens) revealed the presence of laminin in the fibrillar component of exfoliation material. The immunogold label was uniformly distributed on the exfoliation fibres. Deposition of laminin labelled exfoliation material in the dilator muscle was a noteworthy feature, as was an apparent depletion of laminin in the basement membranes of ostensibly unaffected vessels. In control iris tissue (five enucleated eyes) laminin was identified in the basement membrane round vascular contractile cells, but not beneath the endothelium.

Scheie's syndrome. An ultrastructural analysis of the cornea

The histopathology of a corneal graft specimen obtained from a patient with Scheie's syndrome (systemic mucopolysaccharidosis, type IS) is described with particular emphasis on the ultrastructural findings. Numerous vacuoles containing fibrillogranular material were found in the corneal epithelial cells, the keratocytes, and the endothelial cells. The basement membrane of the epithelium contained frequent breaks and peg-like undulations, and Bowman's layer was markedly attenuated. Fibrous long-spacing (FLS) collagen featured prominently in the stroma. Descemet's membrane was normal. The findings of a markedly attenuated Bowman's layer and FLS collagen may be abnormalities specific to Scheie's syndrome resulting from the altered glycosaminoglycan composition of the extracellular matrix.

Basement membrane heterogeneity and variation in corneal epithelial differentiation

We have previously shown that the expression of a major 64-Kda keratin (K3) in corneal epithelium is site-related. It is found suprabasally in limbal epithelium, but uniformly (basal cells included) in central corneal epithelium. In the present study, we used a panel of antibodies against various components of corneal epithelial basement membrane to investigate a possible correlation between basement membrane heterogeneity and differential (basal vs. suprabasal) K3 keratin expression. One of these antibodies, AE27, stains human conjunctival basement membrane weakly, limbal basement membrane heterogeneously, and central corneal basement membrane strongly. Basal cells resting on basement membrane that stains strongly with AE27 tend to stain with monoclonal antibody AE5, which recognizes keratin K3. Basal cells on basement membrane staining weakly with AE27 tend not to stain with AE5. No such correlation exists between AE5 staining and type IV collagen, which is detectable immunohistochemically in conjunctival and limbal basement membrane, but not in corneal basement membrane overlying Bowman's layer. These results suggest that basement membrane of human corneal/conjunctival epithelium can be divided into at least three domains: the conjunctival basement membrane (type IV collagen-positive, AE27-weak), the limbal basement membrane (type IV collagen-positive, AE27-strong), and corneal basement membrane (type IV collagen-negative, AE27-strong). The results also raise the possibility that basement membrane heterogeneity may play a functional role in regulating keratin expression and other aspects of differentiation of corneal epithelium; more experiments are needed to test this hypothesis.

Immunohistochemical studies of Peters' anomaly

Peters' anomaly is a congenital abnormality that affects mainly the posterior cornea. The pathogenesis is unclear. By immunohistochemical methods, the authors determined the presence of extracellular matrix proteins, including fibronectin, laminin, and collagen types I, III, IV, V, and VI, in seven corneal buttons from five patients with Peters' anomaly. Type III collagen was the only protein absent in either the pathologic or normal human corneas. Compared with normal controls, the stromal lamellae in Peters' anomaly corneas were more irregular and more closely packed. In addition, the fibronectin staining was markedly enhanced in all seven Peters' anomaly corneas. Staining with collagen type VI was also mildly increased. These results suggest that extracellular matrix elements such as fibronectin may be important factors in this developmental disorder.