Electron-microscopic studies on the presence of gap junctions between corneal fibroblasts in rabbits

Focus on seed cells: stem cells in 3D bioprinting of corneal grafts

Corneal opacity is one of the leading causes of severe vision impairment. Corneal transplantation is the dominant therapy for irreversible corneal blindness. However, there is a worldwide shortage of donor grafts and consequently an urgent demand for alternatives. Three-dimensional (3D) bioprinting is an innovative additive manufacturing technology for high-resolution distribution of bioink to construct human tissues. The technology has shown great promise in the field of bone, cartilage and skin tissue construction. 3D bioprinting allows precise structural construction and functional cell printing, which makes it possible to print personalized full-thickness or lamellar corneal layers. Seed cells play an important role in producing corneal biological functions. And stem cells are potential seed cells for corneal tissue construction. In this review, the basic anatomy and physiology of the natural human cornea and the grafts for keratoplasties are introduced. Then, the applications of 3D bioprinting techniques and bioinks for corneal tissue construction and their interaction with seed cells are reviewed, and both the application and promising future of stem cells in corneal tissue engineering is discussed. Finally, the development trends requirements and challenges of using stem cells as seed cells in corneal graft construction are summarized, and future development directions are suggested.

Morphological analysis of cell distribution and network structure via gap junctions in swine corneal stroma

Because of corneal transplantation limitations, there is a need for cornea-specific regenerative medicine. The development of such regenerative medicine has been delayed because of the complex and unique structure of the corneal stroma. Few studies have explored the corneal stroma cell distribution and cell types in vivo. This study investigated regional differences in morphological characteristics and distributions of corneal keratocytes and immunocompetent cells in the corneal stroma to clarify their functions and structural characteristics. The porcine eyeballs were subjected to light microscopy, transmission electron microscopy, scanning electron microscopy, and immunofluorescence staining analyses. Corneal cells were primarily located in the limbus, rather than the center of the cornea; the long keratocyte diameter was largest on the epithelial side of the corneal limbus, while the short diameter was largest on the endothelial side of the central cornea. Moreover, there were significantly more corneal cells on the epithelial side than on the endothelial side in both the central and limbus areas. Gap junctions between cells in the corneal stroma were present on the surfaces of cytoplasmic processes. Many cytoplasmic processes were scattered throughout the corneal stroma; they were connected both vertically and horizontally, forming an intercellular network. Additionally, immunocompetent cells on the epithelial side suggested to participate in this network via gap junctions. The morphology of keratocytes and immunocompetent cells on the epithelial side suggests that they play important roles in corneal homeostasis.

Urokinase-type plasminogen activator (uPA) negatively regulates α-smooth muscle actin expression via Endo180 and the uPA receptor in corneal fibroblasts

Corneal fibroblasts are embedded within an extracellular matrix composed largely of collagen type 1, proteoglycans, and other proteins in the corneal stroma, and their morphology and function are subject to continuous regulation by collagen. During wound healing and in various pathological conditions, corneal fibroblasts differentiate into myofibroblasts characterized by the expression of α-smooth muscle actin (α-SMA). Endo180, also known as urokinase-type plasminogen activator (uPA) receptor-associated protein (uPARAP), is a collagen receptor. Here we investigated whether targeting of Endo180 and the uPA receptor (uPAR) by uPA might play a role in the regulation of α-SMA expression by culturing corneal fibroblasts derived from uPA-deficient (uPA^-/-) or wild-type (uPA^+/+) mice in a collagen gel or on plastic. The expression of α-SMA was upregulated, the amounts of full-length Endo180 and uPAR were increased, and the levels of both transforming growth factor-b (TGF-β) expression and Smad3 phosphorylation were higher in uPA^-/- corneal fibroblasts compared with uPA^+/+ cells under the collagen gel culture condition. Antibodies to Endo180 inhibited these effects of uPA deficiency on a-SMA and TGF-b expression, whereas a TGF-b signaling inhibitor blocked the effects on Smad3 phosphorylation and a-SMA expression. Our results suggest that uPA deficiency might promote the interaction between collagen and Endo180 and thereby increase a-SMA expression in a manner dependent on TGF-β signaling. Expression of α-SMA is thus negatively regulated by uPA through targeting of Endo180 and uPAR.

Pivotal Role of Corneal Fibroblasts in Progression to Corneal Ulcer in Bacterial Keratitis

The shape and transparency of the cornea are essential for clear vision. However, its location at the ocular surface renders the cornea vulnerable to pathogenic microorganisms in the external environment. Pseudomonas aeruginosa and Staphylococcus aureus are two such microorganisms and are responsible for most cases of bacterial keratitis. The development of antimicrobial agents has allowed the successful treatment of bacterial keratitis if the infection is diagnosed promptly. However, no effective medical treatment is available after progression to corneal ulcer, which is characterized by excessive degradation of collagen in the corneal stroma and can lead to corneal perforation and corneal blindness. This collagen degradation is mediated by both infecting bacteria and corneal fibroblasts themselves, with a urokinase-type plasminogen activator (uPA)-plasmin-matrix metalloproteinase (MMP) cascade playing a central role in collagen destruction by the host cells. Bacterial factors stimulate the production by corneal fibroblasts of both uPA and pro-MMPs, released uPA mediates the conversion of plasminogen in the extracellular environment to plasmin, and plasmin mediates the conversion of secreted pro-MMPs to the active form of these enzymes, which then degrade stromal collagen. Bacterial factors also stimulate expression by corneal fibroblasts of the chemokine interleukin-8 and the adhesion molecule ICAM-1, both of which contribute to recruitment and activation of polymorphonuclear neutrophils, and these cells then further stimulate corneal fibroblasts via the secretion of interleukin-1. At this stage of the disease, bacteria are no longer necessary for collagen degradation. In this review, we discuss the pivotal role of corneal fibroblasts in corneal ulcer associated with infection by P. aeruginosa or S. aureus as well as the development of potential new modes of treatment for this condition.

Corneal fibroblasts: Function and markers

Corneal stromal keratocytes contribute to the maintenance of corneal transparency and shape by synthesizing and degrading extracellular matrix. They are quiescent in the healthy cornea, but they become activated in response to insults from the external environment that breach the corneal epithelium, with such activation being associated with phenotypic transformation into fibroblasts. Corneal fibroblasts (activated keratocytes) act as sentinel cells to sense various external stimuli-including damage-associated molecular patterns derived from injured cells, pathogen-associated molecular patterns of infectious microorganisms, and inflammatory mediators such as cytokines-under pathological conditions such as trauma, infection, and allergy. The expression of various chemokines and adhesion molecules by corneal fibroblasts determines the selective recruitment and activation of inflammatory cells in a manner dependent on the type of insult. In infectious keratitis, the interaction of corneal fibroblasts with various components of microbes and with cytokines derived from infiltrated inflammatory cells results in excessive degradation of stromal collagen and consequent corneal ulceration. Corneal fibroblasts distinguish between type 1 and type 2 inflammation through recognition of corresponding cytokines, with their activation by type 2 cytokines contributing to the pathogenesis of corneal lesions in severe ocular allergic diseases. Pharmacological targeting of corneal fibroblast function is thus a potential novel therapeutic approach to prevention of excessive corneal stromal inflammation, damage, and scarring.

Keratocyte biology

The study of corneal stromal keratocytes is motivated by its strong association with corneal health and visual function. They play a dominant role in the maintenance of corneal homeostasis and transparency through the production of collagens, proteoglycans and corneal crystallins. Trauma-induced apoptosis of keratocytes and replacement by fibroblasts and myofibroblasts disrupt the stromal matrix organization, resulting in corneal haze formation and vision loss. It is, therefore, important to understand the biology and behaviours of keratocytes and the associated stromal cell types (like fibroblasts, myofibroblasts, stromal stem cells) in wound healing, corneal pathologies (including keratoconus, keratitis, endothelial disorders) as well as different ophthalmic situations (such as collagen crosslinking/photodynamic treatment, keratoplasty and refractive surgery, and topical medications). The recent development of ex vivo propagation of keratocytes and stromal stem cells, and their translational applications, either via stromal injection or incorporated in bioscaffold, have been shown to restore the corneal transparency and regenerate native stromal tissue in animal models of corneal haze and other disorders.

Transient Cell Membrane Disruptions induce Calcium Waves in Corneal Keratocytes

The purpose of this study was to determine if transient cell membrane disruptions (TPMDs) in single keratocytes can trigger signaling events in neighboring keratocytes. Stromal cells were cultured from human corneas (HCSC) and mouse corneas (MCSC). TPMDs were produced using a multiphoton microscope in Cal-520-AM loaded cells. TPMD-induced calcium increases (Ca⁺⁺_i) were measured in Ca⁺⁺-containing and Ca⁺⁺-free solutions containing thapsigargin, ryanodine, BAPTA-AM, 18-α-glycyrrhetinic acid (18α-GA), apyrase, BCTC, AMG 9810, or AMTB. Fluorescence intensity was recorded as the number of cells responding and the area under the fluorescence versus time curve. The maximum distance of responding neighboring cells in ex vivo human corneas was measured. Connexin 43 protein in HCSC and MCSC was examined using immunofluorescence staining, and corneal rubbing was applied to confirm whether TPMDs occur following mechanical manipulation. Our results demonstrate that single cell TPMDs result in Ca⁺⁺ waves in neighboring keratocytes both in culture and within ex vivo corneas. The source of Ca⁺⁺ is both intra-and extra-cellular, and the signal can be mediated by ATP and/or gap junctions, and is species dependent. Stromal rubbing confirmed that TPMDs do occur following mechanical manipulation. Keratocyte TPMDs and their associated signaling events are likely common occurrences following minor or major corneal trauma.

Design and development of artemisinin and dexamethasone loaded topical nanodispersion for the effective treatment of age-related macular degeneration

Age-related macular degeneration (AMD) is a disease affecting the macula by the new blood vessels formation. AMD is widely treated with a combination of anti-angiogenic and anti-vascular endothelial growth factor (VEGF) agents. The topical administration of nanodispersions showed enhanced ocular residence time with controlled and prolonged drug delivery to the disease site at the back of the eye. In the present study we developed and characterized nanodispersion containing anti-angiogenic (artemisinin) and anti-VEGF agent (dexamethasone) for the topical ocular administration in order to obtain a required drug concentration in the posterior part of the eye. The nanodispersions were prepared with varying concentration of polymer, polyvinyl pyrrolidone K90 and polymeric surfactant, Poloxamer 407. The nanodispersions were found to be smooth and spherical in shape with a size range of 12-26 nm. In-vitro drug release studies showed the 90-101% of artemisinin and 55-103% of dexamethasone release from the nanodispersions. The blank formulation with a high concentration of polymer and polymeric surfactant showed an acceptable level of haemolysis and DNA damage. The chorioallantoic membrane assay suggested that the nanodispersion possess good anti-angiogenic effect. Hence the formulated artemisinin and dexamethasone nanodispersion may have the great potential for the AMD treatment.

Clinical and in vivo confocal microscopic features of neuropathic corneal pain

Aims

To describe clinical and in vivo confocal microscopy (IVCM) features of neuropathic corneal pain (NCP) without clinically visible signs.

Methods

Prospective, observational study of 27 eyes of 14 patients who had continuous severe ocular pain for one or more years, with minimal or no ocular surface signs and were non-responsive to topical lubricants, steroids and/or ciclosporin. All patients were evaluated using Ocular Surface Disease Index, Oxford grading scale, Schirmer test 1, Cochet Bonnet esthesiometry and response to topical anaesthesia. Central and paracentral regions of the cornea of patients and seven healthy controls were studied by IVCM. Corneal epithelial thickness and sub-basal nerve density were measured in patients and controls.

Results

Four patients responded to topical anaesthesia (responsive group (RG)), indicating peripheral NCP while 10 patients did not show any improvement (non-responsive group (NRG)), indicating central NCP. Schirmer-1 test was within normal limits in the RG but significantly greater in the NRG (p<0.001). None of the other clinical parameters nor corneal epithelial thickness were statistically significantly different. The sub-basal nerve density was significantly reduced (p<0.008) in patients compared with controls. Stroma of all patients demonstrated activated keratocytes and spindle, lateral and stump microneuromas. There was a statistically significant greater number of microneuromas (p<0.0001) and activated keratocytes in RG compared with NRG.

Conclusion

NCP without visible clinical signs does not represent typical dry eye disease. Distinct signs demonstrated on IVCM suggest that peripheral NCP, which responds to topical anaesthesia, and central NCP, which does not, are separate entities.

Contrasting cellular damage after Blue-IRIS and Femto-LASIK in cat cornea

Blue-intra-tissue refractive index shaping (Blue-IRIS) is a new approach to laser refractive correction of optical aberrations in the eye, which alters the refractive index of the cornea rather than changing its shape. Before it can be implemented in humans, it is critical to establish whether and to what extent, Blue-IRIS damages the cornea. Here, we contrasted the impact of -1.5 D cylinder refractive corrections inscribed using either Blue-IRIS or femtosecond laser in-situ keratomileusis (femto-LASIK) on corneal cell viability. Blue-IRIS was used to write a -1.5 D cylinder gradient index (GRIN) lens over a 2.5 mm by 2.5 mm area into the mid-stromal region of the cornea in six freshly-enucleated feline eyes. The same correction (-1.5 D cylinder) was inscribed into another four cat eyes using femto-LASIK. Six hours later, all corneas were processed for histology and stained for terminal deoxynucleotidyl transferase-mediated dUTP-digoxigenin nick end labeling (TUNEL) and p-γ-H2AX to label damaged cells. In Blue-IRIS-treated corneas, no tissue was removed and TUNEL-stained cells were confined to the laser focal zone in the stroma. In femto-LASIK, photoablation removed 14 μm of anterior stroma, but in addition, TUNEL-positive cells clustered across the femto-flap, the epithelium at the flap edges and the stroma below the ablation zone. Keratocytes positive for p-γ-H2AX were seen adjacent to all Blue-IRIS focal zones, but were completely absent from femto-LASIK-treated corneas. Unlike femto-LASIK, Blue-IRIS attains refractive correction in the cornea without tissue removal and only causes minimal, localized keratocyte death within the laser focal zones. In addition, Blue-IRIS induced DNA modifications associated with phosphorylation of γ-H2AX in keratocytes adjacent to the laser focal zones. We posit that this p-γ-H2AX response is related to alterations in chromatin structure caused by localized changes in osmolarity, a possible mechanism for the induced refractive index changes.

Interaction between Macrophages and Fibroblasts during Wound Healing of Burn Injuries in Rats

Analysis of the structural changes and cell-to-cell interactions occurring during wound healing of burn injuries is essential to elucidate the morphological characteristics of the reconstitution of tissue architecture. However, conventional approaches do not provide sufficient information with respect to cell-to-cell interactions during wound healing. The aim of this study was to evaluate the interaction between bone marrow-derived cells and resident stromal cells throughout the wound healing of burn injuries, using immunohistochemistry and focused ion beam/scanning electron microscope tomography. We induced third-degree burn injuries on the backs of Wistar rats with a heated cylindrical aluminum block (2.0 cm in diameter). At 7 and 14 days after the burn injuries, the burned skin was immunostained with anti-Iba1 and anti-HSP47 antibodies for visualization of bone marrow-derived cells/macrophages and resident stromal cells/fibroblasts, respectively. Normal skin tissue was used as a control. Double-staining immunohistochemistry revealed frequent contacts between macrophages and fibroblasts and a higher contact ratio in the 3 normal skin compared with burned skin, particularly in the areas of granuloma. Three-dimensional ultrastructural analysis with focused ion beam/scanning electron microscope tomography revealed that macrophages and fibroblasts were located closer together in the normal skin than in the burned skin, confirming the analysis by light microscopic observations and ultrastructural analysis from single sections. These results highlight the importance of contact between macrophages and fibroblasts in the maintenance of skin tissue structure and during wound healing.

Commanding roles of keratocytes in health and disease

The healthy corneal stroma comprises a 3-dimensional network of keratocytes interconnected by cellular processes divided by gap junctions. The shape of the corneal stromal cells is regulated by the presence of extracellular collagen matrix. Furthermore, if compact collagen are present around keratocytes, they decrease their mitotic activity so that the cells in normal healthy cornea are quiescent with low proliferative activity. However, when extracellular collagen is disrupted such as when inflammatory cells infiltrate the corneal stroma and release proinflammatory cytokines including interleukin 1 and tumor necrosis factor α, keratocytes are released from their network structure and undergo active proliferation. However, culturing keratocytes in the presence of tumor necrosis factor α dose dependently downregulates the presence and function of gap functions. Keratocytes also have phagocytic activities; foreign particles are internalized into these cells. In corneal ulceration, microorganisms invading the stroma secrete collagen-degrading enzymes and signaling molecules such as elastase and lipopolysaccharide that activate corneal keratocytes to secrete stroma-degrading matrix metalloproteinase and various chemokines to attract immune cells to the focus area of corneal stroma. Thus, keratocytes actively participate in collagen degradation of corneal ulceration. The Chinese herbal extract triptolide has been demonstrated significantly and dose dependently to decrease collagen degradation by condition medium-induced corneal fibroblasts. In ocular health, keratocytes respond to signals to and from epithelial cells. In ocular disease, keratocytes become activated cells that respond to various cytokines. Hence, keratocytes play commanding roles in ocular health and disease.

Keeping an eye on decellularized corneas: a review of methods, characterization and applications

The worldwide limited availability of suitable corneal donor tissue has led to the development of alternatives, including keratoprostheses (Kpros) and tissue engineered (TE) constructs. Despite advances in bioscaffold design, there is yet to be a corneal equivalent that effectively mimics both the native tissue ultrastructure and biomechanical properties. Human decellularized corneas (DCs) could offer a safe, sustainable source of corneal tissue, increasing the donor pool and potentially reducing the risk of immune rejection after corneal graft surgery. Appropriate, human-specific, decellularization techniques and high-resolution, non-destructive analysis systems are required to ensure reproducible outputs can be achieved. If robust treatment and characterization processes can be developed, DCs could offer a supplement to the donor corneal pool, alongside superior cell culture systems for pharmacology, toxicology and drug discovery studies.

Characterization of corneal stromal stem cells with the potential for epithelial transdifferentiation

INTRODUCTION

The corneal stroma is being increasingly recognized as a repository for stem cells. Like the limbal and endothelial niches, stromal stem cells often reside in the peripheral cornea and limbus. These peripheral and limbal corneal stromal cells (PLCSCs) are known to produce mesenchymal stem cells in vitro. Recently, a common corneal stromal and epithelial progenitor was hinted at. This study aims to examine the stem cell potential of corneal stromal cells and to investigate their epithelial transdifferentiation ability.

METHODS

PLCSCs were grown in traditional Dulbecco modified Eagle medium (DMEM)-based keratocyte culture medium and an M199-based medium and analyzed for a profile of cell-surface markers by using flow cytometry and differentiated into mesenchymal phenotypes analyzed with quantitative polymerase chain reaction (qPCR) and histologic staining. PLCSCs in M199 were subsequently divided into subpopulations based on CD34 and CD105 expression by using fluorescence- activated cell sorting (FACS). Subpopulations were characterized by marker profile and mesenchymal differentiation ability. Both whole PLCSCs and subpopulations were also cultured for epithelial transdifferentiation.

RESULTS

Cells cultured in M199 demonstrated a more stem-like cell-surface marker profile, and the keratocyte marker CD34 was retained for several passages but absent in cells cultured in DMEM. Cells cultured in M199 also exhibited a greater mesenchymal differentiation potential, compared with DMEM. PLCSCs could be divided into CD34(+)CD105(+), CD34-CD105(+), and CD34-CD105- subpopulations, of which CD34(+)CD105(+) cells were the most stemlike with regard to marker expression and mesenchymal differentiation potential. Subpopulations of PLCSCs exhibited differing abilities to transdifferentiate into epithelial phenotypes. Cells that were initially CD34(+)CD105(+) showed the greatest differentiation potential, producing CK3(+) and CK19(+) cells, and expressed a range of both epithelial progenitor (HES1, FRZB1, DCT, SOD2, ABCG2, CDH1, KRT19) and terminally differentiated (DSG3, KRT3, KRT12, KRT24) genes.

CONCLUSIONS

Culture medium has a significant effect on the phenotype and differentiation capacity of PLCSCs. The stroma contains a heterogeneous cell population in which we have identified CD34(+) cells as a stem cell population with a capacity for mesenchymal and epithelial differentiation.

Human mesenchymal stem cells differentiate into keratocyte-like cells in keratocyte-conditioned medium

Culturing corneal keratocytes is difficult because keratocytes growing in a monolayer rapidly lose their stellate morphology and cease to express keratocyte markers such as keratocan, lumican and aldehyde dehydrogenase 1 family, member A1 (ALDH1A1). Conversely, mesenchymal stem cells (MSCs) can be easily expanded in cell culture, and they have a variety of differentiation pathways. We studied the feasibility of using MSCs as a source for corneal tissue engineering. Based on the observation that keratocytes have MSC-like properties, similar to bone marrow-derived MSCs (BM-MSCs), we hypothesized that MSCs would differentiate into corneal keratocyte-like cells in keratocyte-conditioned medium (KCM). We measured changes in the expression of keratocyte markers through quantitative real-time polymerase chain reaction (qRT-PCR) and found that human MSC's cultured in KCM expressed both keratocan and ALDH1A1. Western blot analysis demonstrated that human MSCs cultured in KCM steadily increased their expression of lumican and ALDH1A1, while they lost expression of α-smooth muscle actin (α-SMA). Immunocytochemistry indicated that human MSCs grown in KCM acquired characteristics similar to those of keratocytes. These results suggest that KCM can direct human MSCs to differentiate into keratocyte-like cells.

Cardiac fibroblasts in cell culture systems: myofibroblasts all along?

The cytoarchitecture of the working myocardium is characterized by densely packed cardiomyocytes that are embedded in a three-dimensional network of numerous fibroblasts. Although the importance of cardiac fibroblasts in maintaining an orderly structured extracellular matrix is well recognized, less is known about their potential paracrine and electrotonic interactions with cardiomyocytes. This is partly the result of the complex intermingling of both cell types in vivo that tends to preclude a direct investigation of heterocellular crosstalk. It is for that reason that most of our present knowledge regarding stromal-parenchymal cell interactions is based on culture systems that permit direct access to either cell type. An often disregarded feature of such studies is that cardiac fibroblasts in standard two-dimensional cell culture have a pronounced tendency to undergo a phenotype switch to myofibroblasts. This cell type typically appears in injured hearts where it contributes importantly to fibrotic remodeling. The present review focuses on recent insights into electrical and paracrine crosstalk between myofibroblasts and cardiomyocytes while acknowledging that a comprehensive understanding of stromal-parenchymal cell interactions will depend on future methodological developments that permit retaining the fibroblast phenotype in cell culture systems and that will, most importantly, allow direct investigations of heterocellular crosstalk in intact tissue.

Corneal stroma regeneration in felines after supradescemetic keratoprosthesis implantation

PURPOSE

To show corneal regeneration in 3 cats that underwent lamellar keratectomy (90%) depth during supradescemetic keratoprosthetic implantation.

METHODS

Three 2-year-old cats that underwent spontaneous keratoprosthesis extrusion between 15 and 150 days after implanting a supradescemetic prosthesis into their right eyes were studied. Corneal structures and stroma thickness were evaluated by slit-lamp photographs, pachymetry, and confocal microscopy. Regenerated corneal epithelial cells, stroma matrix, and keratocyte morphology were studied with histology and transmission electron microscopy. Epithelial and stromal cell immunocharacterization was performed.

RESULTS

Corneas progressively regained normal thickness and improved clarity within 40 to 60 days. Slit-lamp photographs and pachymetry showed gains in stromal thickness until 600 microm or more. In vivo confocal microscopy showed the restoration of normal epithelium and stroma in all cats. Corneal nerves were seen in the regenerated stroma of 2 cats. Immunostaining showed absent alpha-smooth muscle actin (SMA) expression and a keratin K3-expressing epithelium. Electron microscopy showed regeneration of normal epithelium with a well-formed basement membrane, organized corneal lamellae, and the presence of normal keratocytes.

CONCLUSION

Felines are capable of regenerating corneal structures including epithelium and reinnervated stroma matrix after deep lamellar keratectomy. The use of feline models in corneal keratoprosthesis is therefore questionable.

Expression of haematopoietic stem cell markers, CD133 and CD34 on human corneal keratocytes

AIM

To study the expression of CD133 and CD34 antigens on cultured human keratocytes over time.

METHODS

Primary cultures of human corneal stromal cells were established from explants derived from cadaver eye donors. The cultures were sorted for CD133+ and CD34+ cells using magnetic beads. Both the primary cultures and secondary passages of sorted cells were further analysed by flow cytometry and western blot analysis for expression of the same antigens over time.

RESULTS

Four different cell populations-namely, CD133+, CD133-, CD34+ and CD34-, were identified in the culture samples. Two further specific subgroups were identified by flow cytometry: CD133+/CD34- cells and CD133+/CD34+ cells. Expression of CD133 declines more than CD34 with time in cell cultures. Although most cells lost expression of these markers, small populations retained staining up to 5 weeks in culture.

CONCLUSION

Human keratocytes express the haematopoietic stem cell markers CD133 and CD34. This expression decreases with time in culture, with most but not all cells losing expression. On the basis of these markers, the corneal stroma shows a heterogeneous population of cells. Expression or down regulation of expression of these molecules could represent different stages of activation of these cells.

Preservation and expansion of the primate keratocyte phenotype by downregulating TGF-beta signaling in a low-calcium, serum-free medium

PURPOSE

To demonstrate whether the original keratocyte phenotype is maintained with proliferative activity by suppressing TGF-beta signaling in rhesus monkey keratocytes expanded in a serum-free and low-[Ca2+] medium.

METHODS

Rhesus monkey keratocytes were isolated from central corneal buttons by collagenase digestion for 16 hours, seeded on plastic in Dulbecco's modified Eagle's medium (DMEM) containing insulin-transferrin-sodium selenite (ITS) supplement (DMEM/ITS) or 10% fetal bovine serum (DMEM/10% FBS), or in a defined keratinocyte serum-free medium (KSFM). After confluence, cells in KSFM were continuously subcultured at a 1-to-3 split. Cellular proliferation was analyzed by immunostaining for Ki67 and the MTT assay. The cellular phenotype was determined by immunostaining for aldehyde dehydrogenase (ALDH), keratocan, and CD34 and by the expression of keratocan promoter-driven enhanced cyan fluorescent protein (ECFP). The stability of the keratocyte phenotype was examined by switching KSFM to DMEM/ITS and DMEM/10% FBS. TGF-beta signaling was monitored by measuring the promoter activity of TGF-beta1, -beta2, and -beta RII after transient adenoviral transfection, and cytolocalization of Smad2 and Smad4.

RESULTS

In KSFM, monkey keratocytes proliferated while maintaining the expression of keratocan, CD34, and ALDH proteins and keratocan promoter-driven ECFP for at least 15 passages. The nuclear accumulation of Smad2 and Smad4 and the promoter activities of TGF-beta1 and -beta RII were significantly downregulated in KSFM compared with DMEM/10% FBS. In KSFM, an increase of [Ca2+] to 1.8 mM and addition of 10% FBS synergistically downregulated the keratocan promoter activity, facilitated Smad2 and Smad4 nuclear translocation, and upregulated TGF-beta1 and -beta RII promoter activities.

CONCLUSIONS

The normal monkey keratocyte phenotype can be maintained in a low-calcium, serum-free medium by downregulating Smad-mediated TGF-beta signaling.

Effects of Antiglaucoma Drugs on Collagen Gel Contraction Mediated by Human Corneal Fibroblasts

PURPOSE

Measurement of intraocular pressure is affected by the shape and thickness of the cornea, and corneal shape is thought to be maintained by contraction of corneal fibroblasts. The effects of the antiglaucoma drugs latanoprost, timolol maleate, and pilocarpine on the contraction of corneal fibroblasts cultured in a 3-dimensional collagen gel were investigated. The effects of these drugs on collagen degradation by corneal fibroblasts and their possible cytotoxicity were also examined.

MATERIALS AND METHODS

Human corneal fibroblasts were cultured in a 3-dimensional gel of type I collagen and in the presence of various concentrations of latanoprost, timolol maleate, or pilocarpine for various times. Collagen gel contraction was evaluated by daily measurement of gel diameter. The extent of collagen degradation was determined by measurement of the amount of hydroxyproline generated by acid-heat hydrolysis of culture supernatants. The release of lactate dehydrogenase from corneal fibroblasts was determined as an index of drug cytotoxicity.

RESULTS

Latanoprost stimulated collagen gel contraction mediated by corneal fibroblasts in a concentration- and time-dependent manner, whereas timolol maleate and pilocarpine had no such effect. None of the 3 drugs affected collagen degradation by corneal fibroblasts or exhibited cytotoxicity at concentrations as high as 100 muM.

CONCLUSIONS

Among the antiglaucoma drugs examined, only latanoprost stimulated collagen gel contraction mediated by human corneal fibroblasts. This action of latanoprost might affect corneal shape and thereby influence measurement of intraocular pressure.

Topical therapeutic agents that modulate corneal wound healing

In summary, corneal wound healing is a complex phenomenon that involves interplay between the cellular elements of the cornea, numerous soluble factors, and the constituents of the ECM. Unfortunately, many studies that demonstrate marked alteration on cell behavior in vitro and even in in vivo experiments are often not helpful in the diseased patient, as shown by the results of careful clinical trials. Future work that addresses the complex milieu of the corneal wound healing environment by addressing the interaction of many of these factors will be more likely to be successful than seeking a single agent that will enhance wound healing in all situations. Modulation of wound healing processes by the application of topical therapeutic agents is, however, an expanding field of study sure to produce clinically significant improvements in the management of veterinary patients with corneal defects. The judicious use of topical cytoactive compounds has a place in the clinician's armamentarium integrated into a therapeutic plan that decreases the mechanical stresses imposed on the wound bed as well as removal of any underlying inciting cause.

Enhancement by neutrophils of collagen degradation by corneal fibroblasts

Activated corneal fibroblasts and infiltrated leukocytes are thought to contribute to corneal ulceration. The potential roles of neutrophil-fibroblast and cell-matrix interactions in the degradation of stromal collagen associated with corneal ulceration have now been investigated with the use of three-dimensional cultures of rabbit cells in collagen gels. Degradation of collagen fibrils during culture was measured by spectrophotometric determination of released hydroxyproline. Whereas corneal fibroblasts alone degraded collagen fibrils to a small extent, neutrophils did not. However, the addition of neutrophils or neutrophil-conditioned medium (CM) to cultures of corneal fibroblasts resulted in a marked increase in the amount of collagen degraded by the fibroblasts. The effect of CM from neutrophils cultured in collagen gels on collagen degradation by corneal fibroblasts was greater than that of medium conditioned by neutrophils in monolayer culture. Immunoblot as well as reverse transcription and real-time polymerase chain reaction analyses revealed that neutrophil-CM stimulated the synthesis of matrix metalloproteinase (MMP)-1 and MMP-3 by corneal fibroblasts. The stimulatory effect of neutrophils on collagen degradation by corneal fibroblasts was inhibited by the synthetic MMP inhibitor ilomastat and by interleukin-1 (IL-1) receptor antagonist. These results suggest that factors secreted by collagen-stimulated neutrophils augment collagen degradation by corneal fibroblasts through a stimulatory effect on MMP synthesis and that IL-1 released by neutrophils may contribute to this effect.

Role of protein kinase C signaling in collagen degradation by rabbit corneal fibroblasts cultured in three-dimensional collagen gels

PURPOSE

To understand the mechanism of corneal ulceration by characterizing the intracellular signaling pathways that regulate collagen degradation by corneal fibroblasts cultured in three-dimensional type I collagen gels. Specifically, the potential roles of protein kinase C (PKC) and protein kinase A (PKA) in collagen degradation were investigated.

METHODS

Rabbit corneal fibroblasts were cultured in three-dimensional type I collagen gels for 24 hours in the presence of plasminogen and in the absence or presence of activators or inhibitors of PKC or PKA. Degradation of collagen fibrils was then evaluated by measurement of released hydroxyproline, and the production of matrix metalloproteinases (MMPs) was assessed by gelatin zymography and immunoblot analysis.

RESULTS

The PKC activator phorbol 12-myristate 13-acetate (PMA) increased the extent of collagen degradation by corneal fibroblasts in a dose-dependent manner, with the maximal effect apparent at a concentration of 0.1 microM. The inactive analog 4alpha-PMA had no effect on collagen degradation. The PKC inhibitor H-7 reduced the extent of collagen degradation by corneal fibroblasts in the absence or presence of PMA. Phorbol 12-myristate 13-acetate also increased the production of proMMP-1, -3, and -9 by corneal fibroblasts, whereas H-7 inhibited this effect. Neither the PKA activators 8-bromo-cAMP, isobutylmethylxanthine, and forskolin nor the PKA inhibitor HA1004 affected collagen degradation by corneal fibroblasts.

CONCLUSION

These results demonstrate that PKC plays an important role in collagen degradation by corneal fibroblasts in three-dimensional type I collagen gels, whereas PKA does not appear to participate in this process.

A descriptive and quantitative study of the keratocytes of the corneal stroma of albino rabbits using transmission electron microscopy

The present morphometric study was designed to assess the dimensions and shape of keratocytes and their nuclei by transmission electron microscopy, and to assess these features in relation to the stromal lamellae. Corneas from 10 albino rabbits were fixed in 2% glutaraldehyde in cacodylate buffer (pH 7.4, 300 mOsm/kg) and embedded in Spurr's epoxy resin. Both transverse and coronal thin sections through the corneal stroma were prepared. The stromal lamellae had an average thickness of 2.45+/-1.15 microm. The average cell thickness of the keratocytes was 1.34+/-0.46 microm (range 0.49-4.76 microm), with the apparent cell thickness being related to the average anterior-posterior thickness of the adjacent lamellae (r = 0.424, P = 0.001)). The relative length and thickness of the cell nucleus, in transverse section, was measured to be 0.65+/-0.13 and 0.76+/-0.10 of the cell body section respectively. As assessed by planimetry, the area of the keratocyte cell body viewed in coronal section was 292+/-118 microm2, with a nucleus-to-cytoplasm ratio of 0.437+/-0.295. The electron micrographs confirmed the presence of gap junctions between keratocyte cell processes, and the occasional presence of centrioles in the cells. Some keratocyte processes were observed to extend from one face of the lamellae to the other, suggesting anterior-to-posterior cell communication. These studies indicate that the keratocyte cell thickness is influenced by the physical pressure exerted by adjacent stromal lamellae. The cell nucleus, while a dominant feature in transverse section, has a normal size in relation to the cell cytoplasm when viewed in coronal section.

The keratocyte network of human cornea: a three-dimensional study using confocal laser scanning fluorescence microscopy

PURPOSE

Keratocytes of the living human cornea were examined to compare quantitatively spatial arrangement and cell volume of the stromal layers. This knowledge is required for further studies toward a quantitative understanding of cellular alterations in corneal pathology.

METHODS

Three human corneas were stained with calcein AM and ethidium homodimer (Live/Dead Kit) directly after enucleation. The fluorescent cells were examined with confocal laser scanning fluorescence microscopy. High-resolution three-dimensional (3-D) volumes of < or =270 microm in the z-axis were reconstructed. Cell density and volume density were determined by computer-aided morphometry.

RESULTS

Three keratocyte subpopulations were distinguished. Their spatial arrangement was visualized by 3-D reconstructions of the scanned volumes. Whereas cell density decreased progressively from the anterior (100%) to posterior (53.7%) stroma, volume density was highest in the posterior stroma (17.03 +/- 5.05%) and lowest in the central stroma (9.31 +/- 1.09%). In the anterior stroma, volume density was found to be 10.19 +/- 4.37%.

CONCLUSION

Confocal laser scanning fluorescence microscopy allowed quantitative analysis and the visualization of the spatial arrangement of the keratocyte network in the living human corneal tissue for the first time. The results provide a basis for further studies of alterations of the normal cellular arrangements in corneal disease.

Effect of galardin on collagen degradation by Pseudomonas aeruginosa

The authors examined the effect of a synthetic peptidyl hydroxamate inhibitor of matrix metalloproteinase, Galardin, on collagen degradation by Pseudomonas aeruginosa (P. aeruginosa) in the presence or absence of keratocytes. Type I collagen gels, with or without suspended keratocytes, were incubated under medium containing sterile P. aeruginosa culture broth and/or Galardin for 24 hr. Degradation of collagen fibrils during culture was measured by the release of hydroxyproline. The conditioned media were also subjected to gelatin zymography and Western blotting to analyse the activation, by P. aeruginosa factor(s), of matrix metalloproteinases (MMPs) released by keratocytes. The effects of protease inhibitors, aprotinin, leupeptin and pepstatin, on collagen degradation by P. aeruginosa were also examined. P. aeruginosa broth by itself induced collegen gel degradation. When keratocytes were present, P. aeruginosa broth increased the amount of degraded collagen even further. Galardin significantly reduced the amounts of collagen degraded by P. aeruginosa culture broth, whether keratocytes were present or absent in the gel. However, the protease inhibitors had no inhibitory effects on collagen degradation. Gelatin zymography and Western blotting revealed that inactive proMMP-1, -2 and -3, released by keratocytes, were converted to active forms in the presence of P. aeruginosa broth. Galardin decreased the amounts of active MMPs and increased those of inactive proMMPs, suggesting that Galardin inhibited the activation of proMMPs by P. aeruginosa. The present results suggest that Galardin inhibits the keratocyte-mediated collagen degradation by P. aeruginosa culture broth, resulting from preventing the conversion of proMMPs to active MMPs.

Corneal stromal wound healing in refractive surgery: the role of myofibroblasts

While laser and incisional refractive surgery offer the promise to correct visual refractive errors permanently and predictably, variability and complications continue to hinder wide-spread acceptance. To explain variations, recent studies have focused on the role of corneal wound healing in modulating refractive outcomes. As our understanding of the corneal response to refractive surgery broadens, it has become apparent that the response of one cell, the corneal stromal keratocyte, plays a pivotal role in defining the results of refractive surgery. Studies reviewed herein demonstrate that injury-induced activation and transformation of keratocytes to myofibroblasts control the deposition and organization of extracellular matrix in corneal wounds. Myofibroblasts establish an interconnected meshwork of cells and extracellular matrix that deposits new matrix and contracts wounds using a novel and unexpected "shoe-string-like" mechanism. Transformation of keratocytes to myofibroblasts is induced in culture by transforming growth factor beta (TGFbeta) and blocked in vivo by antibodies to TGFbeta. Overall, myofibroblast appearance in corneal wounds is associated with wound contraction and regression following incisional keratotomy and the development of "haze" or increased scattered light following laser photorefractive keratectomy (PRK). By contrast, absence of myofibroblasts is associated with continued widening of wound gape and progressive corneal flattening after incisional procedures. Based on these studies, we have arrived at the inescapable conclusion that a better understanding of the cellular and molecular biology of this one cell is required if refractive surgery is ever to achieve predictable and safe refractive results.

Galardin inhibits collagen degradation by rabbit keratocytes by inhibiting the activation of pro-matrix metalloproteinases

We investigated the inhibitory action of a synthetic peptidyl hydroxamate inhibitor of matrix metalloproteinase (MMP), Galardin (GM6001), on collagen degradation by rabbit corneal stromal fibroblasts (keratocytes) cultured three-dimensionally in the type I collagen gel with medium containing interleukin 1alpha (IL-1alpha) and/or plasminogen. Degradation of collagen fibrils during culture was measured by the release of hydroxyproline, and activation of MMPs was also analyzed by gelatin zymography and Western blotting. Plasmin activity was measured using a synthetic substrate. In the absence of plasminogen, treatment of the cells with IL-1alpha in collagen gel greatly enhanced the production of proMMP-1, -3 and -9, but no significant degradation of collagen was detected. In the presence of plasminogen, IL-1alpha stimulated collagen degradation by keratocytes in a dose-dependent manner. This resulted from the plasminogen activator-plasmin system-dependent activation of proMMP-1, -3 and -9. Galardin inhibited the collagen degradation in a dose-dependent fashion in the presence of plasminogen, whether IL-1alpha was present or not. Galardin inhibited the activation of proMMP-3, and also prevented the activation of proMMP-9 and the conversion of MMP-1 intermediates to the fully active MMP-1. Galardin did not affect plasmin activity. The present results suggest that Galardin inhibits IL-1alpha-stimulated collagen degradation in the presence of plasminogen, resulting from not only inhibiting active MMPs but also preventing the conversion of proMMPs to active MMPs.

Fluorescence microscopy and three-dimensional imaging of the porcine corneal keratocyte network

BACKGROUND

Little is known about the spatial arrangement and the corresponding morphometric data describing the living keratocyte network. For determination of alterations in corneal diseases it is crucial to know the morphology of the keratocyte network in the healthy state. Porcine cornea was used as a model tissue because it allows the study of species differences.

METHODS

Corneas from freshly enucleated pig eyes were stained with calcein AM and ethidium homodimer and examined by confocal laser scanning microscopy. High-resolution fluorescence images were used for three-dimensional reconstructions from which cell density and volume density were determined by computer-aided morphometry.

RESULTS

Three keratocyte subpopulations were distinguished and visualized in their spatial arrangement. Significant differences with respect to both shape and fluorescence intensity distribution of the cell bodies were found. Cell volume density was 7.7% in the anterior stroma, 13.7% in the central stroma and 11.8% in the posterior stroma.

CONCLUSION

The technique described allowed good visualization of the spatial arrangement of the keratocyte network. Combined with morphometric methods, the analysis of the state of the cornea yields a quantitative description. The method is expected to be useful for the determination of morphological alterations in corneal disease or following surgical treatment.

Keratocyte networks visualised in the living cornea using vital dyes

Fluorescent viability probes have been used to visualise and investigate the viability, morphology and organisation of the keratocyte within the stroma of the intact living cornea. The live cell probe, calcien-AM, in combination with a dead cell probe, ethidium homodimer (Live/Dead Assay, Molecular Probes, U.S.A.) proved superior to earlier generation vital dyes such as fluorescein diacetate or 5,6-carboxyfluorescein diacetate, initially used in combination with ethidium bromide. The ubiquitous distribution of esterase enzymes that cleave calcien-AM within the keratocyte cytoplasm produced a high concentration of fluorescently active calcein throughout the cell, including fine cell processes. Epi-illuminated fluorescence microscopy on transparent corneal dissections subsequently revealed details of keratocyte microanatomy and three-dimensional network organisation in situ. Three morphologically discrete subpopulations of keratocytes were identified: two formed relatively small bands of cells, immediately subjacent to either Bowman's or Descemet's membranes, the third subpopulation constituting the majority of keratocytes typically located within the corneal stroma. The results indicate that calcein-AM is able to penetrate intact living cornea revealing cell viability, and it also has the capacity to ‘trace’ cellular elements and reveal fine structure within a dense connective tissue matrix.

Spatial and functional relationship between myocytes and fibroblasts in the rabbit sinoatrial node

In an attempt to understand better the directional differences in conduction velocity in the rabbit sinoatrial node, a possible conductive role of the abundant connective tissue surrounding the myocytes has been investigated. In particular, starting from the finding of communicating junctions between heart muscle cells and fibroblasts in tissue culture, heterologous gap junctions were searched for in thin sections of the rabbit sinoatrial node. Within and at the edge of nodal cell clusters, fibroblasts often show thin sheet-like extensions parallel to the surface of myocytes. In contrast to the intimately contacting myocytes, fibroblast extensions are kept separated from the myocytes by the basement membrane of the latter. Besides some rare undefined membrane appositions a single tiny gap junction-like structure was found between a fibroblast and a myocyte in a tissue area in which the calculated number of gap junctions between myocytes amounts from 1.10(4) to 3.10(4). Yet, fibroblasts are linked together regularly by small gap junctions containing a wider gap than the junctions between the myocytes (1.4 +/- 0.4 nm vs. 1.0 +/- 0.4 nm, resp., P less than 0.05). As an alternative to direct electrical coupling, the possibility of interaction between fibroblasts and nodal cells by capacitive coupling has been considered. Model calculations based on the reconstruction of some fibroblast extensions parallel to nodal cells show that the current which can be transmitted from discharging nodal cells to fibroblasts is negligible. It is concluded that fibroblasts do not participate in the impulse conduction within the sinoatrial node. The origin of the directional differences in conduction velocity in the sinoatrial node must be found in the spatial arrangement of the myocytes and the distribution of the gap junctions between these cells only.

Initial characterization of whole-cell currents from freshly dissociated corneal keratocytes

The perforated patch technique was utilized to obtain whole-cell currents from freshly dissociated rabbit corneal keratocytes. We describe and provide the initial characterization of two distinct whole cell currents in rabbit keratocytes: a K(+)-selective delayed rectifier and a voltage-sensitive, tetrodotoxin blockable Na+ current. The voltage-sensitive Na+ current is of sufficient magnitude to allow us to initiate action potentials when current-clamping the cells. This is the first detailed electrophysiological study of corneal keratocytes.

Interactions of extracellular collagen and corneal fibroblasts: morphologic and biochemical changes of rabbit corneal cells cultured in a collagen matrix

Corneal fibroblasts, also known as keratocytes are surrounded by an extracellular matrix of collagen in vivo. To understand the physiology and pathology of these corneal fibroblasts, it is important to study their interactions with this extracellular matrix. We cultured rabbit corneal fibroblasts on tissue culture plastic dishes or in a hydrated collagen gel and compared the changes in morphology and mitotic activity. Corneal fibroblasts on plastic dishes were flattened and widely spread, whereas those in collagen gel became spindle-shaped with long processes. Examination with an electron microscope revealed that the corneal fibroblasts in collagen gel formed gap junctions with neighboring cells. Gap junctions were hardly ever observed between corneal fibroblasts cultured on plastic dishes. Corneal fibroblasts cultured in a collagen matrix showed much less incorporation of [3H]thymidine than did corneal fibroblasts cultured on plastic, and this incorporation decreased with increasing concentration of collagen. Our present results suggest that the morphologic and biochemical characteristics of corneal fibroblasts cultured in collagen gel are different from those cultured on plastic.