Descemet's membrane development, structure, function and regeneration

Silk biomaterials for corneal tissue engineering: From research approaches to therapeutic potentials; A review

The corneal complications can result in opacity and eventual blindness. Furthermore, a shortage of available donors constrains the existing therapeutic options. Therefore, one of the most promising strategies involves the application of biomaterials, particularly silk. Silk has garnered significant attention among these biomaterials due to its natural origin and diverse features derived from different sources. One of the most critical factors of silk is its transparency, which is crucial for the cornea, and there are no concerns about infection. This material also possesses several advantages, including cost-effectiveness in production, biocompatibility in vivo and in vitro, biodegradation, and desirable mechanical characteristics. Modifications in the topographical structure, porosity, and crystallinity of silk enhance its properties and optimize its suitability for wound dressing, efficient drug delivery systems, and various cornea-related treatments. In each layer, silk was examined as a single biomaterial or blended with the others, so, this review aims to explore silk as a potential material for corneal regenerative medicine from a novel viewpoint. By considering a range of studies, a classification system has been developed that categorizes the utilization of silk in the various layers of the cornea and sub-categorizes the different modifications and applications of silk.

Long-term alterations of collagen reconstruction and basement membrane regeneration after corneal full-thickness penetrating injury in rabbits

PURPOSE

To investigate the long-term alterations of collagen reconstruction and basement membrane (BM) regeneration after corneal full-thickness penetrating injury in rabbits.

METHODS

The corneal full-thickness penetrating injury model was established in the left eye of New Zealand White rabbits using a 2.0 mm trephine. All corneas were evaluated using slit-lamp photography, hematoxylin and eosin staining, immunofluorescent staining for collagen types I and III (Col I, III), and transmission electron microscopy for collagen fibers and basement membrane.

RESULTS

Between 3 days and 3 weeks, Col I and III expression were documented, exhibiting a largely disorganized distribution throughout the stromal thickness. At 3 weeks, the epithelial basement membrane (EBM) partially regenerated. From 3 weeks to 2 months, Col III was undetectable in the anterior stroma but present in the posterior stroma; Col I was disorganized in the posterior stroma. At 2 months, Descemet's membrane (DM) exhibited incomplete regeneration. From 3 to 4 months, Col I was disorganized in only a small part of the posterior stroma; Col III persisted in the posterior stroma; the EBM fully regenerated while DM exhibited incomplete regeneration.

CONCLUSIONS

Following full-thickness corneal injury, persistent fibrosis within the posterior stroma appears to be primarily responsible for the persistence of corneal scarring. Notably, regeneration of the EBM coincides with remodeling of the anterior stroma, whereas incomplete regeneration of DM is associated with posterior stromal fibrosis.

Topical Ocular Drug Delivery: The Impact of Permeation Enhancers

Topical ophthalmic drug delivery targeting the posterior segment of the eye has become a key area of interest due to its non-invasive nature, safety, ease of application, patient compliance, and cost-effectiveness. However, achievement of effective drug bioavailability in the posterior ocular segment is a significant challenge due to unique ocular barriers, including precorneal factors and anatomical barriers, like the cornea, the conjunctiva, and the sclera. Successful ocular drug delivery systems require increased precorneal residence time and improved corneal penetration to enhance intraocular bioavailability. A promising strategy to overcome these barriers is incorporating drug penetration enhancers (DPEs) into formulations. These compounds facilitate drug delivery by improving permeability across otherwise impermeable or poorly permeable membranes. At the ocular level, they act through three primary mechanisms: breaking tear film stability by interfering with the mucous layer; disrupting membrane components such as phospholipids and proteins; and loosening epithelial cellular junctions. DPEs offer significant potential to improve bioavailability and therapeutic outcomes, particularly for drugs targeting the posterior segment of the eye. This review is focused on analyzing the current literature regarding the use of penetration enhancers in topical ocular drug delivery, highlighting their mechanisms of action and potential to revolutionize ophthalmic treatments.

In Vitro Evaluation of Gelatin-Based Hydrogels as Potential Fillers for Corneal Wounds

Corneal persistent epithelial defects are common ophthalmic injuries that can cause significant visual and structural damage. While diagnosis is straightforward, treatment remains challenging. Noninvasive therapies like eye drops are preferred, but severe neurotrophic keratopathy may require surgical interventions. This study explores gelatin-based hydrogels as noninvasive alternatives for corneal repair. Four photo-cross-linkable hydrogels with gelatin and riboflavin phosphate (RFP) were evaluated: a control and variants incorporating 2.5% dextran (D), 0.4% hyaluronic acid (HA), or 1% methylcellulose (MC). In vitro assessments included physicochemical properties, biocompatibility, and release kinetics alongside ex vivo wound healing assays. The gelatin-RFP hydrogel maintained corneal transparency, while additives reduced it. Dextran slowed compound release, and HA and MC reduced the release rate of larger molecules. All hydrogels showed excellent biocompatibility, and ex vivo models confirmed re-epithelialization, though slower than controls. The unmodified gelatin-RFP hydrogel demonstrated the best potential for corneal tissue engineering, supporting its future clinical translation.

Intracameral Enoxaparin for Descemet Membrane Endothelial Keratoplasty: A Pilot Safety Study

PURPOSE

The purpose of this study was to investigate the safety and outcomes of Descemet membrane endothelial keratoplasty (DMEK) performed with intracameral enoxaparin.

METHODS

Two arms were used: a clinical multicenter retrospective cohort arm (CA) and an ex vivo basic science arm (BSA). In CA, DMEKs were performed by 6 experienced corneal surgeons at multiple sites. Intracameral enoxaparin (40 mg/500 mL) was added to the irrigation fluid for all cases. Primary outcomes were measured at 6 and 12 months. In BSA, mated graft pairs were randomized to control or enoxaparin exposure (0.8 mg/mL × 1 hour) and assessed for endothelial cell death count at 0-, 1-, and 24-hour intervals and cellular stress by ELISA Annexin V protein quantification.

RESULTS

In the cohort arm, the mean age of 159 eyes of 134 patients was 69.3 years with Fuchs dystrophy as the primary diagnosis. Mean BCVA improved from 0.42 ± 0.3 logMAR preoperatively to 0.13 ± 0.1 logMAR postoperatively at 6 months (P  < 0.001) and to 0.1 ± 0.1 logMAR at 12 months (P < 0.001). At 6 months, 58.4% of patients achieved a final BCVA of 20/25 or better and 91% improved to 20/40 or better. Rebubble rate was 13% (n = 21), with 6 of these 21 eyes requiring more than 1 rebubble. One total graft detachment was noted with no reports of intraoperative or postoperative hemorrhage. PGF occurred in 0 of 159 eyes. In BSA, enoxaparin had no significant effect on endothelial cell death count or cellular apoptosis compared with control.

CONCLUSIONS

Enoxaparin can safely be used in DMEK surgery without apparent increased risk of intraoperative hemorrhage, graft detachment/failure, or endothelial cell toxicity.

Biomaterial-based strategies for primary human corneal endothelial cells for therapeutic applications: from cell expansion to transplantable carrier

The treatment of corneal blindness due to corneal diseases and injuries often requires the transplantation of healthy cadaveric corneal endothelial graft tissue to restore corneal clarity and visual function. However, the limited availability of donor corneas poses a significant challenge in meeting the demand for corneal transplantation. As a result, there is a growing interest in developing strategies alleviate this unmet need, and one of the postulated approaches is to isolate and expand primary human corneal endothelial cells (HCECs) in vitro for use in cell therapy. This review summarizes the recent advancements in the expansion of HCECs using biomaterials. Two principal biomaterial-based approaches, including extracellular matrix (ECM) coating and functionalized synthetic polymers, have been investigated to create an optimal microenvironment for the expansion and maintenance of corneal endothelial cells (CECs). This review highlights the challenges and opportunities in expanding primary HCECs using biomaterials. It emphasizes the importance of optimizing biomaterial properties, cell culture conditions, and the roles of biophysical cues to achieve efficient expansion and functional maintenance of CECs. Biomaterial-based strategies hold significant promise for expanding primary HCECs and improving the outcomes of CEC transplantation. The integration of biomaterials as cell culture substrates and transplantable carriers offers a comprehensive approach to address the limitations associated with current corneal tissue engineering techniques.

Histological analysis of anterior eye development in the brown anole lizard (Anolis sagrei)

For all vertebrates, the anterior eye structures work together to protect and nourish the eye while ensuring that light entering the eye is correctly focused on the retina. However, the anterior eye structure can vary significantly among different vertebrates, reflecting how the structures of the anterior eye have evolved to meet the specific visual needs of different vertebrate species. Although conserved pathways regulate fundamental aspects of anterior eye development in vertebrates, there may also be species-specific differences underlying structural variation. Our knowledge of the cellular and molecular mechanisms underlying the development of structures of the anterior eye comes mainly from work in mammals, chicks, some amphibians, and small teleosts such as zebrafish. Our understanding of anterior eye development would benefit from comparative molecular studies in diverse vertebrates. A promising lizard model is the brown anole, Anolis sagrei, which is easily raised in the laboratory and for which genome editing techniques exist. Here, we provide a detailed histological analysis of the development of the anterior structures of the eye in A. sagrei, which include the cornea, iris, ciliary body, lens, trabecular meshwork, and scleral ossicles. The development of the anterior segment in anoles follows a pattern similar to other vertebrates. The lens forms first, followed by the cornea, iris, ciliary body, and tissues involved in the outflow of the aqueous humor. The development of the iris and ciliary body begins temporally and then proceeds nasally. Scleral ossicle development is generally comparable to that reported for chicks and turtles. Anoles have a remarkably thin cornea and a flat ciliary body compared to the eyes of mammals and birds. This study highlights several features in anoles and represents a deeper understanding of reptile eye development.

Anatomic Relationship Among Descemet's Membrane, Trabecular Meshwork, Scleral Spur, and Ciliary Muscle

Purpose

The purpose of this study was to examine anatomic relationships between Descemet's membrane (DM) and neighboring tissues.

Methods

Human eyes enucleated due to malignant uveal melanoma were histomorphometrically examined.

Results

In all 50 eyes included in the study (age = 59.2 ± 13.3 years), a cellular tissue (mean thickness at the DM end = 14.2 ± 19.5 µm), originating in the transitional zone, extended into the space between the peripheral DM and corneal stroma for a length of 142 ± 71 µm. This length decreased with the longer part of the DM covered with Hassall-Henle-bodies (beta = -0.30, B = -0.28, 95% confidence interval [CI] = -0.54 to -0.03, P = 0.03) and was not related with age (P = 0.84), axial length (P = 0.94), transitional zone length (P = 0.51), and the DM-scleral spur distance (P = 0.72). The DM thinned toward its end and the DM-like components appeared getting together and merging with transitional zone tissue fibers. The transitional zone length (mean = 267 ± 115 µm) varied from 82 µm to 586 µm, increased with longer DM-scleral spur distance (beta = 0.80, B = 0.67, 95% CI = 0.52 to 0.82, P < 0.001), and was not related with axial length (P = 0.74) or age (P = 0.18). The DM-scleral spur distance (mean = 660 ± 136 µm, range = 302 µm-979 µm) was not related with axial length (P = 0.60) or age (P = 0.06). The scleral spur dimensions (basis = 193 ± 103 µm, height = 151 ± 46 µm, length = 231 ± 75 µm) were not related with the DM-scleral spur distance (all P > 0.35), axial length (all P > 0.07), and age (all P > 0.25). In all eyes, the scleral spur tip was orientated anteriorly, with the longitudinal ciliary muscle inserting at its posterior side.

Conclusions

DM is firmly connected with the transitional zone tissue, and indirectly through the corneoscleral TM, with the scleral spur. With the latter connected through the longitudinal ciliary muscle and Bruch's membrane with the optic disc, the DM is part of an anatomic spherical unit of the globe.

Decellularized Descemet Membrane Anterior Keratoplasty With Allogeneic Simple Limbal Epithelial Transplantation for Partial Limbal Stem Cell Deficiency Following Partial Keratolimbal Allograft Failure

PURPOSE

The purpose of this study was to describe the use of Descemet membrane anterior keratoplasty (DMAK) with modified allogeneic simple limbal epithelial transplantation to treat a case of partial limbal stem cell deficiency (LSCD) following keratolimbal allograft failure.

METHODS

Case report.

RESULTS

A 21-year-old woman with autoimmune polyglandular syndrome-related LSCD presented with pain and decreased vision. There was partial failure and recurrence of LSCD after a severe/acute keratolimbal allograft rejection that led to persistent epithelial defects refractory to conservative therapy. This was treated with a superficial keratectomy and placement of a DMAK. A modified allogeneic simple limbal epithelial transplantation was performed with an overlying amniotic membrane and temporary tarsorrhaphy. There was epithelialization of the corneal surface by 3 to 4 weeks with an improved ocular surface. Despite partial recurrence of late staining, the cornea has remained epithelized, vision has improved, and the patient has remained pain-free more than 1.5 years following the procedure.

CONCLUSIONS

DMAK may be a long-term substrate to help improve and maintain epithelization of the cornea up to 1.5 years. DMAK may be a viable alternative to using amniotic membrane as a scaffold in allogeneic simple limbal epithelial transplantation for treatment of partial LSCD. While late epithelial staining recurred in our patient, DMAK appears to prevent recurrent epithelial defects and reduce ocular surface pain, conveying an improvement in quality of life in patients at high risk of rejection/failure.

Descemet's membrane transplantation for the treatment of recurrent high myopic macular hole associated with retinal detachment : Descemet's membrane for myopic macular hole

PURPOSE

To report the efficacy of Descemet's Membrane (DM) transplantation over the macular hole in patients with recurrent high myopic macular hole (HMMH) associated with retinal detachment (RD).

METHODS

Six eyes of six patients with wide posterior staphyloma including MH and recurrent HMMH associated with RD were included to this retrospective study. All patients underwent pars plana vitrectomy and DM obtained from eye bank was placed over the macular hole during the surgery. Silicone oil endotamponade was used as endotamponade and removed within 6 months following surgery. Pre-operative and post-operative ophthalmologic examination and optical coherence tomography findings were recorded.

RESULTS

The mean follow-up time was 18.53 ± 7.36 months. Macular hole closure was achieved in all patients (100%). Best-corrected visual acuity was improved from 1.51 ± 0.55 logMAR to 1.08 ± 0.50 logMAR (p = 0.043). No complications due to surgery or DM during follow-up. No DM dislocation or hole re-opening occurred after surgery.

CONCLUSION

DM transplantation during vitrectomy may be an effective treatment for the recurrent HMMH associated with RD.

KEY MESSAGES

What is known Various surgical techniques have been tried for recurrent high myopic macular hole associated with retinal detachment, but satisfactory anatomical and functional success rates have still not been achieved.

WHAT IS NEW

The study demonstrates that Descemet's membrane transplantation is a safe and effective option for treating recurrent high myopic macular hole associated with retinal detachment. This is a novel technique that may overcome the limitations of existing approaches. The findings suggest that Descemet's membrane transplantation could become a promising addition to the surgical options for recurrent high myopic macular hole associated with retinal detachment.

Characterization of the Ocular Phenotype in a Col4a3 Knockout Mouse Model of Alport Syndrome

Purpose

Alport syndrome (AS) is a genetic condition caused by a dysfunctional collagen (IV) α3α4α5 heterotrimer, leading to basement membrane instability and, ultimately, abnormalities in the kidney, inner ear, and eyes. This study aimed to characterize ocular pathology of AS by focusing on inflammatory and fibrotic markers.

Methods

Col4a3tm1Dec knockout (KO) mice eyes were evaluated for the localization of collagen (IV) α3 and collagen (IV) α4, then stained for transforming growth factor-β1 (TGF-β1), TGF-β2, connective tissue growth factor (CTGF), and β-catenin. mRNA levels of the profibrotic genes S100a4, Acta2, Col1a1, Snai1, Snai2, and Twist1 were assessed using real-time reverse transcription quantitative PCR (RT-qPCR).

Results

Collagen (IV) α3 and collagen (IV) α4 were co-expressed in Descemet's and Bruch's membrane but not in the retina, lens, or other corneal substructures. Immunofluorescence quantitation revealed upregulation of TGF-β1 in the anterior lens and TGF-β2 in the retina of KO eyes. Conversely, CTGF and β-catenin were shown to be elevated in the corneal epithelium but not the retina or lens. RT-qPCR showed an increase in the transcription of Acta2, Col1a1, and Snai2 in the retinas and Snai2 in anterior segments of KO mice.

Conclusions

Col4a3 KO mice exhibited a differential inflammatory and profibrotic response in the cornea, retina, and lens, which may play a role in the ocular pathology of AS.

The Corneal Structure of the Yellow-Legged Gull, Larus michahellis (Naumann, 1840)

The cornea is the transparent part of the eye's outer sheath and the primary refractive element in the optical system of all vertebrates allowing light to focus on the central part of the retina. Maintenance of its curvature and clarity is therefore essential, providing a smooth optical surface and a protective goggle to ensure a focused image on the retina. However, the corneas of birds have been largely overlooked and the structures and mechanisms controlling corneal shape and hence visual acuity remain unknown. In this work, the cornea of a seabird, that is, the yellow-legged gull, has been investigated using light and electron microscopy. Histological examination reveals that, as in other vertebrates, the cornea consists of five layers: outer epithelium, Bowman's layer, stroma, Descemet's membrane, and endothelium. The corneal epithelium is a nonkeratinized, stratified squamous epithelium approximately 3-4 cells thick that covers the front of the cornea. The surface of the cornea features two types of microprojections, microridges and microvilli. The acellular Bowman's layer is difficult to define because of its gradual transition into the more regularly arranged stroma, which constitute the bulk of the cornea, a collagen-rich central layer that comprises nearly 90% of the thickness of the cornea. The collagen fibrils are of uniform diameter and, within a given lamella, are all parallel to each other and run the entire breadth of the cornea. The lamellae are oriented at various angles with respect to each other. Between the lamellae, most of the keratocytes were concentrated in the central region of the corneal stroma. Desçemet's membrane is well-developed. The endothelium is a single cell-layer thick of approximately 3 µm in depth. The endothelial cells are polygonal and display irregular and interdigitating borders in basolateral plasma membranes. The results shown different diurnal lifestyle characteristics in the yellow-legged gull cornea.

Corneal Treatment, Repair, and Regeneration: Exosomes at Rescue

Exosomes are extracellular vesicles within the nanosized range that play roles in intercellular communication and thus have certain biological activities. The secretory signaling communication mechanism is an efficient way of exchanging information between cells and has been investigated as nature's therapeutic drug carriers. This review will summarize the potential of exosomes as therapeutic tools and drug delivery vehicles for corneal pathologies. The cornea is an avascular ocular tissue, and its healing is a complex process including cell death and migration, cell proliferation and differentiation, and extracellular matrix remodeling. Here, we discussed the structure, barrier, phases, and healing cascade of cornea. We briefly reviewed the immunogenicity and toxicity of exosomes and role of exosomes in preserving cornea. Additionally, we provided combining exosome strategies with hydrogels, gene and stem cells therapy focused on corneal treatment, repair, and regeneration.

Advancements in Hydrogels for Corneal Healing and Tissue Engineering

Hydrogels have garnered significant attention for their versatile applications across various fields, including biomedical engineering. This review delves into the fundamentals of hydrogels, exploring their definition, properties, and classification. Hydrogels, as three-dimensional networks of crosslinked polymers, possess tunable properties such as biocompatibility, mechanical strength, and hydrophilicity, making them ideal for medical applications. Uniquely, this article offers original insights into the application of hydrogels specifically for corneal tissue engineering, bridging a gap in current research. The review further examines the anatomical and functional complexities of the cornea, highlighting the challenges associated with corneal pathologies and the current reliance on donor corneas for transplantation. Considering the global shortage of donor corneas, this review discusses the potential of hydrogel-based materials in corneal tissue engineering. Emphasis is placed on the synthesis processes, including physical and chemical crosslinking, and the integration of bioactive molecules. Stimuli-responsive hydrogels, which react to environmental triggers, are identified as promising tools for drug delivery and tissue repair. Additionally, clinical applications of hydrogels in corneal pathologies are explored, showcasing their efficacy in various trials. Finally, the review addresses the challenges of regulatory approval and the need for further research to fully realize the potential of hydrogels in corneal tissue engineering, offering a promising outlook for future developments in this field.

Advancements in Polymer Biomaterials as Scaffolds for Corneal Endothelium Tissue Engineering

Corneal endothelial dysfunction is a leading cause of vision loss globally, frequently requiring corneal transplantation. However, the limited availability of donor tissues, particularly in developing countries, has spurred on the exploration of tissue engineering strategies, with a focus on polymer biomaterials as scaffolds for corneal endotlhelium regeneration. This review provides a comprehensive overview of the advancements in polymer biomaterials, focusing on their role in supporting the growth, differentiation, and functional maintenance of human corneal endothelial cells (CECs). Key properties of scaffold materials, including optical clarity, biocompatibility, biodegradability, mechanical stability, permeability, and surface wettability, are discussed in detail. The review also explores the latest innovations in micro- and nano-topological morphologies, fabrication techniques such as electrospinning and 3D/4D bioprinting, and the integration of drug delivery systems into scaffolds. Despite significant progress, challenges remain in translating these technologies to clinical applications. Future directions for research are highlighted, including the need for improved biomaterial combinations, a deeper understanding of CEC biology, and the development of scalable manufacturing processes. This review aims to serve as a resource for researchers and clinician-scientists seeking to advance the field of corneal endothelium tissue engineering.

Exploring single-cell RNA sequencing as a decision-making tool in the clinical management of Fuchs' endothelial corneal dystrophy

Single-cell RNA sequencing (scRNA-seq) has enabled the identification of novel gene signatures and cell heterogeneity in numerous tissues and diseases. Here we review the use of this technology for Fuchs' Endothelial Corneal Dystrophy (FECD). FECD is the most common indication for corneal endothelial transplantation worldwide. FECD is challenging to manage because it is genetically heterogenous, can be autosomal dominant or sporadic, and progress at different rates. Single-cell RNA sequencing has enabled the discovery of several FECD subtypes, each with associated gene signatures, and cell heterogeneity. Current FECD treatments are mainly surgical, with various Rho kinase (ROCK) inhibitors used to promote endothelial cell metabolism and proliferation following surgery. A range of emerging therapies for FECD including cell therapies, gene therapies, tissue engineered scaffolds, and pharmaceuticals are in preclinical and clinical trials. Unlike conventional disease management methods based on clinical presentations and family history, targeting FECD using scRNA-seq based precision-medicine has the potential to pinpoint the disease subtypes, mechanisms, stages, severities, and help clinicians in making the best decision for surgeries and the applications of therapeutics. In this review, we first discuss the feasibility and potential of using scRNA-seq in clinical diagnostics for FECD, highlight advances from the latest clinical treatments and emerging therapies for FECD, integrate scRNA-seq results and clinical notes from our FECD patients and discuss the potential of applying alternative therapies to manage these cases clinically.

Factors and outcomes associated with corneal edema and Haabs striae in primary congenital glaucoma

PURPOSE

To identify specific factors and outcomes associated with corneal edema and Haabs striae in primary congenital glaucoma (PCG).

METHODS

The medical records of patients with PCG from 2011 to 2023 with >3 months' follow-up were reviewed retrospectively. Preoperative details and final outcomes were compared between eyes with and without corneal findings. The right eye of bilateral cases and the affected eye in unilateral cases were included.

RESULTS

A total of 58 patients (104 eyes, 69% male) underwent initial angle surgery at an average age of 297 ± 368 (median, 134) days. Corneal edema and Haabs striae were present preoperatively in 72 (69%) eyes of 41 patients and 68 (65%) eyes of 39 patients, respectively. Patients with corneal edema presented at a younger age (P < 0.0001) and with shorter axial length (P = 0.01) than those without edema. Univariate analysis showed that corneal edema was associated with worse visual acuity at final follow-up (OR = 4.4; 95% CI, 1.2-25.3). Patients with Haabs striae were older than those without striae (P = 0.04). After angle surgery, corneal edema was present at 1 month in 71% (95% CI, 52-84), at 2 months in 26% (95% CI, 12-42), at 3 months in 16% (95% CI, 6-30), and at 1 year in 3% (95% CI, 0-13). Corneal opacification did not resolve in 4 eyes of 3 patients after >4 years of follow-up.

CONCLUSIONS

In our study cohort, corneal edema resolved in the majority of PCG cases within 2-3 months of initial angle surgery but was associated with younger age at presentation and worse visual acuity at final follow-up.

Fabrication of bioengineered corneal endothelial grafts using an allogeneic cornea-derived matrix

Corneal endothelial keratoplasty has been the primary treatment method of endothelial decompensation, but it is often limited in clinical practice due to global shortage of donor cornea. Here, we explored using an ultra-thin allogeneic cornea-derived matrix (uACM) films as a substrate for constructing bioengineered corneal endothelial grafts. We evaluated the films' optical, mechanical, and structural properties, and measured the composition of the extracellular matrix. The uACM was an ultrathin and curved cornea-shaped film with favorable optical and mechanical properties. The fabrication process efficiently preserved corneal extracellular matrix composition and significantly decreased cellular components. Moreover, human corneal endothelial cells and rabbit corneal endothelial cells (RCECs) can adhere and grow on the uACM films with a positive expression of the corneal endothelial functional markers Na+/K+-ATPase and ZO-1. The successful transplantation of uACM with RCECs grafts into the rabbit model of endothelial dysfunction via Descemet membrane endothelial keratoplasty resulted in prompt restoration of corneal transparency and thickness. During the four-week follow-up period, the uACM with RCECs implanted eyes exhibited comparable corneal transparency, central corneal thickness, and endothelial cell count to that of the healthy rabbit. Histologic examination revealed that the grafts were successfully attached and integrated onto the posterior surface of the corneal stroma. The uACM achieved biomimetic reconstruction in terms of both composition and structure, and can be used to construct the bioengineered corneal endothelial grafts. These results indicate that constructing bioengineered corneal endothelial grafts from discarded human corneal tissues may pave the way for generating high-quality corneal endothelial grafts for transplantation.

Influence of Donor Factors on Descemet Membrane Endothelial Keratoplasty (DMEK) Graft Preparation Outcome

Purpose

To determine which donor characteristics, like previous diseases and surgeries, influence the severity of the DM/endothelial lamella preparation prior to DMEK-surgery.

Patients and Methods

Retrospective cross-sectional single-center study is presented. Eight hundred and forty-six eyes with DMEK-surgery between 01/2018 and 01/2021 performed at the University Hospital Cologne, Germany, were included. Information regarding the donors' previous diseases and surgeries were provided by a large database of a cornea bank (Multi Tissue Bank Mecklenburg-Vorpommern) and merged with the Cologne DMEK database, which contains information regarding preparation characteristics of the surgeon-prepared graft directly preoperatively. Three preparation groups (easy, difficult and very difficult) were correlated to the donors' previous diseases and surgeries. The following characteristics were used for the assignment in one of the three groups: stripping difficulty, rolling and staining behavior, central and peripheral adherences, tissue fragility and DM-splitting.

Results

Significant risk factors for DM-splitting were diabetes mellitus (DMel) type II, heart failure, chronic kidney disease and previous cataract surgery (p=0.022, p=0.012; p=0.047 and p<0.001 respectively). Previous DMel (especially type 2) was significantly associated with the occurrence of central adherences (p=0.009). Several cardiovascular diseases (p-values between <0.001 and p=0.038), DMel type II, chronic kidney disease and previous cataract-surgery were associated with peripheral adherences (p=0.004; p=0.020 and p<0.001 respectively). Furthermore, pseudophakic donor eyes presented a higher degree of fragility of the graft (p<0.001). Age was a significant risk factor for difficult preparation (p<0.001). The staining of the graft was poorer in donors with chronic kidney disease (p=0.037).

Conclusion

Donor diabetes mellitus type 2, heart failure, previous cataract surgery, chronic kidney disease and age are associated with a difficult DMEK graft preparation. For every one-year increment in donor age, the odds of having very difficult preparation were increased by 3%. Also, chronic kidney disease predisposes to a poor tissue staining with trypan blue during preparation.

Substrate Stiffness Modulates Stemness and Differentiation of Rabbit Corneal Endothelium Through the Paxillin-YAP Pathway

Purpose

To explore the role of substrate stiffness and the mechanism beneath corneal endothelial cells' (CECs') stemness maintenance and differentiation.

Methods

CECs were divided into central zone (8 mm trephined boundary) and peripheral zone (8 mm trephined edge with attached limbal). Two zones were analyzed by hematoxylin-eosin staining and scanning electron microscopy for anatomic structure. The elastic modulus of Descemet's membrane (DM) was analyzed by atomic force microscopy. Compressed type I collagen gels with different stiffness were constructed as an in vitro model system to test the role of stiffness on phenotype using cultured rabbit CECs. Cell morphology, expression and intracellular distribution of Yes-associated protein (YAP), differentiation (ZO-1, Na+/K+-ATPase), stemness (FOXD3, CD34, Sox2, Oct3/4), and endothelial-mesenchymal transition (EnMT) markers were analyzed by immunofluorescence, quantitative RT-PCR, and Western blot.

Results

The results showed that the peripheral area of rabbit and human DM is softer than the central area ex vivo. Using the biomimetic extracellular matrix collagen gels in vitro model, we then demonstrated that soft substrate weakens the differentiation and EnMT in the culture of CECs. It was further proved by the inhibitor experiment that soft substrate enhances stemness maintenance via inhibition of paxillin-YAP signaling, which was activated on a stiff substrate.

Conclusions

Our findings confirm that substrate stiffness modulates the stemness maintenance and differentiation of CECs and suggest a potential strategy for CEC-based corneal tissue engineering.

Evaluation of biophysical alterations in the epithelial and endothelial layer of patients with Bullous Keratopathy

The cornea is a fundamental ocular tissue for the sense of sight. Thanks to it, the refraction of two-thirds of light manages to participate in the visual process and protect against mechanical damage. Because it is transparent, avascular, and innervated, the cornea comprises five main layers: Epithelium, Bowman's layer, stroma, Descemet's membrane, and endothelium. Each layer plays a key role in the functionality and maintenance of ocular tissue, providing unique ultrastructural and biomechanical properties. Bullous Keratopathy (BK) is an endothelial dysfunction that leads to corneal edema, loss of visual acuity, epithelial blisters, and severe pain, among other symptoms. The corneal layers are subject to changes in their biophysical properties promoted by Keratopathy. In this context, the Atomic Force Microscopy (AFM) technique in air was used to investigate the anterior epithelial surface and the posterior endothelial surface, healthy and with BK, using a triangular silicone tip with a nominal spring constant of 0.4 N/m. Six human corneas (n = 6) samples were used for each analyzed group. Roughness data, calculated by third-order polynomial adjustment, adhesion, and Young's modulus, were obtained to serve as a comparison and identification of morphological and biomechanical changes possibly associated with the pathology, such as craters and in the epithelial layer and exposure of a fibrotic layer due to loss of the endothelial cell wall. Endothelial cell membrane area and volume data were calculated, obtaining a relevant comparison between the control and patient. Such results may provide new data on the physical properties of the ocular tissue to understand the physiology of the cornea when it has pathology.

Squishy matters - Corneal mechanobiology in health and disease

The cornea, as a dynamic and responsive tissue, constantly interacts with mechanical forces in order to maintain its structural integrity, barrier function, transparency and refractive power. Cells within the cornea sense and respond to various mechanical forces that fundamentally regulate their morphology and fate in development, homeostasis and pathophysiology. Corneal cells also dynamically regulate their extracellular matrix (ECM) with ensuing cell-ECM crosstalk as the matrix serves as a dynamic signaling reservoir providing biophysical and biochemical cues to corneal cells. Here we provide an overview of mechanotransduction signaling pathways then delve into the recent advances in corneal mechanobiology, focusing on the interplay between mechanical forces and responses of the corneal epithelial, stromal, and endothelial cells. We also identify species-specific differences in corneal biomechanics and mechanotransduction to facilitate identification of optimal animal models to study corneal wound healing, disease, and novel therapeutic interventions. Finally, we identify key knowledge gaps and therapeutic opportunities in corneal mechanobiology that are pressing for the research community to address especially pertinent within the domains of limbal stem cell deficiency, keratoconus and Fuchs' endothelial corneal dystrophy. By furthering our understanding corneal mechanobiology, we can contextualize discoveries regarding corneal diseases as well as innovative treatments for them.

Expression of Nephronectin in the Descemet's membrane of mouse corneas during development and adult homeostasis

Nephronectin (Npnt) is an extracellular matrix (ECM) protein with pleiotropic functions during organogenesis, disease, and homeostasis. Although the ECM plays a crucial role during development and homeostasis of the adult cornea, little is known about the expression of Npnt in the mammalian cornea. Here, we investigated the expression of Npnt during early embryonic and postnatal development, and in adult mouse corneas. We combined ultrastructural and immunohistochemical analyses to study the early formation of the Descemet's membrane and how the expression of Npnt relates to key basement membrane proteins. Our section in situ hybridization and immunohistochemical analyses revealed that Npnt mRNA is expressed by the nascent corneal endothelial cells at embryonic day (E) 14.5, whereas the protein is localized in the adjacent extracellular matrix. These expression patterns were maintained in the corneal endothelium and Descemet's membrane throughout development and in adult corneas. Ultrastructural analysis revealed discontinuous electron dense regions of protein aggregates at E18.5 that was separated from the endothelial layer by an electron lucent space. At birth (postnatal day, P0), the Descemet's membrane was a single layer, which continuously thickened throughout P4, P8, P10, and P14. Npnt was localized to the Descemet's membrane by E18.5 and overlapped with Collagens IV and VIII, Laminin, and Perlecan. However, the proteins subsequently shifted and formed distinct layers in the adult cornea, whereby Npnt localized between two Collagen VIII bands and anterior to Collagen IV but overlapped with Laminin and Perlecan. Combined, our results reveal the expression of Npnt in the mouse cornea and define its spatiotemporal localization relative to key basement membrane proteins during the formation of the Descemet's membrane and in the adult cornea. Understanding the spatiotemporal expression of Npnt is important for future studies to elucidate its function in the mammalian cornea.

Current Advances in Corneal Stromal Stem Cell Biology and Therapeutic Applications

Corneal stromal stem cells (CSSCs) are of particular interest in regenerative ophthalmology, offering a new therapeutic target for corneal injuries and diseases. This review provides a comprehensive examination of CSSCs, exploring their anatomy, functions, and role in maintaining corneal integrity. Molecular markers, wound healing mechanisms, and potential therapeutic applications are discussed. Global corneal blindness, especially in more resource-limited regions, underscores the need for innovative solutions. Challenges posed by corneal defects, emphasizing the urgent need for advanced therapeutic interventions, are discussed. The review places a spotlight on exosome therapy as a potential therapy. CSSC-derived exosomes exhibit significant potential for modulating inflammation, promoting tissue repair, and addressing corneal transparency. Additionally, the rejuvenation potential of CSSCs through epigenetic reprogramming adds to the evolving regenerative landscape. The imperative for clinical trials and human studies to seamlessly integrate these strategies into practice is emphasized. This points towards a future where CSSC-based therapies, particularly leveraging exosomes, play a central role in diversifying ophthalmic regenerative medicine.

Morphology of the cornea and iris in the Australian lungfish Neoceratodus forsteri (Krefft 1870) (Dipnoi): Functional and evolutionary perspectives of transitioning from an aquatic to a terrestrial environment

The Australian lungfish, Neoceratodus forsteri (Krefft 1870), is the sole extant member of the Ceratodontidae within the Dipnoi, a small order of sarcopterygian (lobe-finned) fishes, that is thought to be the earliest branching species of extant lungfishes, having changed little over the last 100 million years. To extend studies on anatomical adaptations associated with the fish-tetrapod transition, the ultrastructure of the cornea and iris is investigated using light and electron (transmission and scanning) microscopy to investigate structure-function relationships and compare these to other vertebrate corneas (other fishes and tetrapods). In contrast to previous studies, the cornea is found to have only three main components, comprising an epithelium with its basement membrane, a stroma with a Bowman's layer and an endothelium, and is not split into a dermal (secondary) spectacle and a scleral cornea. The epithelial cells are large, relatively low in density and similar to many species of non-aquatic tetrapods and uniquely possess numerous surface canals that contain and release mucous granules onto the corneal surface to avoid desiccation. A Bowman's layer is present and, in association with extensive branching and anastomosing of the collagen fibrils, may be an adaptation for the inhibition of swelling and/or splitting of the stroma during its amphibious lifestyle. The dorsal region of the stroma possesses aggregations of pigment granules that act as a yellow, short wavelength-absorbing filter during bright light conditions. Desçemet's membrane is absent and replaced by an incomplete basement membrane overlying a monocellular endothelium. The iris is pigmented, well-developed, vascularised and contractile containing reflective crystals anteriorly. Based upon its ultrastructure and functional adaptations, the cornea of N. forsteri is more similar to amphibians than to other bony fishes and is well-adapted for an amphibious lifestyle.

Unveiling Cellular Diversity in the Buffalo Corneal Stroma: Insights into Telocytes and Keratocytes Using Light Microscope, Transmission Electron Microscope, and Immunofluorescence Analysis

Telocytes and keratocytes are important cells that maintain the structure and function of the cornea. The buffalo cornea, known for its resilience in harsh conditions, has not been extensively studied regarding the presence and role of telocytes and keratocytes. We used light microscopy, transmission electron microscopy (TEM), and immunofluorescence assays with platelet-derived growth factor receptor alpha (PDGFRα), CD34, and Vimentin markers to investigate their expression and localization in the cornea. TEM analysis confirmed the presence of spindle-shaped keratocytes with intercellular connections, while telocytes exhibited small spindle-shaped bodies with long, thin branches connecting to corneal keratocytes. Immunofluorescence findings showed that CD34 was more abundant near the endothelium, Vimentin was prominently expressed near the epithelium, and PDGFRα was uniformly distributed throughout the corneal stroma. Co-expression of CD34 and Vimentin, PDGFRα and Vimentin, as well as CD34 and PDGFRα, was observed in keratocytes and telocytes within the stroma, indicating the potential presence of mesenchymal cells. These results suggest the involvement of telocytes and keratocytes in corneal wound healing, transparency maintenance, and homeostasis. The co-expression of these markers highlights the critical role of telocytes and keratocytes in regulating corneal physiological functions, further enhancing our understanding of corneal biology in the buffalo model.

Scheimpflug Corneal Densitometry Patterns at the Graft-Host Interface in DMEK and DSAEK: A 12-Month Longitudinal Comparative Study

BACKGROUND

To compare corneal densitometry (CD) patterns at the graft-host interface between Descemet Membrane Endothelial Keratoplasty (DMEK) and Descemet Stripping Automated Endothelial Keratoplasty (DSAEK). Corneal densitometry is a quantitative assessment that objectively evaluates corneal clarity and optical quality by measuring the light backscatter from the cornea.

METHODS

Fifty-one eyes that received DMEK or DSAEK surgery for corneal endothelium dysfunction were evaluated. The primary endpoint included CD patterns at the graft-host interface, which were assessed by the Pentacam HR device at the center point of the corneal horizontal meridian (CDcentral), and at six points on the central circumference of the cornea (with a total diameter of 4 mm) (CDI,II,III,IV,V,VI). Secondary endpoints included the best-corrected distance visual acuity (BCDVA), central corneal thickness (CCT), and graft thickness (GT). All of the evaluations were performed at follow-up appointments one, three, six and twelve months after the procedure.

RESULTS

DMEK showed a significant overall CD reduction of -7.9 ± 8.5 grayscale unit (GSU) compared to DSAEK (p < 0.001). In addition, the DMEK group showed significantly lower CDCentral,I,II,III,IV,V,VI values at follow-up appointments one, three, six and twelve months after the procedure compared to the DSAEK group (p < 0.001). BCDVA, CCT and GT were in favor of the DMEK group with a mean value of 0.39 ± 0.35 LogMar, 552.2 ± 71.1 µm and 11.03 ± 1.4 µm, respectively (p < 0.001).

CONCLUSIONS

CD patterns at the graft-host interface seem to be different depending on the endothelial keratoplasty procedure. This provides specific insight into CD changes in this critical region of surgery, which may provide a better understanding of the postoperative evolution of these patients.

Born Apart, but Raised Together: Twins Delivered in Different Countries/Twin Research Reviews: Hallermann-Streiff Syndrome in Monozygotic (MZ) Twins; Effects of Technology on Conjoined Twin Separation; Reciprocal DIEP Transplantation Between MZ Twins; Guidelines for Multifetal Management/Media Reports: Book by World's Oldest Auschwitz-Birkenau Twin Survivor; Passing of Ian Wilmut; Zhores Medvedev Was an Identical Twin; More Gay Fathers with Twin Sons; Twins and Siblings Admitted to Medical School; First and Fourth Records for Major League Baseball Twins

The circumstances and subsequent life events of the first twins to have been born in different countries are examined. Given that both twins were born in the United Kingdom, their common citizenship was never questioned. In contrast, twins born in Canada to a legally married gay transnational couple - composed of one American and one Israeli - were assigned as citizens of different nations and their parents were regarded as if unmarried. This essay is followed by reviews of research on the Hallermann-Streiff syndrome in monozygotic (MZ) twins, the effects of technology on conjoined twin separation, reciprocal deep inferior epigastric perforator (DIEP) transplantation in MZ twins and new guidelines for managing multifetal pregnancies. Finally, media reports on a book by the world's oldest Auschwitz-Birkenau twin survivor, the passing of Dr Ian Wilmut, the identical twinship of Zhores Medvedev, another case of gay fathers with twin sons, twins and siblings admitted to medical school, and the first and fourth records for major league baseball twins are presented.

The Innovative Biomaterials and Technologies for Developing Corneal Endothelium Tissue Engineering Scaffolds: A Review and Prospect

Corneal transplantation is the only treatment for corneal endothelial blindness. However, there is an urgent need to find substitutes for corneal endothelium grafts due to the global shortage of donor corneas. An emerging research field focuses on the construction of scaffold-based corneal endothelium tissue engineering (CETE). Long-term success in CETE transplantation may be achieved by selecting the appropriate biomaterials as scaffolds of corneal endothelial cells and adding bioactive materials to promote cell activity. This article reviews the research progress of CETE biomaterials in the past 20 years, describes the key characteristics required for corneal endothelial scaffolds, and summarizes the types of materials that have been reported. Based on these, we list feasible improvement strategies for biomaterials innovation. In addition, we describe the improved techniques for the scaffolds' surface topography and drug delivery system. Some promising technologies for constructing CETE are proposed. However, some questions have not been answered yet, and clinical trials and industrialization should be carried out with caution.

Efficient reduction of the scrolling of Descemet membrane endothelial keratoplasty grafts by engineering the medium

The Descemet Membrane Endothelial Keratoplasty (DMEK) procedure for corneal transplantation is challenging due to the need to unscroll the donor graft within the recipient's eye. This process of unscrolling is complex, time-consuming, leads to a loss of endothelial cells and, most importantly, can negatively impact the graft's adhesion and integration with the host tissue after surgery. This problem is particularly evident when the graft is young. However, the physics behind this scrolling is not well understood, and therefore no sustainable solution is attained. Here, we propose that the concentration gradient of the medium used during transplant leads to a displacement gradient across the graft thickness, resulting in an out-of-plane folding or scrolling of the graft tissue. Using chitosan bilayer-based experimental models, it is experimentally demonstrated that this diffusion-coupled-deformation phenomenon can successfully explain why younger donor grafts tend to scroll tighter than older ones. Most importantly, we illustrate here through experiments that the medium can be engineered to reduce the scroll tightness and thus reduce the surgical inconveniences and improve post-transplant recovery. STATEMENT OF SIGNIFICANCE: This paper addresses a major issue that surgeons face while doing Descemet Membrane Endothelial Keratoplasty (DMEK) in unscrolling grafts during the graft insertion procedure. The currently used tapping method to unscroll the graft inside the patient's eye significantly reduces endothelial cell count, thus affecting its lifetime. Surprisingly, the physics behind graft scrolling is not well understood, so no sustainable solutions are proposed by the medical community. In this work, we present the underlying mechanism of DMEK graft scroll and illustrate experimentally the reason for scroll tightness through a chitosan bilayer based experiment model. Most importantly, we have successfully demonstrated that the preserving medium of the grafts can be engineered to reduce scroll tightness.

Biomaterials used for tissue engineering of barrier-forming cell monolayers in the eye

Cell monolayers that form a barrier between two structures play an important role for the maintenance of tissue functionality. In the anterior portion of the eye, the corneal endothelium forms a barrier that controls fluid exchange between the aqueous humor of the anterior chamber and the corneal stroma. This monolayer is central in the pathogenesis of Fuchs endothelial corneal dystrophy (FECD). FECD is a common corneal disease, in which corneal endothelial cells deposit extracellular matrix that increases the thickness of its basal membrane (Descemet's membrane), and forms excrescences (guttae). With time, there is a decrease in endothelial cell density that generates vision loss. Transplantation of a monolayer of healthy corneal endothelial cells on a Descemet membrane substitute could become an interesting alternative for the treatment of this pathology. In the back of the eye, the retinal pigment epithelium (RPE) forms the blood-retinal barrier, controlling fluid exchange between the choriocapillaris and the photoreceptors of the outer retina. In the retinal disease dry age-related macular degeneration (dry AMD), deposits (drusen) form between the RPE and its basal membrane (Bruch's membrane). These deposits hinder fluid exchange, resulting in progressive RPE cell death, which in turn generates photoreceptor cell death, and vision loss. Transplantation of a RPE monolayer on a Bruch's membrane/choroidal stromal substitute to replace the RPE before photoreceptor cell death could become a treatment alternative for this eye disease. This review will present the different biomaterials that are proposed for the engineering of a monolayer of corneal endothelium for the treatment of FECD, and a RPE monolayer for the treatment of dry AMD.

The extracellular matrix mechanics in the vasculature

Mechanical stimuli from the extracellular matrix (ECM) modulate vascular differentiation, morphogenesis and dysfunction of the vasculature. With innovation in measurements, we can better characterize vascular microenvironment mechanics in health and disease. Recent advances in material sciences and stem cell biology enable us to accurately recapitulate the complex and dynamic ECM mechanical microenvironment for in vitro studies. These biomimetic approaches help us understand the signaling pathways in disease pathologies, identify therapeutic targets, build tissue replacement and activate tissue regeneration. This Review analyzes how ECM mechanics regulate vascular homeostasis and dysfunction. We highlight approaches to examine ECM mechanics at tissue and cellular levels, focusing on how mechanical interactions between cells and the ECM regulate vascular phenotype, especially under certain pathological conditions. Finally, we explore the development of biomaterials to emulate, measure and alter the physical microenvironment of pathological ECM to understand cell-ECM mechanical interactions toward the development of therapeutics.

Corneal gene therapy: Structural and mechanistic understanding

Cornea, a dome-shaped and transparent front part of the eye, affords 2/3rd refraction and barrier functions. Globally, corneal diseases are the leading cause of vision impairment. Loss of corneal function including opacification involve the complex crosstalk and perturbation between a variety of cytokines, chemokines and growth factors generated by corneal keratocytes, epithelial cells, lacrimal tissues, nerves, and immune cells. Conventional small-molecule drugs can treat mild-to-moderate traumatic corneal pathology but requires frequent application and often fails to treat severe pathologies. The corneal transplant surgery is a standard of care to restore vision in patients. However, declining availability and rising demand of donor corneas are major concerns to maintain ophthalmic care. Thus, the development of efficient and safe nonsurgical methods to cure corneal disorders and restore vision in vivo is highly desired. Gene-based therapy has huge potential to cure corneal blindness. To achieve a nonimmunogenic, safe and sustained therapeutic response, the selection of a relevant genes, gene editing methods and suitable delivery vectors are vital. This article describes corneal structural and functional features, mechanistic understanding of gene therapy vectors, gene editing methods, gene delivery tools, and status of gene therapy for treating corneal disorders, diseases, and genetic dystrophies.

Current microfluidic platforms for reverse engineering of cornea

According to the World Health Organization, corneal blindness constitutes 5.1% of global blindness population. Surgical outcomes have been improved significantly in the treatment of corneal blindness. However, corneal transplantation is limited by global shortage of donor tissue, prompting researchers to explore alternative therapies such as novel ocular pharmaceutics to delay corneal disease progression. Animal models are commonly adopted for investigating pharmacokinetics of ocular drugs. However, this approach is limited by physiological differences in the eye between animals and human, ethical issues and poor bench-to-bedside translatability. Cornea-on-a-chip (CoC) microfluidic platforms have gained great attention as one of the advanced in vitro strategies for constructing physiologically representative corneal models. With significant improvements in tissue engineering technology, CoC integrates corneal cells with microfluidics to recapitulate human corneal microenvironment for the study of corneal pathophysiological changes and evaluation of ocular drugs. Such model, in complement to animal studies, can potentially accelerate translational research, in particular the pre-clinical screening of ophthalmic medication, driving clinical treatment advancement for corneal diseases. This review provides an overview of engineered CoC platforms with respect to their merits, applications, and technical challenges. Emerging directions in CoC technology are also proposed for further investigations, to accentuate preclinical obstacles in corneal research.

Corneal optical density: Structural basis, measurements, influencing factors, and roles in refractive surgery

The cornea is the main refractive medium of the human eye, and its clarity is critical to visual acuity. Corneal optical density (COD) is an important index to describe corneal transparency. Intact corneal epithelial and endothelial cells, regular arrangement of collagen fibers in the stroma, and normal substance metabolism are all integral for the cornea to maintain its transparency. In the last two decades, the Pentacam Scheimpflug imaging system has emerged as a breakthrough for the measurement of COD (also called corneal densitometry). It has been found that a wide variety of factors such as age, refractive status, and corneal diseases can affect COD. Different corneal refractive surgery methods also change COD in different corneal regions and layers and affect visual acuity following the surgery. Thus, COD has gradually become a significant indicator to evaluate corneal health, one on which the attention of clinicians has been increasingly focused.

Topical Losartan: Practical Guidance for Clinical Trials in the Prevention and Treatment of Corneal Scarring Fibrosis and Other Eye Diseases and Disorders

Losartan is an angiotensin II receptor blocker (ARB) that impedes transforming growth factor (TGF) beta signaling by inhibiting activation of signal transduction molecule extracellular signal-regulated kinase (ERK). Studies supported the efficacy of topical losartan in decreasing scarring fibrosis after rabbit Descemetorhexis, alkali burn, and photorefractive keratectomy injuries, and in case reports of humans with scarring fibrosis after surgical complications. Clinical studies are needed to explore the efficacy and safety of topical losartan in the prevention and treatment of corneal scarring fibrosis, and other eye diseases and disorders where TGF beta has a role in pathophysiology. These include scarring fibrosis associated with corneal trauma, chemical burns, infections, surgical complications, and persistent epithelial defects, as well as conjunctival fibrotic diseases, such as ocular cicatricial pemphigoid and Stevens-Johnson syndrome. Research is also needed to explore the efficacy and safety of topical losartan for hypothesized treatment of transforming growth factor beta-induced (TGFBI)-related corneal dystrophies (Reis-Bu¨cklers corneal dystrophy, lattice corneal dystrophy type 1, and granular corneal dystrophies type 1 and type 2) where deposited mutant protein expression is modulated by TGF beta. Investigations could also explore the efficacy and safety of topical losartan treatments to reduce conjunctival bleb scarring and shunt encapsulation following glaucoma surgical procedures. Losartan and sustained release drug delivery devices could be efficacious in treating intraocular fibrotic diseases. Dosing suggestions and precautions that should be considered in trials of losartan are detailed. Losartan, as an adjuvant to current treatments, has the potential to augment pharmacological therapeutics for many ocular diseases and disorders where TGF beta plays a central role in pathophysiology.

Denuded Descemet's membrane supports human embryonic stem cell-derived retinal pigment epithelial cell culture

Recent clinical studies suggest that retinal pigment epithelial (RPE) cell replacement therapy may preserve vision in retinal degenerative diseases. Scaffold-based methods are being tested in ongoing clinical trials for delivering pluripotent-derived RPE cells to the back of the eye. The aim of this study was to investigate human embryonic stem cell-derived retinal pigment epithelial (hESC-RPE) cells survival and behaviour on a decellularized Descemet's Membrane (DM), which may be of clinical relevance in retinal transplantation. DMs were isolated from human donor corneas and treated with thermolysin. The DM surface topology and the efficiency of the denudation method were evaluated by atomic force microscope, scanning electron microscopy and histology. hESC-RPE cells were seeded onto the endothelial-side surface of decellularized DM in order to determine the potential of the membrane to support hESC-RPE cell culture, alongside maintaining their viability. Integrity of the hESC-RPE monolayer was assessed by measuring transepithelial resistance. RPE-specific gene expression and growth factors secretion were assessed to confirm maturation and functionality of the cells over the new substrate. Thermolysin treatment did not affect the integrity of the tissue, thus ensuring a reliable method to standardize the preparation of decellularized DM. 24 hours post-seeding, hESC-RPE cell attachment and initial proliferation rate over the denuded DM were higher than hESC-RPE cells cultured on tissue culture inserts. On the new matrix, hESC-RPE cells succeeded in forming an intact monolayer with mature tight junctions. The resulting cell culture showed characteristic RPE cell morphology and proper protein localization. Gene expression analysis and VEGF secretion demonstrate DM provides supportive scaffolding and inductive properties to enhance hESC-RPE cells maturation. Decellularized DM was shown to be capable of sustaining hESC-RPE cells culture, thus confirming to be potentially a suitable candidate for retinal cell therapy.

Unintentional Descemet Cleft Introduces Novel Mechanism of Maintenance of Corneal Clarity

PURPOSE

The purpose of this study was to report a case of corneal transparency for 7 months after the formation and persistence of a Descemet fluid cleft.

METHODS

We report the case of a 64-year-old woman undergoing anterior chamber reformation 2 months after Baerveldt implantation of the right eye. During the procedure, the ophthalmic viscoelastic was inadvertently injected into the posterior stroma, nearly isolating the corneal endothelial cell layer, and creating a Descemet fluid cleft filled with viscoelastic. The patient was managed conservatively and monitored near monthly for corneal decompensation.

RESULTS

The cornea remained centrally clear for 207 days after the initial anterior chamber reformation until the collapse of the viscoelastic cleft. When the cleft completely collapsed, the cornea became diffusely edematous, and the patient underwent cataract removal and intraocular lens placement without need for Descemet stripping endothelial keratoplasty to maintain vision. During these 7 months, the patient's best-corrected visual acuity remained stable at 20/25 +2 or better. Intraocular pressure was also stable, averaging 18.2 mm Hg.

CONCLUSIONS

We hypothesize that corneal transparency can be maintained in the absence of endothelial cell function provided that aqueous humor cannot reach the cornea and disrupt the arrangement of the interfibrillar space.

Two-Photon Imaging for Non-Invasive Corneal Examination

Two-photon imaging (TPI) microscopy, namely, two-photon excited fluorescence (TPEF), fluorescence lifetime imaging (FLIM), and second-harmonic generation (SHG) modalities, has emerged in the past years as a powerful tool for the examination of biological tissues. These modalities rely on different contrast mechanisms and are often used simultaneously to provide complementary information on morphology, metabolism, and structural properties of the imaged tissue. The cornea, being a transparent tissue, rich in collagen and with several cellular layers, is well-suited to be imaged by TPI microscopy. In this review, we discuss the physical principles behind TPI as well as its instrumentation. We also provide an overview of the current advances in TPI instrumentation and image analysis. We describe how TPI can be leveraged to retrieve unique information on the cornea and to complement the information provided by current clinical devices. The present state of corneal TPI is outlined. Finally, we discuss the obstacles that must be overcome and offer perspectives and outlooks to make clinical TPI of the human cornea a reality.

Novel Homozygous ADAMTS2 Variants and Associated Disease Phenotypes in Dogs with Dermatosparactic Ehlers-Danlos Syndrome

Tissue fragility, skin hyperextensibility and joint hypermobility are defining characteristics of Ehlers-Danlos syndrome (EDS). Human EDS is subclassified into fourteen types including dermatosparactic EDS, characterized by extreme skin fragility and caused by biallelic ADAMTS2 mutations. We report two novel, ADAMTS2 variants in DNA from EDS-affected dogs. Separate whole-genome sequences from a Pit Bull Terrier and an Alapaha Blue Blood Bulldog each contained a rare, homozygous variant (11:2280117delC, CanFam3.1), predicted to produce a frameshift in the transcript from the first coding ADAMTS2 exon (c.10delC) and a severely truncated protein product, p.(Pro4ArgfsTer175). The clinical features of these dogs and 4 others with the same homozygous deletion included multifocal wounds, atrophic scars, joint hypermobility, narrowed palpebral fissures, skin hyperextensibility, and joint-associated swellings. Due to severe skin fragility, the owners of all 6 dogs elected euthanasia before the dogs reached 13 weeks of age. Cross sections of collagen fibrils in post-mortem dermal tissues from 2 of these dogs showed hieroglyphic-like figures similar to those from cases of severe dermatosparaxis in other species. The whole-genome sequence from an adult Catahoula Leopard Dog contained a homozygous ADAMTS2 missense mutation, [11:2491238G>A; p.(Arg966His)]. This dog exhibited multifocal wounds, atrophic scars, and joint hypermobility, but has survived for at least 9 years. This report expands the spectrum of clinical features of the canine dermatosparactic subtype of EDS and illustrates the potential utility of subclassifying canine EDS by the identity of gene harboring the causal variant.

Collagen as a Biomaterial for Skin and Corneal Wound Healing

The cornea and the skin are two organs that form the outer barrier of the human body. When either is injured (e.g., from surgery, physical trauma, or chemical burns), wound healing is initiated to restore integrity. Many cells are activated during wound healing. In particular, fibroblasts that are stimulated often transition into repair fibroblasts or myofibroblasts that synthesize extracellular matrix (ECM) components into the wound area. Control of wound ECM deposition is critical, as a disorganized ECM can block restoration of function. One of the most abundant structural proteins in the mammalian ECM is collagen. Collagen type I is the main component in connective tissues. It can be readily obtained and purified, and short analogs have also been developed for tissue engineering applications, including modulating the wound healing response. This review discusses the effect of several current collagen implants on the stimulation of corneal and skin wound healing. These range from collagen sponges and hydrogels to films and membranes.

Single cell RNA-seq of human cornea organoids identifies cell fates of a developing immature cornea

The cornea is a protective and refractive barrier in the eye crucial for vision. Understanding the human cornea in health, disease, and cell-based treatments can be greatly advanced with cornea organoids developed in culture from induced pluripotent stem cells. While a limited number of studies have investigated the single-cell transcriptomic composition of the human cornea, its organoids have not been examined similarly. Here, we elucidated the transcriptomic cell fate map of 4-month-old human cornea organoids and human donor corneas. The organoids harbor cell clusters that resemble cells of the corneal epithelium, stroma, and endothelium, with subpopulations that capture signatures of early developmental states. Unlike the adult cornea where the largest cell population is stromal, the organoids contain large proportions of epithelial and endothelial-like cells. These corneal organoids offer a 3D model to study corneal diseases and integrated responses of different cell types.

Alginate-Based Composites for Corneal Regeneration: The Optimization of a Biomaterial to Overcome Its Limits

For many years, corneal transplantation has been the first-choice treatment for irreversible damage affecting the anterior part of the eye. However, the low number of cornea donors and cases of graft rejection highlighted the need to replace donor corneas with new biomaterials. Tissue engineering plays a fundamental role in achieving this goal through challenging research into a construct that must reflect all the properties of the cornea that are essential to ensure correct vision. In this review, the anatomy and physiology of the cornea are described to point out the main roles of the corneal layers to be compensated and all the requirements expected from the material to be manufactured. Then, a deep investigation of alginate as a suitable alternative to donor tissue was conducted. Thanks to its adaptability, transparency and low immunogenicity, alginate has emerged as a promising candidate for the realization of bioengineered materials for corneal regeneration. Chemical modifications and the blending of alginate with other functional compounds allow the control of its mechanical, degradation and cell-proliferation features, enabling it to go beyond its limits, improving its functionality in the field of corneal tissue engineering and regenerative medicine.

Fuchs endothelial corneal dystrophy: current perspectives on diagnostic pathology and genetics-Bowman Club Lecture

Fuchs endothelial corneal dystrophy (FECD) was first described over a century ago. Since then, we have learnt much about its clinical manifestations, surgical and non-surgical treatment, microscopic appearance and pathogenesis. Over the past decade, significant advances have been made with respect to our understanding of FECD genetics. This progress now enables us to appreciate that FECD in fact describes multiple entities with distinct underlying genetic causes. For example, an early-onset and rare form of the disease has been attributed to missense mutations in the COL8A2 gene, whereas the vast majority of late-onset cases can be attributed to a non-coding repeat expansion within the TCF4 gene.FECD is one of the most common indications for corneal transplantation. In recent years, attention has turned to alternative treatment techniques that do not depend on donor tissue supply. The design and development of these non-surgical treatment approaches have benefited from increased knowledge of pathogenesis.This review will cover our current knowledge about the histology and genetics of FECD, and how combining these interdisciplinary approaches might may improve diagnostic accuracy and aid the development of therapeutics for this common and visually disabling disease.

Descemet membrane endothelial keratoplasty in complex eyes

PURPOSE OF REVIEW

To review the current literature on Descemet membrane endothelial keratoplasty (DMEK) in complex eyes.

RECENT FINDINGS

DMEK surgery has become a standardized procedure in Fuchs endothelial dystrophy and simple bullous keratopathy. But eyes with more complex disease present unique intraoperative and postoperative challenges to the DMEK surgeon. Poor visualization during surgery, complex anterior segment anatomy, altered anterior chamber dynamics, glaucoma shunts, and congenital or iatrogenic missing or altered iris and lens make DMEK surgery extremely difficult to accomplish.

SUMMARY

DMEK is feasible in complex eyes, including advanced bullous keratopathy, eyes with history of glaucoma or vitreoretinal surgery, previous penetrating keratoplasty, uveitis, pediatric, and congenital anterior segment disorders. The tools and methods reported in the literature to accomplish DMEK in complex eyes vary widely with no particular consensus or standardization of techniques. The outcomes noted for some of these conditions demonstrate the difficulty of the surgery and the uncertainty of long-term graft survival in complex eyes. Both surgical standardization and randomized prospective data will better help elucidate DMEK's role in the corneal rehabilitation of complex eyes.

Tissue engineered scaffolds for corneal endothelial regeneration: a material's perspective

Currently, the treatment of corneal diseases caused by damage to the corneal endothelium requires a donor cornea. Because of their limited availability (1 donor cornea for 70 patients in need), researchers are investigating alternative approaches that are independent of donor tissue. One of them includes the development of a tissue engineered scaffold onto which corneal endothelial cells are seeded. In order to function as a suitable substrate, some of its essential properties including thickness, permeability, transparency and mechanical strength should meet certain demands. Additionally, the membrane should be biocompatible and allow the formation of a functional endothelium on the surface. Many materials have already been investigated in this regard including natural, semi-synthetic and synthetic polymers. In the current review, we present an overview of their characteristics and provide a critical view on the methods exploited for material characterization. Next, also the suitability of scaffolds to serve their purpose is discussed along with an overview of natural tissues (e.g. amniotic membrane and lens capsule) previously investigated for this application. Eventually, we propose a consistent approach to be exploited ideally for membrane characterization in future research. This will allow a scientifically sound comparison of materials and membranes investigated by different research groups, hence benefitting research towards the creation of a suitable/optimal tissue engineered endothelial graft.

Tissue-Engineered Corneal Endothelial Sheets Using Ultrathin Acellular Porcine Corneal Stroma Substrates for Endothelial Keratoplasty

Tissue-engineered cornea endothelial sheets (TECES), created using a biocompatible thin and transparent carrier with corneal endothelial cells, could alleviate the shortage of donor corneas and provide abundant functional endothelial cells. In our previous clinical trials, the effectiveness and safety of the acellular porcine corneal stroma (APCS) applied in lamellar keratoplasty have been confirmed. In this study, we optimized the method to cut APCS into multiple 20 μm ultrathin lamellae by a cryostat microtome and investigated the feasibility of TECES by seeding rabbit corneal endothelial cells (RCECs) on ultrathin APCS. Cell adhesion, proliferation, and functional gene expression of RCECs on tissue-culture plastic and APCS of different thicknesses were compared. The results indicated that ultrathin lamellae were superior in increasing cell viability and maintaining cell functions. Analyzing with histology, electron microscopy, and immunofluorescence, we found that RCECs cultured on 20 μm ultrathin APCS for 5 days grew into a confluent monolayer with a density of 3726 ± 223 cells/mm² and expressed functional biomarkers Na⁺/K⁺-ATPase and zonula occludens. After 14 days, RCECs formed an early stage of Descemet's membrane-like structure by synthesizing collagen IV and laminin. Human corneal endothelial cells were also used to further validate the supportive effect of ultrathin APCS on cells. The resulting constructs were flexible and tough enough to implant into rabbits' anterior chambers through small incisions. TECES adhered to the posterior corneal stroma, and the thickness of cornea gradually reduced to normal after grafting. These results indicate that the ultrathin APCS can serve as a tissue engineering carrier and might be a suitable alternative for endothelial cells expansion in endothelial keratoplasty.

Endophthalmitis secondary to donor to host infection in posterior lamellar keratoplasties: A systematic review

Donor to host transmission of infectious agents is a well-recognized entity, more commonly related to Endothelial Keratoplasty (EK) than to Penetrating Keratoplasty (PK), that involves complications ranging from interface keratitis to endophthalmitis. A systematic review of the literature was conducted to identify the published articles until December 2020 reporting cases of endophthalmitis secondary to corneal graft contamination in posterior lamellar keratoplasties (DSAEK and DMEK) along with donor characteristics, microbiological profile, treatment and outcomes. Each identified article was assessed to meet donor to host infection criteria, defined as a post-procedural infection in which the same microbiological agent was identified in both the donor corneoscleral rim or preservation medium and receptor ocular tissue. From 23 research articles, eight reports of endophthalmitis in nine patients following DSAEK or DMEK secondary to donor to host infection fulfilled the inclusion criteria. The majority were male and the median age was 72.0 (45.0-81.0) years old. Indications of surgery were primarily pseudophakic bullous keratopathy and Fuchs dystrophy. A fungal pathogen was identified in eight of nine patients. All the cases underwent surgical management with lenticule removal or endothelial plaque aspiration. The final corrected distance visual acuity (CDVA) in all cases was 20/200 or better. Endophthalmitis after an EK procedure is a rare complication whose outcome depends on the aggressive and precocious treatment. Identification of early signs of interface keratitis and lenticule removal seems mandatory in patients undergoing DSAEK or DMEK to prevent further involvement of the globe.

[Morphological features of the cornea in eyes with pseudophakic bullous keratopathy and Fuchs endothelial corneal dystrophy]

PURPOSE

To evaluate the morphological features of posterior corneal layers in pseudophakic bullous keratopathy (PBK) and Fuchs endothelial corneal dystrophy (FECD).

MATERIAL AND METHOD

The study included 20 patients (20 eyes) aged 73.8±7.8 with PBK and 24 patients (25 eyes) aged 72.5±10.6 with FECD. Central corneal thickness was measured in all eyes before the surgery by anterior segment optical coherence tomography (AS-OCT, RTVue-100, Optovue, USA). All patients underwent corneal transplantation (DSAEK or DMEK). Postoperative donor graft status was assessed as attached or detached by AS-OCT (RTVue-100, Optovue, USA). Specimens of Descemet's membranes (DMs) were obtained intraoperatively and investigated histologically (hematoxylin and eosin staining). Morphometric analysis was performed using optical microscope Leica DM-2500 (Digital Color Camera Leica DFC295) and ImageScope M software.

RESULTS

There was no difference in thickness of DMs' prenatal anterior banded layer (2.58±0.49 μm) and postnatal posterior non-banded layer (11.30±2.64 μm) between PBK and normal cornea. In FECD eyes, DM was significantly thickened by additional collagen layer and guttate excrescences (postnatal layer thickness - 16.62±4.34 μm). Morphological study of the DMs of patients with graft detachment in the early postoperative period revealed residual stromal collagen fibrils that were fixed to the area with dense extracellular matrix.

CONCLUSION

The study shows that chronic corneal edema is associated with pathological morphological changes in the posterior corneal layers in PBK and FECD. Individual features of the dense extracellular matrix in the DMs may play a key role in the attachment of donor graft to the corneal stroma and affect the postoperative outcome.