Biomechanical relationships between the corneal endothelium and Descemet's membrane

Corneal endothelial cell morphology in children with autosomal recessive Alport syndrome: a longitudinal study

PURPOSE

To evaluate the corneal endothelial cell morphology in children with autosomal recessive Alport syndrome (ARAS).

METHODS

This is a longitudinal, prospective cohort study that evaluated pediatric patients with genetically diagnosed ARAS. Fifty-eight eyes of 29 pediatric patients (12 patients, 17 controls) underwent a full ophthalmic examination. Corneal endothelial cell density (ECD) (cells/mm²), coefficient variation (CV) of cell area (polymegathism), the percentage of hexagonal cells (HEX) (pleomorphism), and central corneal thickness (CCT) were analyzed automatically using a noncontact specular microscopy.

RESULTS

The mean ECD was 2904 ± 355.48 cell/mm² in the ARAS group and 3263.20 ± 261.71 cell/mm² in the control group (p = 0.004). In the ARAS group, the mean CV was 46.53 ± 10.43, which was significantly higher than that in controls (p = 0.026). The mean HEX was 48.86 ± 14.71 in the ARAS group and 59.06 ± 10.64 in the control group (p = 0.038). The mean CCT was 565.26 ± 39.77 µm in the ARAS group and 579.66 ± 31.65 µm in the control group (p = 0.282). The comparison of endothelial cell characteristic of the ARAS group with 1-year follow-up is as follows: The mean ECD decreased from 2904 ± 355.48 cell/mm² to 2735 ± 241.58 cell/mm² (p = 0.003). The mean CV increased from 46.53 ± 10.43 to 47.93 ± 10.50 (p = 0.471). The mean HEX decreased from 48.86 ± 14.71 to 48.50 ± 10.06 (p = 0.916). The mean CCT decreased from 565.26 ± 39.77 µm to 542.86 ± 40.39 µm (p = 0.000).

CONCLUSION

Measurement of ECD and percentage of hexagonality can also be used as an indicator of the health of the corneal endothelium. In this study, the mean ECD and HEX were significantly lower in ARAS group than in age-matched pediatric controls. Polymegathism, which reflects cellular stress, was statistically significantly higher in ARAS group. The mean ECD and CCT decreased significantly at 1-year follow-up. This study may demostrated that endothelial damages and stress in ARAS patients appear in childhood and show a rapid increase with age.

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.

How Would Nature See Our Corneal Triumphs? The LXXIX Edward Jackson Lecture

PURPOSE

To describe discrepancies between clinical observation and current teachings in corneal endothelial disease, particularly in Fuchs endothelial dystrophy and its potential association with primary open angle glaucoma.

DESIGN

Perspective.

METHODS

A perspective is presented on Fuchs dystrophy, a disorder that commonly presents with a compromised endothelium but minimal stromal edema, indicating that the corneal imbibition pressure is relatively "too high."

RESULTS

The discrepancy between the relative lack of stromal edema in the absence of an endothelial cell layer cannot be explained by the current theories involving a circulatory pumping mechanism over the endothelial cell layer, but may point to the following: (1) secondary involvement of the corneal endothelium in Fuchs dystrophy; (2) separate hydration systems for maintaining the imbibition pressure (vertical static hydration) and corneal nutrition (horizontal dynamic hydration); (3) the cornea as net contributor of aqueous humor; (4) a close relationship between the corneal imbibition and intraocular pressure, with potentially a shared regulatory system; and (5) a potential steroid-type hormone dependency of this regulatory system.

CONCLUSIONS

Clinical observation shows that the stromal imbibition pressure is "too high" in Fuchs endothelial dystrophy, indicating that it may not primarily be an endothelial disease, but a type of "corneal glaucoma."

Type II Diabetes Mellitus Causes Extracellular Matrix Alterations in the Posterior Cornea That Increase Graft Thickness and Rigidity

Purpose

There is a pressing need to investigate the impact of type II diabetes mellitus on the posterior cornea in donor tissues given its increasing prevalence and potential impact on endothelial keratoplasty surgical outcomes.

Methods

Immortalized human cultured corneal endothelial cells (CECs; HCEC-B4G12) were grown in hyperglycemic media for 2 weeks. Extracellular matrix (ECM) adhesive glycoprotein expression and advanced glycation end products (AGEs) in cultured cells and corneoscleral donor tissues, as well as the elastic modulus for the Descemet membrane (DMs) and CECs of diabetic and nondiabetic donor corneas, were measured.

Results

In CEC cultures, increasing hyperglycemia resulted in increased transforming growth factor beta-induced (TGFBI) protein expression and colocalization with AGEs in the ECM. In donor corneas, the thicknesses of the DM and the interfacial matrix (IFM) between the DM and stroma both increased from 8.42 ± 1.35 µm and 0.504 ± 0.13 µm in normal corneas, respectively, to 11.13 ± 2.91 µm (DM) and 0.681 ± 0.24 µm (IFM) in non-advanced diabetes (P = 0.013 and P = 0.075, respectively) and 11.31 ± 1.76 µm (DM) and 0.744 ± 0.18 µm (IFM) in advanced diabetes (AD; P = 0.0002 and P = 0.003, respectively). Immunofluorescence in AD tissues versus controls showed increased AGEs (P < 0.001) and markedly increased labeling intensity for adhesive glycoproteins, including TGFBI, that colocalized with AGEs. The elastic modulus significantly increased between AD and control tissues for the DMs (P < 0.0001) and CECs (P < 0.0001).

Conclusions

Diabetes and hyperglycemia alter human CEC ECM structure and composition, likely contributing to previously documented complications of endothelial keratoplasty using diabetic donor tissue, including tearing during graft preparation and reduced graft survival. AGE accumulation in the DM and IFM may be a useful biomarker for determining diabetic impact on posterior corneal tissue.

Modified gellan gum-based hydrogel with enhanced mechanical properties for application as a cell carrier for cornea endothelial cells

Recently, the number of people suffering from visual loss due to eye diseases is increasing rapidly around the world. However, due to the severe donor shortage and the immune response, corneal replacement is needed. Gellan gum (GG) is biocompatible and widely used for cell delivery or drug delivery, but its strength is not suitable for the corneal substitute. In this study, a GM hydrogel was prepared by blending methacrylated gellan gum with GG (GM) to give suitable mechanical properties to the corneal tissue. In addition, lithium phenyl-2,4,6-trimethylbenzoylphosphinate (LAP), a crosslinking initiator, was added to the GM hydrogel. After the photo-crosslinking treatment, it was named GM/LAP hydrogel. GM and GM/LAP hydrogels were analyzed for physicochemical properties, mechanical characterization, and transparency tests to confirm their applicability as carriers for corneal endothelial cells (CEnCs). Also, in vitro studies were performed with cell viability tests, cell proliferation tests, cell morphology, cell-matrix remodeling analysis, and gene expression evaluation. The compressive strength of the GM/LAP hydrogel was improved compared to the GM hydrogel. The GM/LAP hydrogel showed excellent cell viability, proliferation, and cornea-specific gene expression than the GM hydrogel. Crosslinking-improved GM/LAP hydrogel can be applied as a promising cell carrier in corneal tissue engineering.

GSK-3 inhibition reverts mesenchymal transition in primary human corneal endothelial cells

Human corneal endothelial cells are organized in a tight mosaic of hexagonal cells and serve a critical function in maintaining corneal hydration and clear vision. Regeneration of the corneal endothelial tissue is hampered by its poor proliferative capacity, which is partially retrieved in vitro, albeit only for a limited number of passages before the cells undergo mesenchymal transition (EnMT). Although different culture conditions have been proposed in order to delay this process and prolong the number of cell passages, EnMT has still not been fully understood and successfully counteracted. In this perspective, we identified herein a single GSK-3 inhibitor, CHIR99021, able to revert and avoid EnMT in primary human corneal endothelial cells (HCEnCs) from old donors until late passages in vitro (P8), as shown from cell morphology analysis (circularity). In accordance, CHIR99021 reduced expression of α-SMA, an EnMT marker, while restored endothelial markers such as ZO-1, Na⁺/K⁺ ATPase and N-cadherin, without increasing cell proliferation. A further analysis on RNA expression confirmed that CHIR99021 induced downregulation of EnMT markers (α-SMA and CD44), upregulation of the proliferation repressor p21 and revealed novel insights into the β-catenin and TGFβ pathways intersections in HCEnCs. The use of CHIR99021 sheds light on the mechanisms involved in EnMT, providing a substantial advantage in maintaining primary HCEnCs in culture until late passages, while preserving the correct morphology and phenotype. Altogether, these results bring crucial advancements towards the improvement of the corneal endothelial cells based therapy.

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.

The soil and the seed: The relationship between Descemet's membrane and the corneal endothelium

Descemet's membrane (DM), the basement membrane of the corneal endothelium, is formed from the extracellular matrix (ECM) secreted by corneal endothelial cells. The ECM supports the growth and function of the corneal endothelial cells. Changes to DM are central to the diagnosis of the most common corneal endothelial disease, Fuchs endothelial corneal dystrophy (FECD). Changes in DM are also noted in systemic diseases such as diabetes mellitus. In FECD, the DM progressively accumulates guttae, "drop-like deposits" that disrupt the corneal endothelial cell monolayer. While the pathophysiologic changes to corneal endothelial cells in the course of FECD have been well described and reviewed, the changes to DM have received limited attention. The reciprocity of influence between the corneal endothelial cells and DM demands full attention to the latter in our search for novel treatment and preventive strategies. In this review, we discuss what is known about the formation and composition of DM and how it changes in FECD and other conditions. We review characteristics of guttae and the interplay between corneal endothelial cells and guttae, particularly as it might apply to future cell-based and genetic therapies for FECD.

Cell derived matrices from bovine corneal endothelial cells as a model to study cellular dysfunction

PURPOSE

Fuchs endothelial corneal dystrophy (FECD) is a progressive corneal disease that impacts the structure and stiffness of the Descemet's membrane (DM), the substratum for corneal endothelial cells (CECs). These structural alterations of the DM could contribute to the loss of the CECs resulting in corneal edema and blindness. Oxidative stress and transforming growth factor-β (TGF-β) pathways have been implicated in endothelial cell loss and endothelial to mesenchymal transition of CECs in FECD. Ascorbic acid (AA) is found at high concentrations in FECD and its impact on CEC survival has been investigated. However, how TGF-β and AA effect the composition and rigidity of the CEC's matrix remains unknown.

METHODS

In this study, we investigated the effect of AA, TGF-β1 and TGF-β3 on the deposition, ultrastructure, stiffness, and composition of the extracellular matrix (ECM) secreted by primary bovine corneal endothelial cells (BCECs).

RESULTS

Immunofluorescence and electron microscopy post-decellularization demonstrated a robust deposition and distinct structure of ECM in response to treatments. AFM measurements showed that the modulus of the matrix in BCECs treated with TGF-β1 and TGF-β3 was significantly lower than the controls. There was no difference in the stiffness of the matrix between the AA-treated cell and controls. Gene Ontology analysis of the proteomics results revealed that AA modulates the oxidative stress pathway in the matrix while TGF-β induces the expression of matrix proteins collagen IV, laminin, and lysyl oxidase homolog 1.

CONCLUSIONS

Molecular pathways identified in this study demonstrate the differential role of soluble factors in the pathogenesis of FECD.

Late Descemet membrane detachment after uneventful cataract surgery

Purpose

To report 5 patients with late Descemet membrane (DM) detachment after uneventful cataract surgery.

Observations

After a retrospective chart analysis of consecutive patients that developed DM detachment after uneventful cataract surgery, six eyes of five patients were enrolled. In all cases, patients reported good vision initially after cataract surgery. Within days to months, these patients developed late DM detachment with decreased vision. In one patient, the detachment affected both eyes. Filtered air or diluted sulfur hexafluoride were injected in the anterior chamber to tamponade the DM detachment. In five eyes, the cornea cleared after DM reattachment. In two eyes of the same patient, DM reattached spontaneously requiring no further surgical intervention. In one patient, the Descemet failed to reattach and required an endothelial keratoplasty.

Conclusion and importance

Descemet membrane detachment may occur after uneventful cataract surgery. Filtered air or long-lasting intraocular gas may be used to reattach DM. Spontaneous DM reattachment may also occur and surgeons should be aware of this to avoid unnecessary procedures.

Effect of Collagenase A on Descemet Membrane Endothelial Keratoplasty Scroll Tightness

PURPOSE

The scrolling properties of the Descemet membrane endothelial keratoplasty (DMEK) graft are essential for surgical success. Currently, there is limited knowledge on what dictates the tightness of the DMEK scroll. The purpose of this study was to determine the impact of temperature and protein digestion on DMEK graft scroll tightness.

METHODS

For the temperature experiment, a total of 28 eyes were used for this study. Scrolls in the cold group were kept at 4°C while scrolls in the hot group were kept at 37°C. Scroll width was recorded at the 5-, 15-, and 30-minute mark. For the protein digestion experiment, a total of 18 eyes were exposed to collagenase A (10 CDU/mL) in Optisol solution. Scroll width was recorded at the time points of 1, 3, 5, 10, and 20 minutes.

RESULTS

The results of the temperature experiment did not yield any statistically significant changes in the mean scroll width of the DMEK scrolls across both temperature ranges and observation times. For the protein digestion experiment, the mean scroll width grew from 1.85 mm to 2.13 mm from the beginning of the experiment until the final observation at 20 minutes. This is a 14.7% change over 20 minutes with a P value (<0.001), exemplifying a statistically significant change in scroll width.

CONCLUSIONS

Temperature did not have any significant effect over scroll tightness, but scroll tightness decreased with collagenase exposure.

Design of functional biomaterials as substrates for corneal endothelium tissue engineering

Corneal endothelium defects are one of the leading causes of blindness worldwide. The actual treatment is transplantation, which requires the use of human cadaveric donors, but it faces several problems, such as global shortage of donors. Therefore, new alternatives are being developed and, among them, cell therapy has gained interest in the last years due to its promising results in tissue regeneration. Nevertheless, the direct administration of cells may sometimes have limited success due to the immune response, hence requiring the combination with extracellular mimicking materials. In this review, we present different methods to obtain corneal endothelial cells from diverse cell sources such as pluripotent or multipotent stem cells. Moreover, we discuss different substrates in order to allow a correct implantation as a cell sheet and to promote an enhanced cell behavior. For this reason, natural or synthetic matrixes that mimic the native environment have been developed. These matrixes have been optimized in terms of their physicochemical properties, such as stiffness, topography, composition and transparency. To further enhance the matrixes properties, these can be tuned by incorporating certain molecules that can be delivered in a sustained manner in order to enhance biological behavior. Finally, we elucidate future directions for corneal endothelial regeneration, such as 3D printing, in order to obtain patient-specific substrates.

Eye bank versus surgeon prepared Descemet stripping automated endothelial keratoplasty tissues: Influence on adhesion force in a pilot study

PURPOSE

To evaluate and compare the biomechanical properties of the eye bank-prepared and surgeon prepared Descemet stripping automated endothelial keratoplasty (DSAEK) tissues.

METHODS

In this laboratory study, corneal tissues for research were randomly allocated in the following groups: a) surgeon-cut DSAEK and b) eye bank-prepared (pre-cut and pre-loaded) DSAEK. Endothelial cell loss (ECL), immunostaining for tight junction protein ZO-1, elastic modulus, and adhesion force were investigated.

RESULTS

ECL was not found to be significantly different between surgeon-cut DSAEK (7.8% ±6.5%), pre-cut DSAEK (8.6% ±2.3%), and pre-loaded DSAEK (11.1% ±4.8%) (P = 0.5910). ZO-1 was expressed equally across all groups. Surgeon-cut DSAEK grafts showed a significantly higher elastic modulus compared to pre-cut and pre-loaded DSAEK groups (P = 0.0047 and P < 0.0001, respectively). Adhesion force was significantly greater in the surgeon-cut DSAEK compared to pre-cut (P < 0.0001) or pre-loaded DSAEK groups (P = 0.0101).

CONCLUSION

The laboratory data on the biomechanics of DSAEK grafts suggests that surgeon-cut DSAEK grafts present higher elastic modulus and adhesion force compared to eye bank-prepared DSAEK grafts.

Fourier-Domain Optical Coherence Tomographic Assessment of Changes in the Schlemm's Canal of Nonglaucomatous Subjects After Keratoplasty

Purpose: This study aimed to evaluate the impact of keratoplasty on the in vivo anatomical structures in the Schlemm's canal (SC) of nonglaucomatous subjects using Fourier-domain optical coherence tomography (FD-OCT). Methods: Sixty-six nonglaucomatous eyes that underwent penetrating keratoplasty (PK), deep anterior lamellar keratoplasty (DALK), or triple surgery were enrolled in this prospective, comparative, observational study. The SC imaging was performed using FD-OCT before and after surgery in both the nasal and temporal quadrants. Patient demographics, SC parameters [e.g., cross-sectional area (CSA), meridional diameter of SC (MSC), sagittal diameter of SC (SSC), and circumference (CCF)], and the correlations between the variation of SC parameters and intraocular pressure (IOP) were analyzed. Results: The mean age of all subjects was 40.27 ± 18.97 years. Among all cases, the nasal, temporal, and mean MSC significantly decreased on the first day after surgery and then increased at 1 week (p = 0.04, 0.017, and 0.01, respectively). Temporal CSA (tCSA), temporal MSC (tMSC), and temporal circumference (tCCF) after PK (p = 0.017, 0.020, and 0.018, respectively) and nasal MSC (nMSC) after DALK (p = 0.025) decreased significantly on the first day after surgery. The shift in mean IOP was significantly correlated with the changes in tMSC (r = 0.341, p = 0.003) and CCF (r = 0.207, p = 0.048). Conclusion: SC had significant in vivo morphological changes in the early period after keratoplasty in nonglaucomatous eyes, accompanied with elevation of IOP. Early intervention might be necessary to prevent secondary glaucoma early after keratoplasty.

The Response of Corneal Endothelial Cells to Shear Stress in an In Vitro Flow Model

Purpose

Corneal endothelial cells are usually exposed to shear stress caused by the aqueous humour, which is similar to the exposure of vascular endothelial cells to shear stress caused by blood flow. However, the effect of fluid shear stress on corneal endothelial cells is still poorly understood. The purpose of this study was to explore whether the shear stress that results from the aqueous humour influences corneal endothelial cells.

Methods

An in vitro model was established to generate fluid flow on cells, and the effect of fluid flow on corneal endothelial cells after exposure to two levels of shear stress for different durations was investigated. The mRNA and protein expression of corneal endothelium-related markers in rabbit corneal endothelial cells was evaluated by real-time PCR and western blotting.

Results

The expression of the corneal endothelium-related markers ZO-1, N-cadherin, and Na⁺-K⁺-ATPase in rabbit corneal endothelial cells (RCECs) was upregulated at both the mRNA and protein levels after exposure to shear stress.

Conclusion

This study demonstrates that RCECs respond favourably to fluid shear stress, which may contribute to the maintenance of corneal endothelial cell function. Furthermore, this study also provides a theoretical foundation for further investigating the response of human corneal endothelial cells to the shear stress caused by the aqueous humour.

Animal models of corneal endothelial dysfunction to facilitate development of novel therapies

Progressive corneal endothelial disease eventually leads to corneal edema and vision loss due to the limited regenerative capacity of the corneal endothelium in vivo and is a major indication for corneal transplantation. Despite the relatively high success rate of corneal transplantation, there remains a pressing global clinical need to identify improved therapeutic strategies to address this debilitating condition. To evaluate the safety and efficacy of novel therapeutics, there is a growing demand for pre-clinical animal models of corneal endothelial dysfunction. In this review, experimentally induced, spontaneously occurring and genetically modified animal models of corneal endothelial dysfunction are described to assist researchers in making informed decisions regarding the selection of the most appropriate animal models to meet their research goals.

The impact of biomechanics on corneal endothelium tissue engineering

The integrity of innermost layer of the cornea, the corneal endothelium, is key to sustaining corneal transparency. Therefore, disease or injury causing loss or damage to the corneal endothelial cell population may threaten vision. Transplantation of corneal tissue is the standard treatment used to replace malfunctioning corneal endothelial cells. However, this surgery is dependent upon donor tissue, which is limited in supply. Hence, tissue engineers have attempted to construct alternative transplantable tissues or cell therapies to alleviate this problem. Nevertheless, the intrinsic non-dividing nature of corneal endothelial cells continues to foil scientists in their attempts to yield large numbers of cells in the laboratory for use in such novel therapies. Interestingly, the contribution of the biomechanical properties of the underlying extracellular matrix (ECM) on cell division, tissue development and maintenance has been extensively investigated in other many cell types. However, the impact of biomechanics on corneal endothelial cell behaviour is relatively unexplored. Here, we describe contemporary tissue engineering solutions aimed at circumventing donor tissue scarcity. We review the ECM structure and biomechanical features of corneal endothelial cells. We discuss the alterations of ECM in endothelial disease development and progression and point out the role of ECM in developing a tissue-engineered corneal endothelium. We highlight the main biomechanical cues, including topographical and mechanical features, that impact cellular behaviors. Finally, we discuss the influence of biomechanical cues on cell and tissue development, and how corneal endothelial cells response to individual biomechanical stimuli in tissue engineering, which have implications for designing an engineered endothelium and maintaining cell function.

A Retrospective Study of Corneal Endothelial Dystrophy in Dogs (1991-2014)

PURPOSE

To retrospectively evaluate the clinical data, diagnostic tests, treatments, and outcomes for dogs with corneal endothelial dystrophy (CED) and determine risk factors for CED when compared with a canine reference population.

METHODS

Medical records of 99 dogs (1991-2014) diagnosed with CED at the University of California Davis Veterinary Medical Teaching Hospital were reviewed and compared with 458,680 dogs comprising the general hospital population during the study period. Retrieved data included signalment, examination findings, diagnoses, treatments, and outcomes associated with CED. The exact Pearson χ2 test or exact Kruskal-Wallis test was used to compare parameters between the groups. Progression of corneal edema was assessed using 3 independent Kaplan-Meier curves, identifying clinically significant changes in corneal opacity.

RESULTS

Boston terriers, German wirehaired pointers, and Dachshunds were overrepresented in the CED-affected group, whereas Labradors were underrepresented. Dogs older than 11 years were overrepresented in the CED-affected group, whereas intact dogs were underrepresented. Surgical intervention was performed (n = 11) based on the severity of disease and secondary complications from CED. Median time to progression of corneal edema was 1) 368 days when an at-risk eye initially without edema developed edema at a subsequent visit, 2) 701 days when there was progression from mild to marked corneal edema, and 3) 340 days when there was progression from focal to diffuse corneal edema.

CONCLUSIONS

Many CED-affected dogs progress over months to years without surgical intervention, making dogs with CED a useful model for studying genetic predispositions and development of novel therapeutics for Fuchs endothelial corneal dystrophy.

COL8A2 Regulates the Fate of Corneal Endothelial Cells

Purpose

To investigate the effect of COL8A2 repression on corneal endothelial cells (CECs) in vitro and in vivo.

Methods

Cultured human CECs (hCECs) were transfected with COL8A2 siRNA (siCOL8A2), and the cell viability and proliferation rate were measured. The expression of cell proliferation–associated molecules was evaluated by Western blotting and real-time reverse transcription PCR. Cell shape, Wingless-INT (WNT) signaling, and mitochondrial oxidative stress were also measured. For in vivo experiments, siCOL8A2 was transfected into rat CECs (rCECs), and corneal opacity and corneal endothelium were evaluated.

Results

After transfection with siCOL8A2, COL8A2 expression was reduced (80%). Cell viability, cell proliferation rate, cyclin D1 expression, and the number of cells in the S-phase were reduced in siCOL8A2-treated cells. The cell attained a fibroblast-like shape, and SNAI1, pSMAD2, and β-catenin expression, along with mitochondrial mass and oxidative stress levels, were altered. Corneal opacity increased, and the CECs were changed in rats in the siCOL8A2 group.

Conclusions

COL8A2 is required to maintain normal wound healing and CEC function.

Nanoscale Topographies for Corneal Endothelial Regeneration

The corneal endothelium is the innermost layer of the cornea that selectively pumps ions and metabolites and regulates the hydration level of the cornea, ensuring its transparency. Trauma or disease affecting human corneal endothelial cells (hCECs) can result in major imbalances of such transport activity with consequent deterioration or loss of vision. Since tissue transplantation from deceased donors is only available to a fraction of patients worldwide, alternative solutions are urgently needed. Cell therapy approaches, in particular by attempting to expand primary culture of hCECs in vitro, aim to tackle this issue. However, existing cell culture protocols result in limited expansion of this cell type. Recent studies in this field have shown that topographical features with specific dimensions and shapes could improve the efficacy of hCEC expansion. Therefore, potential solutions to overcome the limitation of the conventional culture of hCECs may include recreating nanometer scale topographies (nanotopographies) that mimic essential biophysical cues present in their native environment. In this review, we summarize the current knowledge and understanding of the effect of substrate topographies on the response of hCECs. Moreover, we also review the latest developments for the nanofabrication of such bio-instructive cell substrates.

Development of Animal Models for Lens and Corneal Diseases Using N-Methyl-N-Nitrosourea

Purpose

N-methyl-N-nitrosourea (MNU) is an alkylating toxicant with potent mutagenic ability. This study was designed to induce apoptosis in lens epithelial cells (LECs) and corneal endothelial cells (CECs) via MNU administration. We sought to build ocular disease models of cataract and corneal endothelial decompensation.

Methods

MNU was delivered into the intraperitoneal cavities of neonatal rats and the anterior chambers of adult rabbits. The MNU-treated animals were then subjected to a series of functional and morphological analyses at various time points.

Results

MNU treatment induced pervasive apoptosis of LECs and CECs. These effects were dose and time dependent. Mature cataracts were found in neonatal rats 3 weeks after MNU treatment. Histological analysis revealed that MNU toxicity induced swelling, vacuolation, and liquefaction in lens fibers of MNU-treated rats. Pentacam examination showed that the average density of rat lens increased significantly after MNU administration. Terminal deoxynucleotidyl transferase-mediated nick end labeling (TUNEL) analysis showed pervasive apoptotic staining in the lenses of MNU-treated rats. In rabbit eyes, intracameral treatment with MNU induced corneal edema and significantly increased central corneal thickness, which peaked at P14. Morphological and immunohistochemical analysis showed that CECs were effectively ablated in the MNU-treated rabbits. The expression of 8-OHdG increased significantly in the cornea of MNU-treated rabbits, compared with vehicle-treated controls.

Conclusions

MNU is sufficient to induce ocular cell apoptosis in animal models. These models of MNU-induced cataract and corneal endothelial decompensation represent valuable tools for efforts to develop relevant therapies.

Fuchs endothelial corneal dystrophy: The vicious cycle of Fuchs pathogenesis

Fuchs endothelial corneal dystrophy (FECD) is the most common primary corneal endothelial dystrophy and the leading indication for corneal transplantation worldwide. FECD is characterized by the progressive decline of corneal endothelial cells (CECs) and the formation of extracellular matrix (ECM) excrescences in Descemet's membrane (DM), called guttae, that lead to corneal edema and loss of vision. FECD typically manifests in the fifth decades of life and has a greater incidence in women. FECD is a complex and heterogeneous genetic disease where interaction between genetic and environmental factors results in cellular apoptosis and aberrant ECM deposition. In this review, we will discuss a complex interplay of genetic, epigenetic, and exogenous factors in inciting oxidative stress, auto(mito)phagy, unfolded protein response, and mitochondrial dysfunction during CEC degeneration. Specifically, we explore the factors that influence cellular fate to undergo apoptosis, senescence, and endothelial-to-mesenchymal transition. These findings will highlight the importance of abnormal CEC-DM interactions in triggering the vicious cycle of FECD pathogenesis. We will also review clinical characteristics, diagnostic tools, and current medical and surgical management options for FECD patients. These new paradigms in FECD pathogenesis present an opportunity to develop novel therapeutics for the treatment of FECD.

The human Descemet's membrane and lens capsule: Protein composition and biomechanical properties

The Descemet's membrane (DM) and the lens capsule (LC) are two ocular basement membranes (BMs) that are essential in maintaining stability and structure of the cornea and lens. In this study, we investigated the proteomes and biomechanical properties of these two materials to uncover common and unique properties. We also screened for possible protein changes during diabetes. LC-MS/MS was used to determine the proteomes of both BMs. Biomechanical measurements were conducted by atomic force microscopy (AFM) in force spectroscopy mode, and complemented with immunofluorescence microscopy. Proteome analysis showed that all six existing collagen IV chains represent 70% of all LC-protein, and are thus the dominant components of the LC. The DM on the other hand is predominantly composed of a single protein, TGF-induced protein, which accounted for around 50% of all DM-protein. Four collagen IV-family members in DM accounted for only 10% of the DM protein. Unlike the retinal vascular BMs, the LC and DM do not undergo significant changes in their protein compositions during diabetes. Nanomechanical measurements showed that the endothelial/epithelial sides of both BMs are stiffer than their respective stromal/anterior-chamber sides, and both endothelial and stromal sides of the DM were stiffer than the epithelial and anterior-chamber sides of the LC. Long-term diabetes did not change the stiffness of the DM and LC. In summary, our analyses show that the protein composition and biomechanical properties of the DM and LC are different, i.e., the LC is softer than DM despite a significantly higher concentration of collagen IV family members. This finding is unexpected, as collagen IV members are presumed to be responsible for BM stiffness. Diabetes had no significant effect on the protein composition and the biomechanical properties of both the DM and LC.

Corneal endothelium tissue engineering: An evolution of signaling molecules, cells, and scaffolds toward 3D bioprinting and cell sheets

Cornea is an avascular and transparent tissue that focuses light on retina. Cornea is supported by the corneal-endothelial layer through regulation of hydration homeostasis. Restoring vision in patients afflicted with corneal endothelium dysfunction-mediated blindness most often requires corneal transplantation (CT), which faces considerable constrictions due to donor limitations. An emerging alternative to CT is corneal endothelium tissue engineering (CETE), which involves utilizing scaffold-based methods and scaffold-free strategies. The innovative scaffold-free method is cell sheet engineering, which typically generates cell layers surrounded by an intact extracellular matrix, exhibiting tunable release from the stimuli-responsive surface. In some studies, scaffold-based or scaffold-free technologies have been reported to achieve promising outcomes. However, yet some issues exist in translating CETE from bench to clinical practice. In this review, we compare different corneal endothelium regeneration methods and elaborate on the application of multiple cell types (stem cells, corneal endothelial cells, and endothelial precursors), signaling molecules (growth factors, cytokines, chemical compounds, and small RNAs), and natural and synthetic scaffolds for CETE. Furthermore, we discuss the importance of three-dimensional bioprinting strategies and simulation of Descemet's membrane by biomimetic topography. Finally, we dissected the recent advances, applications, and prospects of cell sheet engineering for CETE.

Ex vivo excimer laser ablation of cornea guttata and ROCK inhibitor-aided endothelial recolonization of ablated central cornea

PURPOSE

To determine whether excimer laser ablation of guttae is a viable strategy for removal of diseased tissue in Fuchs' endothelial corneal dystrophy (FECD) on excised human Descemet membranes and whether an excimer laser-created wound on healthy human corneas ex vivo is recolonized with corneal endothelial cells.

METHODS

Descemet membranes of FECD patients and corneal endothelium of normal human corneas were ablated ex vivo using an excimer laser licensed for glaucoma surgery. Specimens were kept in cell culture medium supplemented with 10 μm of rho-kinase inhibitor ripasudil. Corneal endothelial cell regeneration was observed using light and electron scanning microscopy. Furthermore, the whole corneal samples were evaluated by haematoxylin/eosin staining and immunohistochemical analysis using antibodies against Na⁺ /K⁺ -ATPase.

RESULTS

Guttae and corneal endothelium could be ablated with an excimer laser without total ultrastructural damage to the Descemet membrane or stroma. Nearly complete endothelial wound closure was accomplished after 26-38 days in treated corneas. Light and electron scanning microscopy suggested the establishment of a layer of flat endothelial cells. Additionally, Na⁺ /K⁺ -ATPase expression could only be observed on the inner side of the Descemet membrane.

CONCLUSION

Our proof of concept study demonstrated that excimer lasers can be used to ablate diseased tissue from excised FECD Descemet membranes ex vivo. Additionally, corneal endothelial cells recolonize a previously ablated endothelial area in healthy human corneas ex vivo under treatment with ripasudil. Thus, our results are the first experimental basis to further investigate the feasibility of an excimer laser ablation as a graftless FECD treatment option.

A fine-tuned β-catenin regulation during proliferation of corneal endothelial cells revealed using proteomics analysis

Corneal endothelial (CE) dysfunction is the main indication for corneal transplantation, an invasive procedure with several limitations. Developing novel strategies to re-activate CE regenerative capacity is, therefore, of fundamental importance. This goal has proved to be challenging as corneal endothelial cells (CEnC) are blocked in the G0/G1 phase of the cell cycle in vivo and, albeit retaining proliferative capacity in vitro, this is further hindered by endothelial-to-mesenchymal transition. Herein we investigated the mechanisms regulating CEnC proliferation in vitro. Comparing the proteome of non-proliferating (in vivo-G0/G1) and proliferating (in vitro-G2/M) rabbit CEnC (rCEnC), 77 proteins, out of 3,328 identified, were differentially expressed in the two groups (p < 0.005). Literature and Gene Ontology analysis revealed β-catenin and transforming growth factor (TGF-β) pathways to be correlated with the identified proteins. Treatment of rCEnC with a β-catenin activator and inhibitor showed that β-catenin activation was necessary during rCEnC proliferation, but not sufficient for its induction. Furthermore, both pro-proliferative activity of basic fibroblast growth factor and anti-proliferative effects of TGF-β were regulated through β-catenin. Overall, these results provide novel insights into the molecular basis underlying the proliferation process that CEnC re-activate in vitro, consolidating the role of β-catenin and TGF-β.

Integrin: Basement membrane adhesion by corneal epithelial and endothelial cells

Integrins mediate adhesion of cells to substrates and maintain tissue integrity by facilitating mechanotransduction between cells, the extracellular matrix, and gene expression in the nucleus. Changes in integrin expression in corneal epithelial cells and corneal endothelial cells impacts their adhesion to the epithelial basement membrane (EpBM) and Descemet's membrane, respectively. Integrins also play roles in assembly of basement membranes by both activating TGFβ1 and other growth factors. Over the past two decades, this knowledge has been translated into methods to grow corneal epithelial and endothelial cells in vitro for transplantation in the clinic thereby transforming clinical practice and quality of life for patients. Current knowledge on the expression and function of the integrins that mediate adhesion to the basement membrane expressed by corneal epithelial and endothelial cells in health and disease is summarized. This is the first review to discuss similarities and differences in the integrins expressed by both cell types.

Poly-ε-lysine based hydrogels as synthetic substrates for the expansion of corneal endothelial cells for transplantation

Dysfunction of the corneal endothelium (CE) resulting from progressive cell loss leads to corneal oedema and significant visual impairment. Current treatments rely upon donor allogeneic tissue to replace the damaged CE. A donor cornea shortage necessitates the development of biomaterials, enabling in vitro expansion of corneal endothelial cells (CECs). This study investigated the use of a synthetic peptide hydrogel using poly-ε-lysine (pεK), cross-linked with octanedioic-acid as a potential substrate for CECs expansion and CE grafts. PεK hydrogel properties were optimised to produce a substrate which was thin, transparent, porous and robust. A human corneal endothelial cell line (HCEC-12) attached and grew on pεK hydrogels as confluent monolayers after 7 days, whereas primary porcine CECs (pCECs) detached from the pεK hydrogel. Pre-adsorption of collagen I, collagen IV and fibronectin to the pεK hydrogel increased pCEC adhesion at 24 h and confluent monolayers formed at 7 days. Minimal cell adhesion was observed with pre-adsorbed laminin, chondroitin sulphate or commercial FNC coating mix (fibronectin, collagen and albumin). Functionalisation of the pεK hydrogel with synthetic cell binding peptide H-Gly-Gly-Arg-Gly-Asp-Gly-Gly-OH (RGD) or α2β1 integrin recognition sequence H-Asp-Gly-Glu-Ala-OH (DGEA) resulted in enhanced pCEC adhesion with the RGD peptide only. pCECs grown in culture at 5 weeks on RGD pεK hydrogels showed zonula occludins 1 staining for tight junctions and expression of sodium-potassium adenosine triphosphase, suggesting a functional CE. These results demonstrate the pεK hydrogel can be tailored through covalent binding of RGD to provide a surface for CEC attachment and growth. Thus, providing a synthetic substrate with a therapeutic application for the expansion of allogenic CECs and replacement of damaged CE.

Descemet's Membrane Biomimetic Microtopography Differentiates Human Mesenchymal Stem Cells Into Corneal Endothelial-Like Cells

Supplemental Digital Content is Available in the Text.

Purpose:

Loss of corneal endothelial cells (CECs) bears disastrous consequences for the patient, including corneal clouding and blindness. Corneal transplantation is currently the only therapy for severe corneal disorders. However, the worldwide shortages of corneal donor material generate a strong demand for personalized stem cell–based alternative therapies. Because human mesenchymal stem cells are known to be sensitive to their mechanical environments, we investigated the mechanotransductive potential of Descemet membrane–like microtopography (DLT) to differentiate human mesenchymal stem cells into CEC-like cells.

Methods:

Master molds with inverted DLT were produced by 2-photon lithography (2-PL). To measure the mechanotransductive potential of DLT, mesenchymal stem cells were cultivated on silicone or collagen imprints with DLT. Changes in morphology were imaged, and changes in gene expression of CEC typical genes such as zonula occludens (ZO-1), sodium/potassium (Na/K)-ATPase, paired-like homeodomain 2 (PITX2), and collagen 8 (COL-8) were measured with real-time polymerase chain reaction. At least immunofluorescence analysis has been conducted to confirm gene data on the protein level.

Results:

Adhesion of MSCs to DLT molded in silicone and particularly in collagen initiates polygonal morphology and monolayer formation and enhances not only transcription of CEC typical genes such as ZO-1, Na/K-ATPase, PITX2, and COL-8 but also expression of the corresponding proteins.

Conclusions:

Artificial reproduction of Descemet membrane with respect to topography and similar stiffness offers a potential innovative way to bioengineer a functional CEC monolayer from autologous stem cells.

Biomechanical changes to Descemet's membrane precede endothelial cell loss in an early-onset murine model of Fuchs endothelial corneal dystrophy

Early-onset Fuchs endothelial corneal dystrophy (FECD) has been associated with nonsynonymous mutations in collagen VIII α2 (COL8A2), a key extracellular matrix (ECM) protein in Descemet's membrane (DM). Two knock-in strains of mice have been generated to each express a mutant COL8A2 protein (Col8a2^L450W/L450W and Col8a2^Q455K/Q455K) that recapitulate the clinical phenotype of early-onset FECD including endothelial cell loss, cellular polymegathism and pleomorphism, and guttae. Due to abnormalities in ECM protein composition and structure in FECD, the stiffness of DM in Col8a2 knock-in mice and wildtype (WT) controls was measured using atomic force microscopy at 5 and 10 months of age, coinciding with the onset of FECD phenotypic abnormalities. At 5 months, only sporadic guttae were identified via in vivo confocal microscopy (IVCM) in Col8a2^Q455K/Q455K mice, otherwise both strains of Col8a2 transgenic mice were indistinguishable from WT controls in terms of endothelial cell density and size. By 10 months of age, Col8a2^L450W/L450W and Col8a2^Q455K/Q455K mice developed reduced corneal endothelial density, increased endothelial cell area and guttae, with the Col8a2^Q455K/Q455K strain exhibiting a more severe phenotype. However, at 5 months of age, prior to the development endothelial cell abnormalities, Col8a2^L450W/L450W and Col8a2^Q455K/Q455K mice knock-in mice had reduced tissue stiffness of DM that was statistically significant in the Col8a2^Q455K/Q455K mice when compared with wildtype controls. These data indicate that alterations in the tissue compliance of DM precede phenotypic changes in endothelial cell count and morphology, and may play a role in onset and progression of FECD.

Fish Scale-Derived Scaffolds for Culturing Human Corneal Endothelial Cells

Purpose

To investigate the biocompatibility of fish scale-derived scaffolds (FSS) with primary human corneal endothelial cells (HCEnCs).

Methods

HCEnCs were isolated from 30 donor corneas in a donor-matched study and plated in precoated Lab-Tek slides (n = 15) and FSS (n = 15). Cell morphology, proliferation/migration, and glucose uptake were studied (n = 30). Hoechst, ethidium homodimer, and calcein AM (HEC) staining was performed to determine viability and toxicity (n = 6). The cell surface area was calculated based on calcein AM staining. HCEnCs were stained for ZO-1 (n = 6) to detect tight junctions and to measure cell morphology; Ki-67 (n = 6) to measure proliferating cells; and vinculin to quantify focal adhesions (n = 6). The formation of de novo extracellular matrix was analyzed using histology (n = 6).

Results

HCEnCs attach and grow faster on Lab-Tek slides compared to the undulating topography of the FSS. At day 11, HCEnCs on Lab-Tek slide grew 100% confluent, while FSS was only 65% confluent (p = 0.0883), with no significant difference in glucose uptake between the two (p = 0.5181) (2.2 μg/mL in Lab-Tek versus 2.05 μg/mL in FSS). HEC staining showed no toxicity. The surface area of the cells in Lab-Tek was 409.1 μm² compared to 452.2 μm² on FSS, which was not significant (p = 0.5325). ZO-1 showed the presence of tight junctions in both conditions; however, hexagonality was higher (74% in Lab-Tek versus 45% in FSS; p = 0.0006) with significantly less polymorphic cells on Lab-Tek slides (8% in Lab-Tek versus 16% in FSS; p = 0.0041). Proliferative cells were detected in both conditions (4.6% in Lab-Tek versus 4.2% in FSS; p = 0.5922). Vinculin expression was marginally higher in HCEnCs cultured on Lab-Tek (234 versus 199 focal adhesions; p = 0.0507). Histological analysis did not show the formation of a basement membrane.

Conclusions

HCEnCs cultured on precoated FSS form a monolayer, displaying correct morphology, cytocompatibility, and absence of toxicity. FSS needs further modification in terms of structure and surface chemistry before considering it as a potential carrier for cultured HCEnCs.

Phenotypic Characterization of Corneal Endothelial Dystrophy in German Shorthaired and Wirehaired Pointers Using In Vivo Advanced Corneal Imaging and Histopathology

PURPOSE

To evaluate corneal morphology using ultrasonic pachymetry (USP), Fourier-domain optical coherence tomography (FD-OCT), and in vivo confocal microscopy (IVCM) in 2 related canine breeds-German shorthaired pointers (GSHPs) and German wirehaired pointers (GWHPs)-with and without corneal endothelial dystrophy (CED). This condition is characterized by premature endothelial cell degeneration leading to concomitant corneal edema and is similar to Fuchs endothelial corneal dystrophy.

METHODS

Corneas of 10 CED-affected (4 GSHP and 6 GWHP) and 19 unaffected, age-matched (15 GSHP and 4 GWHP) dogs were examined using USP, FD-OCT, and IVCM. A 2-sample t test or Mann-Whitney rank-sum test was used to statistically compare parameters between both groups. Data are presented as mean ± SD or median (range).

RESULTS

Central corneal thickness determined using USP was significantly greater in CED-affected than in unaffected dogs at 1179 (953-1959) and 646 (497-737) μm, respectively (P < 0.001). Central epithelial thickness was found to be significantly decreased in CED-affected versus unaffected dogs at 47 ± 7.1 and 55 ± 7.1 μm, respectively (P = 0.011), using FD-OCT. With IVCM, corneal endothelial density was significantly less (P < 0.001) in 5 dogs with CED versus 19 unaffected controls at 499 ± 315 versus 1805 ± 298 cells/mm, respectively. CED-affected dogs exhibited endothelial pleomorphism and polymegethism, whereas CED-unaffected dogs had regular hexagonal arrangement of cells.

CONCLUSIONS

GSHPs and GWHPs with CED exhibit marked differences in corneal morphology when compared with age-matched control dogs. These 2 CED-affected breeds represent spontaneous, large animal models for human Fuchs endothelial corneal dystrophy.

Influences on rebubble rate in Descemet's membrane endothelial keratoplasty

Purpose

Descemet’s membrane endothelial keratoplasty (DMEK) is a minimally invasive partial corneal transplant procedure used in patients with failing endothelial membranes. This study aims to identify those factors which influence the need for a rebubble of the corneal graft.

Methods

A total of 94 eyes that received DMEK between March 2014 and January 2016 at Vance Thompson Vision were used in the study. Demographic and graft data were collected from the patients and donors, and perioperative statistics of the procedures. A logistical regression was used to compare eyes that did and did not require a rebubble.

Results

Among those characteristics that were included (patient age/sex, donor age/sex, death to processing time, donation to surgery time, death to procurement time, specular cell count density, burping procedure, postoperative day 1 intraocular pressure [IOP], and postoperative week 1 IOP, concurrent phacoemulsification, and how well the Descemet graft was centered), only a lower specular cell count density of the corneal graft, and a graft that was not well-centered correlated with needing a rebubble due to partial graft detachment (p=0.021) and (p=0.023), respectively.

Conclusion

An increased specular cell count density may allow for better placement of the corneal graft by allowing for better unfolding in DMEK procedures. A well-centered graft may decrease postoperative complications by increasing adherence. Additionally, postoperative management of IOP may not affect the rebubble rate, and therefore should be left to the discretion of the provider to determine whether it is necessary.

Organ sculpting by patterned extracellular matrix stiffness

How organ-shaping mechanical imbalances are generated is a central question of morphogenesis, with existing paradigms focusing on asymmetric force generation within cells. We show here that organs can be sculpted instead by patterning anisotropic resistance within their extracellular matrix (ECM). Using direct biophysical measurements of elongating Drosophila egg chambers, we document robust mechanical anisotropy in the ECM-based basement membrane (BM) but not in the underlying epithelium. Atomic force microscopy (AFM) on wild-type BM in vivo reveals an anterior–posterior (A–P) symmetric stiffness gradient, which fails to develop in elongation-defective mutants. Genetic manipulation shows that the BM is instructive for tissue elongation and the determinant is relative rather than absolute stiffness, creating differential resistance to isotropic tissue expansion. The stiffness gradient requires morphogen-like signaling to regulate BM incorporation, as well as planar-polarized organization to homogenize it circumferentially. Our results demonstrate how fine mechanical patterning in the ECM can guide cells to shape an organ.

DOI:http://dx.doi.org/10.7554/eLife.24958.001

All organs have specific shapes and architectures that are necessary for them to work properly. Many different factors are responsible for arranging the right cells into the correct positions to make an organ. These include physical forces that act within and around cells to pull them into the right shape and location.

A structure called the extracellular matrix surrounds cells and provides them with support; it can also guide cell movements. It is not clear whether the extracellular matrix plays only a passive role or a more active, instructive role in shaping organs, in part, because it is difficult to measure the physical forces within densely packed cells.

The ovaries of the fruit fly Drosophila melanogaster provide a simple system in which to study how organs take their shape. Crest et al. developed a method to measure forces in the fly ovary as it changes from being an initially spherical group of cells to its final elongated tube shape. The results revealed that, during this process, the extracellular matrix becomes gradually stiffer from one end of the ovary to the other. This change is the main factor responsible for the cell rearrangements that shape the developing organ.

This work reveals that, along with providing structural support to cells, the mechanical properties of the matrix also actively guide how organs form. In the future, these findings may aid efforts to grow organs in a laboratory and to regenerate organs in human patients.

DOI:http://dx.doi.org/10.7554/eLife.24958.002