Cell regulation of collagen fibril macrostructure during corneal morphogenesis

Dimensionality Matters: Exploiting UV-Photopatterned 2D and Two-Photon-Printed 2.5D Contact Guidance Cues to Control Corneal Fibroblast Behavior and Collagen Deposition

In the event of disease or injury, restoration of the native organization of cells and extracellular matrix is crucial for regaining tissue functionality. In the cornea, a highly organized collagenous tissue, keratocytes can align along the anisotropy of the physical microenvironment, providing a blueprint for guiding the organization of the collagenous matrix. Inspired by this physiological process, anisotropic contact guidance cues have been employed to steer the alignment of keratocytes as a first step to engineer in vitro cornea-like tissues. Despite promising results, two major hurdles must still be overcome to advance the field. First, there is an enormous design space to be explored in optimizing cellular contact guidance in three dimensions. Second, the role of contact guidance cues in directing the long-term deposition and organization of extracellular matrix proteins remains unknown. To address these challenges, here we combined two microengineering strategies-UV-based protein patterning (2D) and two-photon polymerization of topographies (2.5D)-to create a library of anisotropic contact guidance cues with systematically varying height (H, 0 µm ≤ H ≤ 20 µm) and width (W, 5 µm ≤ W ≤ 100 µm). With this unique approach, we found that, in the short term (24 h), the orientation and morphology of primary human fibroblastic keratocytes were critically determined not only by the pattern width, but also by the height of the contact guidance cues. Upon extended 7-day cultures, keratocytes were shown to produce a dense, fibrous collagen network along the direction of the contact guidance cues. Moreover, increasing the heights also increased the aligned fraction of deposited collagen and the contact guidance response of cells, all whilst the cells maintained the fibroblastic keratocyte phenotype. Our study thus reveals the importance of dimensionality of the physical microenvironment in steering both cellular organization and the formation of aligned, collagenous tissues.

Developmental Changes in Patterns of Distribution of Fibronectin and Tenascin-C in the Chicken Cornea: Evidence for Distinct and Independent Functions during Corneal Development and Morphogenesis

The cornea forms the tough and transparent anterior part of the eye and by accurate shaping forms the major refractive element for vision. Its largest component is the stroma, a dense collagenous connective tissue positioned between the epithelium and the endothelium. In chicken embryos, the stroma initially develops as the primary stroma secreted by the epithelium, which is then invaded by migratory neural crest cells. These cells secrete an organised multi-lamellar collagenous extracellular matrix (ECM), becoming keratocytes. Within individual lamellae, collagen fibrils are parallel and orientated approximately orthogonally in adjacent lamellae. In addition to collagens and associated small proteoglycans, the ECM contains the multifunctional adhesive glycoproteins fibronectin and tenascin-C. We show in embryonic chicken corneas that fibronectin is present but is essentially unstructured in the primary stroma before cell migration and develops as strands linking migrating cells as they enter, maintaining their relative positions as they populate the stroma. Fibronectin also becomes prominent in the epithelial basement membrane, from which fibronectin strings penetrate into the stromal lamellar ECM at right angles. These are present throughout embryonic development but are absent in adults. Stromal cells associate with the strings. Since the epithelial basement membrane is the anterior stromal boundary, strings may be used by stromal cells to determine their relative anterior-posterior positions. Tenascin-C is organised differently, initially as an amorphous layer above the endothelium and subsequently extending anteriorly and organising into a 3D mesh when the stromal cells arrive, enclosing them. It continues to shift anteriorly in development, disappearing posteriorly, and finally becoming prominent in Bowman's layer beneath the epithelium. The similarity of tenascin-C and collagen organisation suggests that it may link cells to collagen, allowing cells to control and organise the developing ECM architecture. Fibronectin and tenascin-C have complementary roles in cell migration, with the former being adhesive and the latter being antiadhesive and able to displace cells from their adhesion to fibronectin. Thus, in addition to the potential for associations between cells and the ECM, the two could be involved in controlling migration and adhesion and subsequent keratocyte differentiation. Despite the similarities in structure and binding capabilities of the two glycoproteins and the fact that they occupy similar regions of the developing stroma, there is little colocalisation, demonstrating their distinctive roles.

Chondroitin Sulphate/Dermatan Sulphate Proteoglycans: Potential Regulators of Corneal Stem/Progenitor Cell Phenotype In Vitro

Chondroitin sulphate (CS) proteoglycans with variable sulphation-motifs along their glycosaminoglycan (GAG) chains are closely associated with the stem cell niche of articular cartilage, where they are believed to influence the characteristics of the resident stem cells. Here, we investigated the immunohistochemical distribution of hybrid CS/dermatan sulphate (DS) GAGs in the periphery of the adult chicken cornea, which is the location of the cornea's stem cell niche in a number of species, using a monoclonal antibody, 6C3, that recognises a sulphation motif-specific CS/DS GAG epitope. This revealed positive labelling that was restricted to the subepithelial corneal stroma, as well as nearby bony structures within the sclera, called ossicles. When cultivated on cell culture dishes coated with 6C3-rich CS/DS, corneal stromal cells (keratocytes) that had been isolated from embryonic chicken corneas formed circular colonies, which took several days to reach confluency. A flow cytometric analysis of these keratocytes revealed changes in their expression levels of the indicative stem cell markers, Connexin 43 (Cx43), Paired Box 6 (PAX6), B-lymphoma Moloney murine leukemia virus insertion region-1 (Bmi-1), and C-X-C Chemokine Receptor 4 (CXCR4) suggestive of a less-differentiated phenotype compared with expression levels in cells not exposed to CS/DS. These findings support the view that CS/DS promotes the retention of a stem cell phenotype in corneal cells, much as it has been proposed to do in other connective tissues.

Mechanochemistry of collagen

The collagen molecular family is the result of nearly one billion years of evolution. It is a unique family of proteins, the majority of which provide general mechanical support to biological tissues. Fibril forming collagens are the most abundant collagens in vertebrate animals and are generally found in positions that resist tensile loading. In animals, cells produce fibril-forming collagen molecules that self-assemble into larger structures known as collagen fibrils. Collagen fibrils are the fundamental, continuous, load-bearing elements in connective tissues, but are often further aggregated into larger load-bearing structures, fascicles in tendon, lamellae in cornea and in intervertebral disk. We know that failure to form fibrillar collagen is embryonic lethal, and excessive collagen formation/growth (fibrosis) or uncontrolled enzymatic remodeling (type II collagen: osteoarthritis) is pathological. Collagen is thus critical to vertebrate viability and instrumental in maintaining efficient mechanical structures. However, despite decades of research, our understanding of collagen matrix formation is not complete, and we know still less about the detailed mechanisms that drive collagen remodeling, growth, and pathology. In this perspective, we examine the known role of mechanical force on the formation and development of collagenous structure. We then discuss a mechanochemical mechanism that has the potential to unify our understanding of collagenous tissue assembly dynamics, which preferentially deposits and grows collagen fibrils directly in the path of mechanical force, where the energetics should be dissuasive and where collagen fibrils are most required. We term this mechanism: Mechanochemical force-structure causality. STATEMENT OF SIGNIFICANCE: Our mechanochemical-force structure causality postulate suggests that collagen molecules are components of mechanochemically-sensitive and dynamically-responsive fibrils. Collagen molecules assemble preferentially in the path of applied strain, can be grown in place by mechanical extension, and are retained in the path of force through strain-stabilization. The mechanisms that drive this behavior operate at the level of the molecules themselves and are encoded into the structure of the biomaterial. The concept might change our understanding of structure formation, enhance our ability to treat injuries, and accelerate the development of therapeutics to prevent pathologies such as fibrosis. We suggest that collagen is a mechanochemically responsive dynamic element designed to provide a substantial "material assist" in the construction of adaptive carriers of mechanical signals.

Dynamic development of the extracellular matrix in chick cornea

Understanding the mechanisms of structural organization of corneal stroma is scientifically interesting and may lead to advances in corneal tissue engineering. In the past, we used the combination of second harmonic generation and fast Fourier transform to study a number of topics including the structural alteration of human keratoconus. Recently, we applied this approach to studying changes of chick embryonic cornea during development. As numerous studies were performed with ex vivo specimens, a thorough understanding of the parameters associated with the development process cannot be achieved until experiments are performed in living chick embryos.

Response to Letter to Editor "Comments on 'Cell regulation of collagen fibril macrostructure during corneal morphogenesis' by Koudouna et al."

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Comments on "Cell regulation of collagen fibril macrostructure during corneal morphogenesis" by Koudouna et al

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Identification of a Primary Stroma and Novel Endothelial Cell Projections in the Developing Human Cornea

Purpose

To investigate the initial events in the development of the human cornea, focusing on cell migration, and extracellular matrix synthesis and organization. To determine whether elastic fibers are present in the extracellular matrix during early human corneal development.

Methods

Human corneas were collected from week 7 to week 17 of development. An elastic fiber-enhancing stain, tannic acid–uranyl acetate, was applied to all tissue. Three-dimensional serial block-face scanning electron microscopy combined with conventional transmission electron microscopy was used to analyze the corneal stroma.

Results

An acellular collagenous primary stroma with an orthogonal arrangement of fibrils was identified in the central cornea from week 7 of corneal development. At week 7.5, mesenchymal cells migrated toward the central cornea and associated with the acellular collagenous matrix. Novel cell extensions from the endothelium were identified. Elastic fibers were found concentrated in the posterior peripheral corneal stroma from week 12 of corneal development.

Conclusions

This study provides novel evidence of an acellular primary stroma in the early development of the embryonic human cornea. Cell extensions exist as part of a communication system and are hypothesized to assist in the migration of the mesenchymal cells and the development of the mature cornea. Elastic fibers identified in early corneal development may play an important role in establishing corneal shape.

Development, structure, and bioengineering of the human corneal stroma: A review of collagen-based implants

Bio-engineering technologies are currently used to produce biomimetic artificial corneas that should present structural, chemical, optical, and biomechanical properties close to the native tissue. These properties are mainly supported by the corneal stroma which accounts for 90% of corneal thickness and is mainly made of collagen type I. The stromal collagen fibrils are arranged in lamellae that have a plywood-like organization. The fibril diameter is between 25 and 35 nm and the interfibrillar space about 57 nm. The number of lamellae in the central stroma is estimated to be 300. In the anterior part, their size is 10-40 μm. They appear to be larger in the posterior part of the stroma with a size of 60-120 μm. Their thicknesses also vary from 0.2 to 2.5 μm. During development, the acellular corneal stroma, which features a complex pattern of organization, serves as a scaffold for mesenchymal cells that invade and further produce the cellular stroma. Several pathways including Bmp4, Wnt/β-catenin, Notch, retinoic acid, and TGF-β, in addition to EFTFs including the mastering gene Pax-6, are involved in corneal development. Besides, retinoic acid and TGF- β seem to have a crucial role in the neural crest cell migration in the stroma. Several technologies can be used to produce artificial stroma. Taking advantage of the liquid-crystal properties of acid-soluble collagen, it is possible to produce transparent stroma-like matrices with native-like collagen I fibrils and plywood-like organization, where epithelial cells can adhere and proliferate. Other approaches include the use of recombinant collagen, cross-linkers, vitrification, plastically compressed collagen or magnetically aligned collagen, providing interesting optical and mechanical properties. These technologies can be classified according to collagen type and origin, presence of telopeptides and native-like fibrils, structure, and transparency. Collagen matrices feature transparency >80% for the appropriate 500-μm thickness. Non-collagenous matrices made of biopolymers including gelatin, silk, or fish scale have been developed which feature interesting properties but are less biomimetic. These bioengineered matrices still need to be colonized by stromal cells to fully reproduce the native stroma.

Recapitulation of normal collagen architecture in embryonic wounded corneas

Wound healing is characterized by cell and extracellular matrix changes mediating cell migration, fibrosis, remodeling and regeneration. We previously demonstrated that chick fetal wound healing shows a regenerative phenotype regarding the cellular and molecular organization of the cornea. However, the chick corneal stromal structure is remarkably complex in the collagen fiber/lamellar organization, involving branching and anastomosing of collagen bundles. It is unknown whether the chick fetal wound healing is capable of recapitulating this developmentally regulated organization pattern. The purpose of this study was to examine the three-dimensional collagen architecture of wounded embryonic corneas, whilst identifying temporal and spatial changes in collagen organization during wound healing. Linear corneal wounds that traversed the epithelial layer, Bowman´s layer, and anterior stroma were generated in chick corneas on embryonic day 7. Irregular thin collagen fibers are present in the wounded cornea during the early phases of wound healing. As wound healing progresses, the collagen organization dramatically changes, acquiring an orthogonal arrangement. Fourier transform analysis affirmed this observation and revealed that adjacent collagen lamellae display an angular displacement progressing from the epithelium layer towards the endothelium. These data indicate that the collagen organization of the wounded embryonic cornea recapitulate the native macrostructure.

Cell-independent matrix configuration in early corneal development

Mechanisms controlling the spatial configuration of the remarkably ordered collagen-rich extracellular matrix of the transparent cornea remain incompletely understood. We previously described the assembly of the emerging corneal matrix in the mid and late stages of embryogenesis and concluded that collagen fibril organisation was driven by cell-directed mechanisms. Here, the early stages of corneal morphogenesis were examined by serial block face scanning electron microscopy of embryonic chick corneas starting at embryonic day three (E3), followed by a Fourier transform analysis of three-dimensional datasets and theoretical considerations of factors that influence matrix formation. Eyes developing normally and eyes that had the lens surgically removed at E3 were studied. Uniformly thin collagen fibrils are deposited by surface ectoderm-derived corneal epithelium in the primary stroma of the developing chick cornea and form an acellular matrix with a striking micro-lamellar orthogonal arrangement. Fourier transform analysis supported this observation and indicated that adjacent micro-lamellae display a clockwise rotation of fibril orientation, depth-wise below the epithelium. We present a model which attempts to explain how, in the absence of cells in the primary stroma, collagen organisation might be influenced by cell-independent, intrinsic mechanisms, such as fibril axial charge derived from associated proteoglycans. On a supra-lamellar scale, fine cords of non-collagenous filamentous matrix were detected over large tissue volumes. These extend into the developing cornea from the epithelial basal lamina and appear to associate with the neural crest cells that migrate inwardly to form, first the corneal endothelium and then keratocytes which synthesise the mature, secondary corneal stroma. In a small number of experimental specimens, matrix cords were present even when periocular neural crest cell migration and corneal morphogenesis had been perturbed following removal of the lens at E3.

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Highly-ordered connective tissue appears early in development of the avian cornea.

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Cell-independent mechanisms may contribute to the organisation of collagen fibrils into an orthogonal array.

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Matrix cords from epithelium into stroma contact invading neural crest cells.

Three-dimensional imaging of the extracellular matrix and cell interactions in the developing prenatal mouse cornea

As the outer lens in the eye, the cornea needs to be strong and transparent. These properties are governed by the arrangement of the constituent collagen fibrils, but the mechanisms of how this develops in mammals is unknown. Using novel 3-dimensional scanning and conventional transmission electron microscopy, we investigated the developing mouse cornea, focusing on the invading cells, the extracellular matrix and the collagen types deposited at different stages. Unlike the well-studied chick, the mouse cornea had no acellular primary stroma. Collagen fibrils initially deposited at E13 from the presumptive corneal stromal cells, become organised into fibril bundles orthogonally arranged between cells. Extensive cell projections branched to adjacent stromal cells and interacted with the basal lamina and collagen fibrils. Types I, II and V collagen were expressed from E12 posterior to the surface ectoderm, and became widespread from E14. Type IX collagen localised to the corneal epithelium at E14. Type VII collagen, the main constituent of anchoring filaments, was localised posterior to the basal lamina. We conclude that the cells that develop the mouse cornea do not require a primary stroma for cell migration. The cells have an elaborate communication system which we hypothesise helps cells to align collagen fibrils.

Quantification of collagen fiber structure using second harmonic generation imaging and two-dimensional discrete Fourier transform analysis: Application to the human optic nerve head

Second harmonic generation (SHG) microscopy is widely used to image collagen fiber microarchitecture due to its high spatial resolution, optical sectioning capabilities and relatively nondestructive sample preparation. Quantification of SHG images requires sensitive methods to capture fiber alignment. This article presents a two-dimensional discrete Fourier transform (DFT)-based method for collagen fiber structure analysis from SHG images. The method includes integrated periodicity plus smooth image decomposition for correction of DFT edge discontinuity artefact, avoiding the loss of peripheral image data encountered with more commonly used windowing methods. Outputted parameters are as follows: the collagen fiber orientation distribution, aligned collagen content and the degree of collagen fiber dispersion along the principal orientation. We demonstrate its application to determine collagen microstructure in the human optic nerve head, showing its capability to accurately capture characteristic structural features including radial fiber alignment in the innermost layers of the bounding sclera and a circumferential collagen ring in the mid-stromal tissue. Higher spatial resolution rendering of individual lamina cribrosa beams within the nerve head is also demonstrated. Validation of the method is provided in the form of correlative results from wide-angle X-ray scattering and application of the presented method to other fibrous tissues.