Alvena Kureshi

Matrix mechanical properties of transversalis fascia in inguinal herniation as a model for tissue expansion

Inguinal herniation represents a common condition requiring surgical intervention. Despite being regarded as a connective tissue disorder of uncertain cause, research has focused predominantly on biochemical changes in the key tissue layer, the transversalis fascia (TF) with little direct analysis of functional tissue mechanics. Connective tissue tensile properties are dominated by collagen fibril density and architecture. This study has correlated mechanical properties of herniated TF (HTF) and non-herniated TF (NHTF) with fibrillar properties at the ultrastructural level by quasi-static tensile mechanical analysis and image analysis of collagen electron micrographs. No significant difference was found between any of the key mechanical properties (break stress, strain or modulus) for HTF and NHTF. In addition, no significant differences were found in average collagen fibril diameter, density or fibre bundle spacing. However, both groups displayed anisotropy with greater break stress (p=0.001) on average in the transverse anatomical plane compared to the longitudinal plane in a mean ratio of 2:1 (anisotropy ratio), though there was no evidence of a difference in this ratio for HTF and NHTF for both break stress and modulus. It was noted that this anisotropy ratio corresponds closely with the expected force distribution on a model cylindrical structure loaded axially. The absence of other functional differences does not support the idea of a failing (injured) tissue but is consistent with it being a tissue undergoing chronic growth/expansion under multi-vectored mechanical loading. These findings provide new clues to collagen tissue herniation for mathematical modelling and model tissue engineering.

Alignment hierarchies: engineering architecture from the nanometre to the micrometre scale

Natural tissues are built of metabolites, soluble proteins and solid extracellular matrix components (largely fibrils) together with cells. These are configured in highly organized hierarchies of structure across length scales from nanometre to millimetre, with alignments that are dominated by anisotropies in their fibrillar matrix. If we are to successfully engineer tissues, these hierarchies need to be mimicked with an understanding of the interaction between them. In particular, the movement of different elements of the tissue (e.g. molecules, cells and bulk fluids) is controlled by matrix structures at distinct scales. We present three novel systems to introduce alignment of collagen fibrils, cells and growth factor gradients within a three-dimensional collagen scaffold using fluid flow, embossing and layering of construct. Importantly, these can be seen as different parts of the same hierarchy of three-dimensional structure, as they are all formed into dense collagen gels. Fluid flow aligns collagen fibrils at the nanoscale, embossed topographical features provide alignment cues at the microscale and introducing layered configuration to three-dimensional collagen scaffolds provides microscale- and mesoscale-aligned pathways for protein factor delivery as well as barriers to confine protein diffusion to specific spatial directions. These seemingly separate methods can be employed to increase complexity of simple extracellular matrix scaffolds, providing insight into new approaches to directly fabricate complex physical and chemical cues at different hierarchical scales, similar to those in natural tissues.

Tissue Engineering the Cornea: The Evolution of RAFT.

Corneal blindness affects over 10 million people worldwide and current treatment strategies often involve replacement of the defective layer with healthy tissue. Due to a worldwide donor cornea shortage and the absence of suitable biological scaffolds, recent research has focused on the development of tissue engineering techniques to create alternative therapies. This review will detail how we have refined the simple engineering technique of plastic compression of collagen to a process we now call Real Architecture for 3D Tissues (RAFT). The RAFT production process has been standardised, and steps have been taken to consider Good Manufacturing Practice compliance. The evolution of this process has allowed us to create biomimetic epithelial and endothelial tissue equivalents suitable for transplantation and ideal for studying cell-cell interactions in vitro.

Human corneal stromal stem cells support limbal epithelial cells cultured on RAFT tissue equivalents.

Human limbal epithelial cells (HLE) and corneal stromal stem cells (CSSC) reside in close proximity in vivo in the corneal limbal stem cell niche. However, HLE are typically cultured in vitro without supporting niche cells. Here, we re-create the cell-cell juxtaposition of the native environment in vitro, to provide a tool for investigation of epithelial-stromal cell interactions and to optimize HLE culture conditions for potential therapeutic application. RAFT (Real Architecture For 3D Tissue) tissue equivalents (TEs) were used as a 3-dimensional substrate for co-culturing HLE and CSSC. Our results demonstrate that a monolayer of HLE that maintained expression of p63α, ABCB5, CK8 and CK15 (HLE markers), formed on the surface of RAFT TEs within 13 days of culture. CSSC remained in close proximity to HLE and maintained expression of mesenchymal stem cell markers. This simple technique has a short preparation time of only 15 days with the onset of HLE layering and differentiation observed. Furthermore, co-cultivation of HLE with another niche cell type (CSSC) directly on RAFT TEs, eliminates the requirement for animal-derived feeder cells. RAFT TEs may be useful for future therapeutic delivery of multiple cell types to restore the limbal niche following ocular surface injury or disease.

Advanced imaging and tissue engineering of the human limbal epithelial stem cell niche.

The limbal epithelial stem cell niche provides a unique, physically protective environment in which limbal epithelial stem cells reside in close proximity with accessory cell types and their secreted factors. The use of advanced imaging techniques is described to visualize the niche in three dimensions in native human corneal tissue. In addition, a protocol is provided for the isolation and culture of three different cell types, including human limbal epithelial stem cells from the limbal niche of human donor tissue. Finally, the process of incorporating these cells within plastic compressed collagen constructs to form a tissue-engineered corneal limbus is described and how immunohistochemical techniques may be applied to characterize cell phenotype therein.

Optimization of optical and mechanical properties of real architecture for 3-dimensional tissue equivalents: Towards treatment of limbal epithelial stem cell deficiency.

Graphical abstract

Challenges in the development of a reference standard and potency assay for the clinical production of RAFT tissue equivalents for the cornea

AIM To develop a reference standard and potency assay for Real Architecture For 3D Tissues (RAFT) tissue equivalents intended for use in limbal epithelial stem cell (LESC) therapy for the cornea. METHODS RAFT, a cell-seeded plastic compressed collagen construct with LESCs cultured on the surface, was manufactured with the goal of achieving GMP compliance. RAFTs were tested for reproducibility of manufacture (reference standard) and subsequently wounded and monitored for re-epithelialization (potency assay). RESULTS RAFT tissue equivalents produced with cells from different biological donors were capable of supporting multilayered epithelium in culture. The potency assay demonstrated re-epithelialization following wounding, indicating the potential efficacy of RAFT constructs. CONCLUSION We have presented our attempts at creating a reference standard and potency assay for the clinical manufacture of RAFT for the treatment of LESC deficiency. However, it remains challenging for adult stem cell therapies (including LESC therapy) to fully meet regulatory requirements when dealing with a limited source of autologous cells with inherent biological variation between donors.

Human Corneal Stromal Stem Cells Exhibit Survival Capacity Following Isolation From Stored Organ-Culture Corneas

PURPOSE To assess the suitability of human donor corneas maintained in long-term organ culture for the isolation and expansion of viable and functional corneal stromal stem cells (CSSCs). These cells display properties similar to mesenchymal stem cells and demonstrate the ability to reproduce an organized matrix in vitro. Therefore, CSSCs have great potential for the development of cell-based therapies for corneal blindness or stromal tissue bioengineering. METHODS Human donor corneas that had been stored either in organ-culture medium (OC) up to 4 weeks (n = 3) or in Optisol medium (OS) up to 6 days (n = 3) were used for isolation of CSSCs and maintained in culture until passage 4. Cell phenotype of isolated CSSCs was assessed with light microscopy and immunocytochemistry (PAX6, CD73, and CD90). PAX6 protein expression was further confirmed with immunoblot analysis. RESULTS A comparison of CSSCs isolated from corneas stored under OC and OS conditions revealed no obvious differences in their morphology. Immunocytochemistry revealed CSSCs from both OC and OS corneas maintained positive staining for PAX6 and mesenchymal stem cell markers CD73 and CD90. Immunoblotting confirmed protein expression of PAX6 in cells from both tissue types. CONCLUSIONS Human CSSCs exhibit survival capacity by retaining their phenotype following isolation from long storage, OC corneas. This advantageous property enables the retrieval of CSSCs from OC corneas that are more abundantly available for research than OS-stored corneas. Organ-culture corneas are also often discarded for retrieval of other cell types, such as corneal epithelial and endothelial cells, which require high tissue quality for their preservation.

Controlling human corneal stromal stem cell contraction to mediate rapid cell and matrix organization of real architecture for 3-dimensional tissue equivalents

The architecture of the human corneal stroma consists of a highly organized extracellular matrix (ECM) interspersed with keratocytes. Their progenitor cells; corneal stromal stem cells (CSSC) are located at the periphery, in the limbal stroma. A highly organized corneal ECM is critical for effective transmission of light but this structure may be compromised during injury or disease, resulting in loss of vision. Re-creating normal organization in engineered tissue equivalents for transplantation often involves lengthy culture times that are inappropriate for clinical use or utilisation of synthetic substrates that bring complications such as corneal melting. CSSC have great therapeutic potential owing to their ability to reorganize a disorganized matrix, restoring transparency in scarred corneas. We examined CSSC contractile behavior to assess whether this property could be exploited to rapidly generate cell and ECM organization in Real Architecture For 3D Tissues (RAFT) tissue equivalents (TE) for transplantation. Free-floating collagen gels were characterized to assess contractile behavior of CSSC and establish optimum cell density and culture times. To mediate cell and collagen organization, tethered collagen gels seeded with CSSC were cultured and subsequently stabilized with the RAFT process. We demonstrated rapid creation of biomimetic RAFT TE with tunable structural properties. These displayed three distinct regions of varying degrees of cellular and collagen organization. Interestingly, increased organization coincided with a dramatic loss of PAX6 expression in CSSC, indicating rapid differentiation into keratocytes. The organized RAFT TE system could be a useful bioengineering tool to rapidly create an organized ECM while simultaneously controlling cell phenotype. STATEMENT OF SIGNIFICANCE For the first time, we have demonstrated that human CSSC exhibit the phenomenon of cellular self-alignment in tethered collagen gels. We found this mediated rapid co-alignment of collagen fibrils and thus subsequently exploited this property in vitro to improve the architecture of engineered RAFT tissue equivalents of the corneal stroma. Existing techniques are extremely lengthy and carry significant risk and cost for GMP manufacture. This rapid and tunable technique takes just 8 h of culture and is therefore ideal for clinical manufacture, creating biomimetic tissue equivalents with both cellular and ECM organization. Thus, cellular self-alignment can be a useful bioengineering tool for the development of organized tissue equivalents in a variety of applications.