Angiogenic response of endothelial cells seeded dispersed versus on beads in fibrin gels

3D Bioprinting for fabrication of tissue models of COVID-19 infection

Over the last few decades, the world has witnessed multiple viral pandemics, the current severe acute respiratory syndrome-related coronavirus 2 (SARS-CoV-2) pandemic being the worst and most devastating one, claiming millions of lives worldwide. Physicians, scientists, and engineers worldwide have joined hands in dealing with the current situation at an impressive speed and efficiency. One of the major reasons for the delay in response is our limited understanding of the mechanism of action and individual effects of the virus on different tissues and organs. Advances in 3D bioprinting have opened up a whole new area to explore and utilize the technology in fabricating models of these tissues and organs, recapitulating in vivo environment. These biomimetic models can not only be utilized in learning the infection pathways and drug toxicology studies but also minimize the need for animal models and shorten the time span for human clinical trials. The current review aims to integrate the existing developments in bioprinting techniques, and their implementation to develop tissue models, which has implications for SARS-CoV-2 infection. Future translation of these models has also been discussed with respect to the pandemic.

Use of Simvastatin, Fibrin Clots, and Their Combination to Improve Human Ovarian Tissue Grafting for Fertility Restoration After Anti-Cancer Therapy

Anticancer treatments, particularly chemotherapy, induce ovarian damage and loss of ovarian follicles. There are limited options for fertility restoration, one of which is pre-chemotherapy cryopreservation of ovarian tissue. Transplantation of frozen-thawed human ovarian tissue from cancer survivors has resulted in live-births. There is extensive follicular loss immediately after grafting, probably due to too slow graft revascularization. To avoid this problem, it is important to develop methods to improve ovarian tissue neovascularization. The study's purpose was to investigate if treatment of murine hosts with simvastatin or/and embedding human ovarian tissue within fibrin clots can improve human ovarian tissue grafting (simvastatin and fibrin clots promote vascularization). There was a significantly higher number of follicles in group A (ungrafted control) than in group B (untreated tissue). Group C (simvastatin-treated hosts) had the highest levels of follicle atresia. Group C had significantly more proliferating follicles (Ki67-stained) than groups B and E (simvastatin-treated hosts and tissue embedded within fibrin clots), group D (tissue embedded within fibrin clots) had significantly more proliferating follicles (Ki67-stained) than group B. On immunofluorescence study, only groups D and E showed vascular structures that expressed both human and murine markers (mouse-specific platelet endothelial cell adhesion molecule, PECAM, and human-specific von Willebrand factor, vWF). Peripheral human vWF expression was significantly higher in group E than group B. Diffuse human vWF expression was significantly higher in groups A and E than groups B and C. When grafts were not embedded in fibrin, there was a significant loss of human vWF expression compared to groups A and E. This protocol may be tested to improve ovarian implantation in cancer survivors.

Multiscale bioprinting of vascularized models

A basic prerequisite for the survival and function of three-dimensional (3D) engineered tissue constructs is the establishment of blood vessels. 3D bioprinting of vascular networks with hierarchical structures that resemble in vivo structures has allowed blood circulation within thick tissue constructs to accelerate vascularization and enhance tissue regeneration. Successful rapid vascularization of tissue constructs requires synergy between fabrication of perfusable channels and functional bioinks that induce angiogenesis and capillary formation within constructs. Combinations of 3D bioprinting techniques and four-dimensional (4D) printing concepts through patterning proangiogenic factors may offer novel solutions for implantation of thick constructs. In this review, we cover current bioprinting techniques for vascularized tissue constructs with vasculatures ranging from capillaries to large blood vessels and discuss how to implement these approaches for patterning proangiogenic factors to maintain long-term, stimuli-controlled formation of new capillaries.

A PEGylated fibrin hydrogel-based antimicrobial wound dressing controls infection without impeding wound healing

Combat injuries are associated with a high incidence of infection, and there is a continuing need for improved approaches to control infection and promote wound healing. Due to the possible local and systemic adverse effects of standard 1% cream formulation (Silvadene), we had previously developed a polyethylene glycol (PEGylated) fibrin hydrogel (FPEG)-based wound dressing for the controlled delivery of silver sulfadiazine (SSD) entrapped in chitosan microspheres (CSM). In this study, we have evaluated the antimicrobial and wound healing efficacy of SSD-CSM-FPEG using a full-thickness porcine wound infected with Pseudomonas aeruginosa. Infected wounds treated with a one-time application of the SSD-CSM-FPEG wound dressing demonstrated significantly reduced bacterial bioburden over time (99·99% of reduction by day 11; P < 0·05) compared with all the other treatment groups. The epithelial thickness and granulation of the wound bed was significantly better on day 7 (150·9 ± 13·12 µm), when compared with other treatment groups. Overall, our findings demonstrate that the SSD-CSM-FPEG wound dressing effectively controls P. aeruginosa infection and promotes wound healing by providing a favourable environment that induces neovascularisation. Collectively, sustained release of SSD using fibrin hydrogel exhibited enhanced benefits when compared with the currently available SSD treatment, and this may have significant implications in the bacterial reduction of infected wounds in military and civilian populations.

What Is the Biological and Clinical Relevance of Fibrin?

As our knowledge of the structure and functions of fibrinogen and fibrin has increased tremendously, several key findings have given some people a superficial impression that the biological and clinical significance of these clotting proteins may be less than earlier thought. Most strikingly, studies of fibrinogen knockout mice demonstrated that many of these mice survive to weaning and beyond, suggesting that fibrin(ogen) may not be entirely necessary. Humans with afibrinogenemia also survive. Furthermore, in recent years, the major emphasis in the treatment of arterial thrombosis has been on inhibition of platelets, rather than fibrin. In contrast to the initially apparent conclusions from these results, it has become increasingly clear that fibrin is essential for hemostasis; is a key factor in thrombosis; and plays an important biological role in infection, inflammation, immunology, and wound healing. In addition, fibrinogen replacement therapy has become a preferred, major treatment for severe bleeding in trauma and surgery. Finally, fibrin is a unique biomaterial and is used as a sealant or glue, a matrix for cells, a scaffold for tissue engineering, and a carrier and/or a vector for targeted drug delivery.

Stromal cells and stem cells in clinical bone regeneration

Stem-cell-mediated bone repair has been used in clinical trials for the regeneration of large craniomaxillofacial defects, to slow the process of bone degeneration in patients with osteonecrosis of the femoral head and for prophylactic treatment of distal tibial fractures. Successful regenerative outcomes in these investigations have provided a solid foundation for wider use of stromal cells in skeletal repair therapy. However, employing stromal cells to facilitate or enhance bone repair is far from being adopted into clinical practice. Scientific, technical, practical and regulatory obstacles prevent the widespread therapeutic use of stromal cells. Ironically, one of the major challenges lies in the limited understanding of the mechanisms via which transplanted cells mediate regeneration. Animal models have been used to provide insight, but these models largely fail to reproduce the nuances of human diseases and bone defects. Consequently, the development of targeted approaches to optimize cell-mediated outcomes is difficult. In this Review, we highlight the successes and challenges reported in several clinical trials that involved the use of bone-marrow-derived mesenchymal or adipose-tissue-derived stromal cells. We identify several obstacles blocking the mainstream use of stromal cells to enhance skeletal repair and highlight technological innovations or areas in which novel techniques might be particularly fruitful in continuing to advance the field of skeletal regenerative medicine.

A two-component pre-seeded dermal-epidermal scaffold

We have developed a bilayered dermal-epidermal scaffold for application in the treatment of full-thickness skin defects. The dermal component gels in situ and adapts to the lesion shape, delivering human dermal fibroblasts in a matrix of fibrin and cross-linked hyaluronic acid modified with a cell adhesion-promoting peptide. Fibroblasts were able to form a tridimensional matrix due to material features such as tailored mechanical properties, presence of protease-degradable elements and cell-binding ligands. The epidermal component is a robust membrane containing cross-linked hyaluronic acid and poly-l-lysine, on which keratinocytes were able to attach and to form a monolayer. Amine-aldehyde bonding at the interface between the two components allows the formation of a tightly bound composite scaffold. Both parts of the scaffold were designed to provide cell-type-specific cues to allow for cell proliferation and form a construct that mimics the skin environment.

Effects of hydroxyapatite on endothelial network formation in collagen/fibrin composite hydrogels in vitro and in vivo

Co-culture of endothelial cells (EC) and mesenchymal stem cells (MSC) results in robust vascular network formation in constrained 3-D collagen/fibrin (COL/FIB) composite hydrogels. However, the ability to form endothelial networks is lost when such gels are allowed to compact via cell-mediated remodeling. In this study, we created co-cultures of human EC and human MSC in both constrained and unconstrained COL/FIB matrices and systematically added nanoparticulate hydroxyapatite (HA, 0-20 mg ml(-1)), a bone-like mineral that has been shown to have pro-vasculogenic effects. Constructs cultured for 7 days were assayed for gel compaction, vascular network formation, and mechanical properties. In vitro, robust endothelial network formation was observed in constrained COL/FIB constructs without HA, but this response was significantly inhibited by addition of 5, 10, or 20 mg ml(-1) HA. In unconstrained matrices, network formation was abolished in pure COL/FIB constructs but was rescued by 1.25 or 2.5 mg ml(-1) HA, while higher levels again inhibited vasculogenesis. HA inhibited gel compaction in a dose-dependent manner, which was not correlated to endothelial network formation. HA affected initial stiffness of the gels, but gel remodeling abrogated this effect. Subcutaneous implantation of COL/FIB with 0, 2.5 or 2 0mg ml(-1) HA in the mouse resulted in increased perfusion at the implant site, with no significant differences between materials. Histology at day 7 showed both host and human CD31-stained vasculature infiltrating the implants. These findings are relevant to the design of materials and scaffolds for orthopedic tissue engineering, where both vasculogenesis and formation of a mineral phase are required for regeneration.

Platelet-derived growth factor and spatiotemporal cues induce development of vascularized bone tissue by adipose-derived stem cells

Vasculature is essential to the functional integration of a tissue-engineered bone graft to enable sufficient nutrient delivery and viability after implantation. Native bone and vasculature develop through intimately coupled, tightly regulated spatiotemporal cell-cell signaling. The complexity of these developmental processes has been a challenge for tissue engineers to recapitulate, resulting in poor codevelopment of both bone and vasculature within a unified graft. To address this, we cultured adipose-derived stromal/stem cells (ASCs), a clinically relevant, single cell source that has been previously investigated for its ability to give rise to vascularized bone grafts, and studied the effects of initial spatial organization of cells, the temporal addition of growth factors, and the presence of exogenous platelet-derived growth factor-BB (PDGF-BB) on the codevelopment of bone and vascular tissue structures. Human ASCs were aggregated into multicellular spheroids via the hanging drop method before encapsulation and subsequent outgrowth in fibrin gels. Cellular aggregation substantially increased vascular network density, interconnectivity, and pericyte coverage compared to monodispersed cultures. To form robust vessel networks, it was essential to culture ASCs in a purely vasculogenic medium for at least 8 days before the addition of osteogenic cues. Physiologically relevant concentrations of exogenous PDGF-BB (20 ng/mL) substantially enhanced both vascular network stability and osteogenic differentiation. Comparisons with the bone morphogenetic protein-2, another pro-osteogenic and proangiogenic growth factor, indicated that this potential to couple the formation of both lineages might be unique to PDGF-BB. Furthermore, the resulting tissue structure demonstrated the close association of mineral deposits with pre-existing vascular structures that have been described for developing tissues. This combination of a single cell source with a potent induction factor used at physiological concentrations can provide a clinically relevant approach to engineering highly vascularized bone grafts.

Matrix composition regulates three-dimensional network formation by endothelial cells and mesenchymal stem cells in collagen/fibrin materials

Co-cultures of endothelial cells (EC) and mesenchymal stem cells (MSC) in three-dimensional (3D) protein hydrogels can be used to recapitulate aspects of vasculogenesis in vitro. MSC provide paracrine signals that stimulate EC to form vessel-like structures, which mature as the MSC transition to the role of mural cells. In this study, vessel-like network formation was studied using 3D collagen/fibrin (COL/FIB) matrices seeded with embedded EC and MSC and cultured for 7 days. The EC:MSC ratio was varied from 5:1, 3:2, 1:1, 2:3 and 1:5. The matrix composition was varied at COL/FIB compositions of 100/0 (pure COL), 60/40, 50/50, 40/60 and 0/100 (pure FIB). Vasculogenesis was markedly decreased in the highest EC:MSC ratio, relative to the other cell ratios. Network formation increased with increasing fibrin content in composite materials, although the 40/60 COL/FIB and pure fibrin materials exhibited the same degree of vasculogenesis. EC and MSC were co-localized in vessel-like structures after 7 days and total cell number increased by approximately 70%. Mechanical property measurements showed an inverse correlation between matrix stiffness and network formation. The effect of matrix stiffness was further investigated using gels made with varying total protein content and by crosslinking the matrix using the dialdehyde glyoxal. This systematic series of studies demonstrates that matrix composition regulates vasculogenesis in 3D protein hydrogels, and further suggests that this effect may be caused by matrix mechanical properties. These findings have relevance to the study of neovessel formation and the development of strategies to promote vascularization in transplanted tissues.

Three-dimensional image quantification as a new morphometry method for tissue engineering

Morphological analysis is an essential step in verifying the success of a tissue engineering strategy where the presence of a desired cellular phenotype must be determined. While morphometry has transitioned from observational grading to computational quantification, established quantitative methods eliminate information by relying on two-dimensional (2D) analysis to describe three-dimensional (3D) niches. In this study, we demonstrate the validity and utility of 3D morphological quantification using two common angiogenesis assays in our fibrin-based in vitro model: (1) the microcarrier bead assay with human mesenchymal stem cells and (2) the rat aortic ring outgrowth assay. The quantification method is based on collecting and segmenting fluorescent confocal z-stacks into 3D models with 3D Slicer, an open-source magnetic resonance imaging/computed tomography analysis program. Data from 3D models are then processed into biologically relevant metrics in MATLAB for statistical analysis. Metrics include descriptive parameters such as vascular network length, volume, number of network segments, and degree of network branching. Our results indicate that 2D measures are significantly different than their 3D counterparts unless the vascular network exhibits anisotropic growth along the plane of imaging. Additionally, the statistical outcomes of 3D morphological quantification agreed with our initial qualitative observations among different test groups. This novel quantification approach generates more spatially accurate and objective measures, representing an important step toward improving the reliability of morphological comparisons.

A bilayer construct controls adipose-derived stem cell differentiation into endothelial cells and pericytes without growth factor stimulation

This work describes the differentiation of adipose-derived mesenchymal stem cells (ASC) in a composite hydrogel for use as a vascularized dermal matrix. Our intent is that such a construct could be utilized following large-surface-area burn wounds that require extensive skin grafting and that are limited by the availability of uninjured sites. To develop engineered skin replacement constructs, we are pursuing the use of ASC. We have established that a PEGylated fibrin gel can provide a suitable environment for the proliferation of ASC over a 7 day time course. Further, we have demonstrated that PEGylated fibrin can be used to control ASC differentiation toward vascular cell types, including cells characteristic of both endothelial cells and pericytes. Gene expression analysis revealed strong upregulation of endothelial markers, CD31, and von Willebrand factor, up to day 11 in culture with corresponding evidence of protein expression demonstrated by immunocytochemical staining. ASC were not only shown to express endothelial cell phenotype, but a subset of the ASC expressed pericyte markers. The NG2 gene was upregulated over 11 days with corresponding evidence for the cell surface marker. Platelet-derived growth factor receptor beta gene expression decreased as the multipotent ASC differentiated up to day 7. Increased receptor expression at day 11 was likely due to the enhanced pericyte gene expression profile, including increased NG2 expression. We have also demonstrated that when cells are loaded onto chitosan microspheres and sandwiched between the PEGylated fibrin gel and a type I collagen gel, the cells can migrate and proliferate within the two different gel types. The matrix composition dictates the lineage specification and is not driven by soluble factors. Utilizing an insoluble bilayer matrix to direct ASC differentiation will allow for the development of both vasculature as well as dermal connective tissue from a single population of ASC. This work underscores the importance of the extracellular matrix in controlling stem cell phenotype. It is our goal to develop layered composites as wound dressings or vascularized dermal equivalents that are not limited by nutrient diffusion.

The effects of co-culture with fibroblasts and angiogenic growth factors on microvascular maturation and multi-cellular lumen formation in HUVEC-oriented polymer fibre constructs

In the present study, polymer monofilaments were embedded in fibrin seeded with human umbilical vein endothelial cells (HUVEC) to guide HUVEC attachment and migration in order to form oriented vessel-like structures between adjacent monofilaments. Histology and fluorescent fibrin experiments confirmed that microvessel-like structures, which were developing between polymer monofilaments embedded in fibrin, contained a lumen. The effect of human fibroblasts and growth factors (VEGF and bFGF) over the microvessel formation process was tested. The effects of VEGF and bFGF were dose-dependent. The effect of VEGF was optimum at the lower concentration tested (2 ng/mL), while that of bFGF was optimum at the higher tested concentration (20 ng/mL). Furthermore, the use of fibroblasts significantly improved the maturation of the microvessels compared to control and to samples cultured with VEGF and bFGF.

Efficient isolation and chondrogenic differentiation of adult mesenchymal stem cells with fibrin microbeads and micronized collagen sponges

Background: Mesenchymal stem cells (MSCs) have been demonstrated to potentially undergo chondrogenic differentiation. We propose a new matrix for stem cell-based chondrogenesis using dense fibrin microbeads (FMBs) combined with grounded dehydrothermally crosslinked collagen sponges (micronized collagen). Methods: In this study, MSCs were isolated from bone marrow of transgenic green fluorescent protein C57/Bl mice by FMBs in high yield. After 48 h in slowly rotating suspension culture, micronized collagen was added. Results: The cells on the FMBs migrated to the collagen pieces and formed aggregates that developed into cartilage-like structures. Following chondrogenic differentiation, alcian blue staining and collagen type II immunohistochemistry demonstrated the presence of chondrocytes in the 3D structures. PCR for the expression of aggrecan and collagen type II genes supported these findings. The in vitro structures that formed were used for ectopic subdermal implantation in wild-type C57/Bl mice. However, the chondrogenic markers faded relative to the pre-implant in vitro structures. Conclusion: We propose that FMBs with micronized collagen could serve as a simple technology for MSC isolation and chondrogenesis as a basis for implantation.

The use of fibrin based matrices and fibrin microbeads (FMB) for cell based tissue regeneration

BACKGROUND

Due to its good cell attachment capabilities and promotion of cell migration, fibrin serves as an interim cell-binding matrix in wounded tissues. Due to their fast degradation, unprocessed fibrin matrices have limited use in tissue engineering.

OBJECTIVE

To describe stable fibrin-based matrices for isolation, growth and delivery of stem cells for implantation to enhance tissue regeneration.

METHODS

Fibrin microbeads (FMB) were produced by moderate-heat condensation of fibrin particles in oil without compromising the cell binding capability of the fibrin.

RESULTS

Mesenchymal stem cells (MSC) were separated from different sources at much higher yields with FMB. They were further expanded on them in suspension without trypsinization and passages. Cells on FMB could be induced to differentiate into different phenotypes, such as bone and cartilage. This enabled implantation of the cells on FMB for cell-based tissue regeneration.

CONCLUSIONS

FMB technology provides a simple and effective method for cell separation, expansion in suspension and delivery for tissue regeneration.

Stem cells and scaffolds for vascularizing engineered tissue constructs

The clinical impact of tissue engineering depends upon our ability to direct cells to form tissues with characteristic structural and mechanical properties from the molecular level up to organized tissue. Induction and creation of functional vascular networks has been one of the main goals of tissue engineering either in vitro, for the transplantation of prevascularized constructs, or in vivo, for cellular organization within the implantation site. In most cases, tissue engineering attempts to recapitulate certain aspects of normal development in order to stimulate cell differentiation and functional tissue assembly. The induction of tissue growth generally involves the use of biodegradable and bioactive materials designed, ideally, to provide a mechanical, physical, and biochemical template for tissue regeneration. Human embryonic stem cells (hESCs), derived from the inner cell mass of a developing blastocyst, are capable of differentiating into all cell types of the body. Specifically, hESCs have the capability to differentiate and form blood vessels de novo in a process called vasculogenesis. Human ESC-derived endothelial progenitor cells (EPCs) and endothelial cells have substantial potential for microvessel formation, in vitro and in vivo. Human adult EPCs are being isolated to understand the fundamental biology of how these cells are regulated as a population and to explore whether these cells can be differentiated and reimplanted as a cellular therapy in order to arrest or even reverse damaged vasculature. This chapter focuses on advances made toward the generation and engineering of functional vascular tissue, focusing on both the scaffolds - the synthetic and biopolymer materials - and the cell sources - hESCs and hEPCs.

Composite fibrin scaffolds increase mechanical strength and preserve contractility of tissue engineered blood vessels

OBJECTIVES

We recently demonstrated that fibrin-based tissue engineered blood vessels (TEV) exhibited vascular reactivity, matrix remodeling and sufficient strength for implantation into the veins of an ovine animal model, where they remained patent for 15 weeks. Here we present an approach to improve the mechanical properties of fibrin-based TEV and examine the relationship between mechanical strength and smooth muscle cell (SMC) function.

MATERIALS AND METHODS

To this end, we prepared TEV that were composed of two layers: a cellular layer containing SMC embedded in fibrin hydrogel to provide contractility and matrix remodeling; and a second cell-free fibrin layer composed of high concentration fibrinogen to provide mechanical strength.

RESULTS

The ultimate tensile force of double-layered TEV increased with FBG concentration in the cell-free layer in a dose-dependent manner. Double-layered TEV exhibited burst pressure that was ten-fold higher than single-layered tissues but vascular reactivity remained high even though the cells were constricting an additional tissue layer.

CONCLUSION

These results showed that mechanical strength results largely from the biomaterial but contractility requires active cellular machinery. Consequently, they may suggest novel approaches for engineering biomaterials that satisfy the requirement for high mechanical strength while preserving SMC function.