Pre-vascularized dermis model for fast and functional anastomosis with host vasculature

Angiogenesis and full thickness wound repair in a cell sheet-based vascularized skin substitute

Skin tissue engineering is undergoing tremendous expansion as a result from clinical needs, mandatory replacement of animal models and development of new technologies. Many approaches have been used to produce vascularized skin substitutes for grafting purposes showing the presence of capillary-like structures but with limited analysis of their in vitro maturation and plasticity. Such knowledge is however important for the development of tissue substitutes with improved implantation success as well as for validation of vascularization in vitro models, including as a readout in pharmacological analyses. For optimal interactions of cells with microenvironment and vasculature, we here used a cell sheet approach consisting in the sole production of matrix by the cells. In this context, we limited the density of endothelial cells seeded for self-assembly and rather relied on the stimulation of angiogenesis for the development of an extensive connected microvascular-like network. After detailed characterization of this network, we challenged its plasticity both during and after establishment of the skin substitute. We show that fine tuning of VEGF concentration and time of application differentially affects formation of capillary-like structures and their perivascular coverage. Furthermore, we performed a deep wound assay that displayed tissue repair and angiogenesis with unique characteristics of the physiological process. These studies demonstrate the importance of cell-derived microenvironment for the establishment of mature yet dynamic vascularized skin models allowing a wide range of pharmacological and basic investigations. STATEMENT OF SIGNIFICANCE: The significant advancements in organ-on-chips and tissue engineering call for more relevant models including microvascularization with remodeling potential. While vascularized skin substitutes have been developed for years, focus has primarily been on the impact of microvascularization on implantation rather than on its in vitro characterization. We here developed a cell sheet-based vascularized skin substitute relying on angiogenesis, i.e. growth of vessel-like structures within the 3D model, rather than solely on endothelial cell self-assembly. We then characterized :1/ vascularization after modulation of angiogenic factor VEGF during the substitute construction; -2/ angiogenesis associated to tissue repair after deep mechanical wounding. These studies establish a solid physiologically relevant model for further investigation of skin cell interactions and in vitro wound healing.

Multiscale Imaging to Monitor Functional SHED-Supported Engineered Vessels

Regeneration of orofacial tissues is hampered by the lack of adequate vascular supply. Implantation of in vitro engineered, prevascularized constructs has emerged as a strategy to allow the rapid vascularization of the entire graft. Given the angiogenic properties of dental pulp stem cells, we hereby established a preclinical model of prevascularized constructs loaded with stem cells from human exfoliating deciduous teeth (SHED) in a 3-dimensional-printed material and provided a functional analysis of their in vivo angiogenesis, vascular perfusion, and permeability. Three different cell-loaded collagen hydrogels (SHED-human umbilical vein endothelial cell [HUVEC], HUVEC with SHED-conditioned medium, and SHED alone) were cast in polylactic acid (PLA) grids and ectopically implanted in athymic mice. At day 10, in vivo positron emission tomography (PETscan) revealed a significantly increased uptake of radiotracer targeting activated endothelial cells in the SHED-HUVEC group compared to the other groups. At day 30, ex vivo micro-computed tomography imaging confirmed that SHED-HUVEC constructs had a significantly increased vascular volume compared to the other ones. Injection of species-specific lectins analyzed by 2-photon microscopy demonstrated blood perfusion of the engineered human vessels in both prevascularized groups. However, in vivo quantification showed increased vessel density in the SHED-HUVEC group. In addition, coinjection of fluorescent lectin and dextran revealed that prevascularization with SHED prevented vascular leakage, demonstrating the active role of SHED in the maturation of human-engineered microvascular networks. This preclinical study introduces a novel PLA prevascularized and implantable construct, along with an array of imaging techniques, to validate the ability of SHED to promote functional human-engineered vessels, further highlighting the interest of SHED for orofacial tissue engineering. Furthermore, this study validates the use of PETscan for the early detection of in vivo angiogenesis, which may be applied in the clinic to monitor the performance of prevascularized grafts.

Bioprinting of human dermal microtissues precursors as building blocks for endogenousin vitroconnective tissue manufacturing

The advent of 3D bioprinting technologies in tissue engineering has unlocked the potential to fabricatein vitrotissue models, overcoming the constraints associated with the shape limitations of preformed scaffolds. However, achieving an accurate mimicry of complex tissue microenvironments, encompassing cellular and biochemical components, and orchestrating their supramolecular assembly to form hierarchical structures while maintaining control over tissue formation, is crucial for gaining deeper insights into tissue repair and regeneration. Building upon our expertise in developing competent three-dimensional tissue equivalents (e.g. skin, gut, cervix), we established a two-step bottom-up approach involving the dynamic assembly of microtissue precursors (μTPs) to generate macroscopic functional tissue composed of cell-secreted extracellular matrix (ECM). To enhance precision and scalability, we integrated extrusion-based bioprinting technology into our established paradigm to automate, control and guide the coherent assembly ofμTPs into predefined shapes. Compared to cell-aggregated bioink, ourμTPs represent a functional unit where cells are embedded in their specific ECM.μTPs were derived from human dermal fibroblasts dynamically seeded onto gelatin-based microbeads. After 9 days,μTPs were suspended (50% v/v) in Pluronic-F127 (30% w/v) (µTP:P30), and the obtained formulation was loaded as bioink into the syringe of the Dr.INVIVO-4D6 extrusion based bioprinter.µTP:P30 bioink showed shear-thinning behavior and temperature-dependent viscosity (gel atT> 30 °C), ensuringµTPs homogenous dispersion within the gel and optimal printability. The bioprinting involved extruding several geometries (line, circle, and square) into Pluronic-F127 (40% w/v) (P40) support bath, leveraging its shear-recovery property. P40 effectively held the bioink throughout and after the bioprinting procedure, untilµTPs fused into a continuous connective tissue.µTPs fusion dynamics was studied over 8 days of culture, while the resulting endogenous construct underwent 28 days culture. Histological, immunofluorescence analysis, and second harmonic generation reconstruction revealed an increase in endogenous collagen and fibronectin production within the bioprinted construct, closely resembling the composition of the native connective tissues.

Rapid innervation and physiological epidermal regeneration by bioengineered dermis implanted in mouse

Tissue-engineered skin substitutes are promising tools to cover large and deep skin defects. However, the lack of a synergic and fast regeneration of the vascular network, nerves, and skin appendages limits complete skin healing and impairs functional recovery. It has been highlighted that an ideal skin substitute should mimic the structure of the native tissue to enhance clinical effectiveness. Here, we produced a pre-vascularized dermis (PVD) comprised of fibroblasts embedded in their own extracellular matrix (ECM) and a capillary-like network. Upon implantation in a mouse full-thickness skin defect model, we observed a very early innervation of the graft in 2 weeks. In addition, mouse capillaries and complete epithelialization were detectable as early as 1 week after implantation and, skin appendages developed in 2 weeks. These anatomical features underlie the interaction with the skin nerves, thus providing a further cue for reinnervation guidance. Further, the graft displays mechanical properties, collagen density, and assembly features very similar to the host tissue. Taken together our data show that the pre-existing ECM components of the PVD, physiologically organized and assembled similarly to the native tissue, support a rapid regeneration of dermal tissue. Therefore, our results suggest a promising potential for PVD in skin regeneration.

A heparin-functionalized bioink with sustained delivery of vascular endothelial growth factor for 3D bioprinting of prevascularized dermal constructs

Skin tissue engineering faces challenges due to the absence of vascular architecture, impeding the development of permanent skin replacements. To address this, a heparin-functionalized 3D-printed bioink (GH/HepMA) was formulated to enable sustained delivery of vascular endothelial growth factor (VEGF), comprising 0.3 % (w/v) hyaluronic acid (HA), 10 % (w/v) gelatin methacrylate (GelMA), and 0.5 % (w/v) heparin methacrylate (HepMA). The bioink was then used to print dermal constructs with angiogenic functions, including fibroblast networks and human umbilical vein endothelial cell (HUVEC) networks. GH/HepMA, with its covalently cross-linked structure, exhibits enhanced mechanical properties and heparin stability, allowing for a 21-day sustained delivery of VEGF. Cytocompatibility experiments showed that the GH/HepMA bioink supported fibroblast proliferation and promoted collagen I production. With VEGF present, the GH/HepMA bioink promoted HUVEC proliferation, migration, as well as the formation of a richer capillary-like network. Furthermore, HA within the GH/HepMA bioink enhanced rheological properties and printability. Additionally, 3D-bioprinted dermal constructs showed significant deposition of collagen I and III and mature stable capillary-like structures along the axial direction. In summary, this study offers a promising approach for constructing biomimetic multicellular skin substitutes with angiogenesis-induced functions.

Sulfated oligosaccharide activates endothelial Notch for inducing macrophage-associated arteriogenesis to treat ischemic diseases

Ischemic diseases lead to considerable morbidity and mortality, yet conventional clinical treatment strategies for therapeutic angiogenesis fall short of being impactful. Despite the potential of biomaterials to deliver pro-angiogenic molecules at the infarct site to induce angiogenesis, their efficacy has been impeded by aberrant vascular activation and off-target circulation. Here, we present a semisynthetic low-molecular sulfated chitosan oligosaccharide (SCOS) that efficiently induces therapeutic arteriogenesis with a spontaneous generation of collateral circulation and blood reperfusion in rodent models of hind limb ischemia and myocardial infarction. SCOS elicits anti-inflammatory macrophages' (Mφs') differentiation into perivascular Mφs, which in turn directs artery formation via a cell-to-cell communication rather than secretory factor regulation. SCOS-mediated arteriogenesis requires a canonical Notch signaling pathway in Mφs via the glycosylation of protein O-glucosyltransferases 2, which results in promoting arterial differentiation and tissue repair in ischemia. Thus, this highly bioactive oligosaccharide can be harnessed to direct efficiently therapeutic arteriogenesis and perfusion for the treatment of ischemic diseases.

Prevascularization techniques for dental pulp regeneration: potential cell sources, intercellular communication and construction strategies

One of the difficulties of pulp regeneration is the rapid vascularization of transplanted engineered tissue, which is crucial for the initial survival of the graft and subsequent pulp regeneration. At present, prevascularization techniques, as emerging techniques in the field of pulp regeneration, has been proposed to solve this challenge and have broad application prospects. In these techniques, endothelial cells and pericytes are cocultured to induce intercellular communication, and the cell coculture is then introduced into the customized artificial vascular bed or induced to self-assembly to simulate the interaction between cells and extracellular matrix, which would result in construction of a prevascularization system, preformation of a functional capillary network, and rapid reconstruction of a sufficient blood supply in engineered tissue after transplantation. However, prevascularization techniques for pulp regeneration remain in their infancy, and there remain unresolved problems regarding cell sources, intercellular communication and the construction of prevascularization systems. This review focuses on the recent advances in the application of prevascularization techniques for pulp regeneration, considers dental stem cells as a potential cell source of endothelial cells and pericytes, discusses strategies for their directional differentiation, sketches the mechanism of intercellular communication and the potential application of communication mediators, and summarizes construction strategies for prevascularized systems. We also provide novel ideas for the extensive application and follow-up development of prevascularization techniques for dental pulp regeneration.

Image-based modeling of vascular organization to evaluate anti-angiogenic therapy

In tumor therapy anti-angiogenic approaches have the potential to increase the efficacy of a wide variety of subsequently or co-administered agents, possibly by improving or normalizing the defective tumor vasculature. Successful implementation of the concept of vascular normalization under anti-angiogenic therapy, however, mandates a detailed understanding of key characteristics and a respective scoring metric that defines an improved vasculature and thus a successful attempt. Here, we show that beyond commonly used parameters such as vessel patency and maturation, anti-angiogenic approaches largely benefit if the complex vascular network with its vessel interconnections is both qualitatively and quantitatively assessed. To gain such deeper insight the organization of vascular networks, we introduce a multi-parametric evaluation of high-resolution angiographic images based on light-sheet fluorescence microscopy images of tumors. We first could pinpoint key correlations between vessel length, straightness and diameter to describe the regular, functional and organized structure observed under physiological conditions. We found that vascular networks from experimental tumors diverted from those in healthy organs, demonstrating the dysfunctionality of the tumor vasculature not only on the level of the individual vessel but also in terms of inadequate organization into larger structures. These parameters proofed effective in scoring the degree of disorganization in different tumor entities, and more importantly in grading a potential reversal under treatment with therapeutic agents. The presented vascular network analysis will support vascular normalization assessment and future optimization of anti-angiogenic therapy.

Tissue Engineering-Based Strategies for Diabetic Foot Ulcer Management

Significance: Diabetic foot ulcers (DFU) are a mounting problem with the increasingly frail population. Injuries that would otherwise heal are kept open by risk factors such as diabetes, obesity, and age-related conditions, which interferes with the natural wound healing processes. Recent Advances: This review summarizes recent advancements in the field of tissue engineering for the treatment of DFUs. FDA-approved approaches, including signaling-based therapies, stem cell therapies, and skin substitutes are summarized and cutting-edge experimental technologies that have the potential to manage chronic wounds, such as skin printing, skin organogenesis, skin self-assembly, and prevascularization, are discussed. Critical Issues: The standard of care for chronic wounds involves wound debridement, wound dressings, and resolving the underlying cause such as lowering the glycemic index and reducing wound pressure. Current DFU treatments are limited by low wound closure rates and poor regrown skin quality. New adjuvant therapies that facilitate wound closure in place of or in conjunction with standard care are critically needed. Future Directions: Tissue engineering strategies are limited by the plasticity of adult human cells. In addition to traditional techniques, genetic modification, although currently an emerging technology, has the potential to unlock human regeneration and can be incorporated in future therapeutics.

Development of fibroblast/endothelial cell-seeded collagen scaffolds for in vitro prevascularization

The development of vascularized scaffolds remains one of the major challenges in tissue engineering, and co-culturing with endothelial cells is known as one of the possible approaches for this purpose. In this approach, optimization of cell culture conditions, scaffolds, and fabrication techniques is needed to develop tissue equivalents that will enable in vitro formation of a capillary network. Prevascularized equivalents will be more physiologically comparable to the native tissues and potentially prevent insufficient vascularization after implantation. This study aimed to culture human umbilical vein endothelial cells (HUVECs), alone or in co-culture with fibroblasts, on collagen scaffolds prepared by simple fabrication approaches for in vitro prevascularization. Different concentrations and ratios of HUVECs and fibroblasts seeded on collagen gel and sponge scaffolds under several culture conditions were examined. Cell viability, scaffolds morphology, and structure were analyzed. Collagen gel scaffolds showed good cell proliferation and viability, with higher proliferation rates for cells cultured in a 2:1 (fibroblasts: HUVECs) ratio and kept in endothelial cell growth medium. However, these matrices were unable to support endothelial cell sprouting. Collagen sponges were highly porous and showed good cell viability. However, they became fragile over time in culture, and they still lack signs of vascularization. Collagen scaffolds were a good platform for cell growth and viability. However, under the experimental conditions of this study, the HUVEC/fibroblast-seeded scaffolds were not suitable platforms to generate in vitro prevascularized equivalents. Our findings will be a valuable starting point to optimize culture microenvironments and scaffolds during fabrication of prevascularized scaffolds.

Understanding the mechanisms of spontaneous and skin-grafted wound repair: the path to engineered skin grafts

Spontaneous wound repair is a complex process that involves overlapping phases of inflammation, proliferation and remodelling, co-ordinated by growth factors and proteases. In extensive wounds such as burns, the repair process would not be achieved in a timely fashion unless grafted. Although spontaneous wound repair has been extensively described, the processes by which wound repair mechanisms mediate graft take are yet to be fully explored. This review describes engraftment stages and summarises current understanding of molecular mechanisms which regulate autologous skin graft healing, with the goal of directing innovation in permanent wound closure with skin substitutes. Graftability and vascularisation of various skin substitutes that are either in the market or in development phase are discussed. In doing so, we cast a spotlight on the paucity of scientific information available as to how skin grafts (both autologous and engineered) heal a wound bed. Better understanding of these processes may assist in developing novel methods of wound management and treatments.

Collagen-Based Biomimetic Systems to Study the Biophysical Tumour Microenvironment

The extracellular matrix (ECM) is a pericellular network of proteins and other molecules that provides mechanical support to organs and tissues. ECM biophysical properties such as topography, elasticity and porosity strongly influence cell proliferation, differentiation and migration. The cell's perception of the biophysical microenvironment (mechanosensing) leads to altered gene expression or contractility status (mechanotransduction). Mechanosensing and mechanotransduction have profound implications in both tissue homeostasis and cancer. Many solid tumours are surrounded by a dense and aberrant ECM that disturbs normal cell functions and makes certain areas of the tumour inaccessible to therapeutic drugs. Understanding the cell-ECM interplay may therefore lead to novel and more effective therapies. Controllable and reproducible cell culturing systems mimicking the ECM enable detailed investigation of mechanosensing and mechanotransduction pathways. Here, we discuss ECM biomimetic systems. Mainly focusing on collagen, we compare and contrast structural and molecular complexity as well as biophysical properties of simple 2D substrates, 3D fibrillar collagen gels, cell-derived matrices and complex decellularized organs. Finally, we emphasize how the integration of advanced methodologies and computational methods with collagen-based biomimetics will improve the design of novel therapies aimed at targeting the biophysical and mechanical features of the tumour ECM to increase therapy efficacy.

Spatial patterning of PCL µ-scaffolds directs 3D vascularized bio-construct morphogenesis in vitro

Modular tissue engineering (mTE) strategies aim to build three-dimensional tissue analogues in vitro by the sapient combination of cells, micro-scaffolds (μ-scaffs) and bioreactors. The translation of these newly engineered tissues into current clinical approaches is, among other things, dependent on implant-to-host microvasculature integration, a critical issue for cells and tissue survival in vivo. In this work we reported, for the first time, a computer-aided modular approach suitable to build fully vascularized hybrid (biological/synthetic) constructs (bio-constructs) with micro-metric size scale control of blood vessels growth and orientation. The approach consists of four main steps, starting with the fabrication of polycaprolactone μ-scaffs by fluidic emulsion technique, which exhibit biomimetic porosity features. In the second step, layers of μ-scaffs following two different patterns, namely ordered and disordered, were obtained by a soft lithography-based process. Then, the as obtained μ-scaff patterns were used as template for human dermal fibroblasts and human umbilical vein endothelial cells co-culture, aiming to promote and guide the biosynthesis of collagenous extracellular matrix and the growth of new blood vessels within the mono-layered bio-constructs. Finally, bi-layered bio-constructs were built by the alignment, stacking and fusion of two vascularized mono-layered samples featuring ordered patterns. Our results demonstrated that, if compared to the disordered pattern, the ordered one provided better control over bio-constructs shape and vasculature architecture, while minor effect was observed with respect to cell colonization and new tissue growth. Furthermore, by assembling two mono-layered bio-constructs it was possible to build 1-mm thick fully vascularized viable bio-constructs and to study tissue morphogenesis during 1 week of in vitro culture. In conclusion, our results highlighted the synergic role of μ-scaff architectural features and spatial patterning on cells colonization and biosynthesis, and pay the way for the possibility to create in silico designed vasculatures within modularly engineered bio-constructs.

Strategies for vascularized skin models in vitro

As the largest organ of the human body, the skin has a complex multi-layered structure. The composition of the skin includes cells, extracellular matrix (ECM), vascular networks, and other appendages. Because of the shortage of donor sites, skin substitutes are of great significance in the field of skin tissue repair. Moreover, skin models for disease research, drug screening, and cosmetic testing fall far short of the demand. Skin tissue engineering has made remarkable progress in developing skin models over the years. However, there are still several problems to be resolved. One of the crucial aspects is the lack of vascular systems for nutrient transport and waste disposal. Here, we will focus on the discussion and analysis of advanced manufacturing strategies for prevascularized skin, such as a scaffold-based method, cell coating technology, cell sheet engineering, skin-on-a-chip, and three-dimensional (3D) bioprinting. These key challenges, which restrict the prevascularized skin and provide perspectives on future directions will also be highlighted.

Bioprinting and plastic compression of large pigmented and vascularized human dermo-epidermal skin substitutes by means of a new robotic platform

Extensive availability of engineered autologous dermo-epidermal skin substitutes (DESS) with functional and structural properties of normal human skin represents a goal for the treatment of large skin defects such as severe burns. Recently, a clinical phase I trial with this type of DESS was successfully completed, which included patients own keratinocytes and fibroblasts. Yet, two important features of natural skin were missing: pigmentation and vascularization. The first has important physiological and psychological implications for the patient, the second impacts survival and quality of the graft. Additionally, accurate reproduction of large amounts of patient’s skin in an automated way is essential for upscaling DESS production. Therefore, in the present study, we implemented a new robotic unit (called SkinFactory) for 3D bioprinting of pigmented and pre-vascularized DESS using normal human skin derived fibroblasts, blood- and lymphatic endothelial cells, keratinocytes, and melanocytes. We show the feasibility of our approach by demonstrating the viability of all the cells after printing in vitro, the integrity of the reconstituted capillary network in vivo after transplantation to immunodeficient rats and the anastomosis to the vascular plexus of the host. Our work has to be considered as a proof of concept in view of the implementation of an extended platform, which fully automatize the process of skin substitution: this would be a considerable improvement of the treatment of burn victims and patients with severe skin lesions based on patients own skin derived cells.

Collagen triple helix repeat containing-1 promotes functional recovery of sweat glands by inducing adjacent microvascular network reconstruction in vivo

Background

Sweat glands (SGs) have low regenerative potential after severe burns or trauma and their regeneration or functional recovery still faces many obstacles. In practice, restoring SG function requires not only the structural integrity of the gland itself, but also its neighboring tissues, especially blood vessels. Collagen triple helix repeat containing-1 (CTHRC1) was first identified in vascular repair, and increasing reports showed a close correlation between cutaneous appendage specification, patterning and regeneration. The purpose of the present study was to clarify the role of CTHRC1 in SGs and their adjacent microvessels and find therapeutic strategies to restore SG function.

Methods

The SGs and their adjacent microvascular network of Cthrc1^−/− mice were first investigated using sweat test, laser Doppler imaging, tissue clearing technique and transcriptome analysis. The effects of CTHRC1 on dermal microvascular endothelial cells (DMECs) were further explored with cell proliferation, DiI-labeled acetylated low-density lipoprotein uptake, tube formation and intercellular junction establishment assays. The effects of CTHRC1 on SG function restoration were finally confirmed by replenishing the protein into the paws of Cthrc1^−/− mice.

Results

CTHRC1 is a key regulator of SG function in mice. At the tissue level, Cthrc1 deletion resulted in the disorder and reduction of the microvascular network around SGs. At the molecular level, the knockout of Cthrc1 reduced the expression of vascular development genes and functional proteins in the dermal tissues. Furthermore, CTHRC1 administration considerably enhanced SG function by inducing adjacent vascular network reconstruction.

Conclusions

CTHRC1 promotes the development, morphogenesis and function execution of SGs and their neighboring vasculature. Our study provides a novel target for the restoration or regeneration of SG function in vivo.

Oxygenation therapies for improved wound healing: Current trends and technologies

Degree of oxygenation is one of the important parameters governing various processes, including cell proliferation, angiogenesis, extracellular matrix production, and even combating the microbial burden at the wound site, all...

3D-printed dermis-specific extracellular matrix mitigates scar contraction via inducing early angiogenesis and macrophage M2 polarization

Scar contraction frequently happens in patients with deep burn injuries. Hitherto, porcine dermal extracellular matrix (dECM) has supplied microenvironments that assist in wound healing but fail to inhibit scar contraction. To overcome this drawback, we integrate dECM into three-dimensional (3D)-printed dermal analogues (PDA) to prevent scar contraction. We have developed thermally gelled, non-rheologically modified dECM powder (dECMp) inks and successfully transformed them into PDA that was endowed with a micron-scale spatial structure. The optimal crosslinked PDA exhibited desired structure, good mechanical properties as well as excellent biocompatibility. Moreover, in vivo experiments demonstrated that PDA could significantly reduced scar contraction and improved cosmetic upshots of split thickness skin grafts (STSG) than the commercially available dermal templates and STSG along. The PDA has also induced an early, intense neovascularization, and evoked a type-2-like immune response. PDA's superior beneficial effects may attribute to their desired porous structure, the well-balanced physicochemical properties, and the preserved dermis-specific ECM cues, which collectively modulated the expression of genes such as Wnt11, ATF3, and IL1β, and influenced the crucial endogenous signalling pathways. The findings of this study suggest that PDA is a clinical translatable material that possess high potential in reducing scar contraction.

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Current dermal analogues have supplied microenvironments that assist in wound healing but cannot inhibit scar contraction.

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dECMp ink was formulated and transformed into PDA endowed with a micron-scale designed spatial structure.

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The PDAs were neatly superior to split thickness skin grafts and commercial dermal templates in hindering scar contraction.

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The transcriptome data may reveal how at the molecular level the IS and skin wounds respond to biomaterial stimuli.

Engineering injectable vascularized tissues from the bottom-up: Dynamics of in-gel extra-spheroid dermal tissue assembly

Modular tissue engineering approaches open up exciting perspectives for the biofabrication of vascularized tissues from the bottom-up, using micro-sized units such as spheroids as building blocks. While several techniques for 3D spheroid formation from multiple cell types have been reported, strategies to elicit the extra-spheroid assembly of complex vascularized tissues are still scarce. Here we describe an injectable approach to generate vascularized dermal tissue, as an example application, from spheroids combining fibroblasts and endothelial progenitors (OEC) in a xeno-free (XF) setting. Short-term cultured spheroids (1 day) were selected over mature spheroids (7 days), as they showed significantly higher angiogenic sprouting potential. Embedding spheroids in fibrin was crucial for triggering cell migration into the external milieu, while providing a 3D framework for in-gel extra-spheroid morphogenesis. Migrating fibroblasts proliferated and produced endogenous ECM forming a dense tissue, while OEC self-assembled into stable capillaries with lumen and basal lamina. Massive in vitro interconnection between sprouts from neighbouring spheroids rapidly settled an intricate vascular plexus. Upon injection into the chorioallantoic membrane of chick embryos, fibrin-entrapped pre-vascularized XF spheroids developed into a macrotissue with evident host vessel infiltration. After only 4 days, perfused chimeric capillaries with human cells were present in proximal areas, showing fast and functional inosculation between host and donor vessels. This method for generating dense vascularized tissue from injectable building blocks is clinically relevant and potentially useful for a range of applications.

Geometrical confinement controls cell, ECM and vascular network alignment during the morphogenesis of 3D bioengineered human connective tissues

Engineered tissues featuring aligned ECM possess superior regenerative capabilities for the healing of damaged aligned tissues. The morphofunctional integration in the host's injury site improves if the aligned ECM elicits the unidirectional growth of vascular network. In this work we used a bottom-up tissue engineering strategy to produce endogenous and highly aligned human connective tissues with the final aim to trigger the unidirectional growth of capillary-like structures. Engineered microtissues, previously developed by our group, were casted in molds featured by different aspect ratio (AR) to obtain final centimeter-sized macrotissues differently shaped. By varying the AR from 1 to 50 we were able to vary the final shape of the macrotissues, from square to wire. We demonstrated that by increasing the AR of the maturation space hosting the microtissues, it was possible to control the alignment of the neo-synthesized ECM. The geometrical confinement conditions at AR = 50, indeed, promoted the unidirectional growth and assembly of the collagen network. The wire-shaped tissues were characterized by parallel arrangement of the collagen fiber bundles, higher persistence length and speed of migrating cells and superior mechanical properties than the square-shaped macrotissues. Interestingly, the aligned collagen fibers elicited the unidirectional growth of capillary-like structures. STATEMENT OF SIGNIFICANCE: Alignment of preexisting extracellular matrices by using mechanical cues modulating cell traction, has been widely described. Here, we show a new method to align de novo synthesized extracellular matrix components in bioengineered connective tissues obtained by means of a bottom-up tissue engineering approach. Building blocks are cast in maturation chambers, having different aspect ratios, in which the in vitro morphogenesis process takes place. High aspect ratio chambers (corresponding to wire-shaped tissues) triggered spontaneous alignment of collagenous network affecting cell polarization, migration and tensile properties of the tissue as well. Aligned ECM provided a contact guidance for the formation of highly polarized capillary-like network suggesting an in vivo possible application to trigger fast angiogenesis and perfusion in damaged aligned tissues.

3D Printing of Strontium Silicate Microcylinder-Containing Multicellular Biomaterial Inks for Vascularized Skin Regeneration

The reconstruction of dermal blood vessels is essential for skin regeneration process. However, the lack of vascular structure, insufficient angiogenesis induction, and ineffective graft-host anastomosis of the existing skin substitutes are major bottle-necks for permanent skin replacement in tissue engineering. In this study, the uniform strontium silicate (SS) microcylinders are successfully synthesized and integrated into the biomaterial ink to serve as stable cell-induced factors for angiogenesis, and then a functional skin substitute based on a vascularization-induced biomimetic multicellular system is prepared via a "cell-writing" bioprinting technology. With an unprecedented combination of vascularized skin-mimicking structure and vascularization-induced function, the SS-containing multicellular system exhibits outstanding angiogenic activity both in vitro and in vivo. As a result, the bioprinted skin substitutes significantly accelerate the healing of both acute and chronic wounds by promoting the graft-host integration and vascularized skin regeneration in three animal models. Therefore, the study provides a referable strategy to fabricate biomimetic multicellular constructs with angiogenesis-induced function for regeneration of vascularized complex and hierarchical tissues.

Cellular human tissue-engineered skin substitutes investigated for deep and difficult to heal injuries

Wound healing is an important function of skin; however, after significant skin injury (burns) or in certain dermatological pathologies (chronic wounds), this important process can be deregulated or lost, resulting in severe complications. To avoid these, studies have focused on developing tissue-engineered skin substitutes (TESSs), which attempt to replace and regenerate the damaged skin. Autologous cultured epithelial substitutes (CESs) constituted of keratinocytes, allogeneic cultured dermal substitutes (CDSs) composed of biomaterials and fibroblasts and autologous composite skin substitutes (CSSs) comprised of biomaterials, keratinocytes and fibroblasts, have been the most studied clinical TESSs, reporting positive results for different pathological conditions. However, researchers' purpose is to develop TESSs that resemble in a better way the human skin and its wound healing process. For this reason, they have also evaluated at preclinical level the incorporation of other human cell types such as melanocytes, Merkel and Langerhans cells, skin stem cells (SSCs), induced pluripotent stem cells (iPSCs) or mesenchymal stem cells (MSCs). Among these, MSCs have been also reported in clinical studies with hopeful results. Future perspectives in the field of human-TESSs are focused on improving in vivo animal models, incorporating immune cells, designing specific niches inside the biomaterials to increase stem cell potential and developing three-dimensional bioprinting strategies, with the final purpose of increasing patient's health care. In this review we summarize the use of different human cell populations for preclinical and clinical TESSs under research, remarking their strengths and limitations and discuss the future perspectives, which could be useful for wound healing purposes.

Mechano-regulation of vascular network formation without branches in 3D bioprinted cell-laden hydrogel constructs

Restoration of a wound is a common surgical procedure in clinic. Currently, the skin required for clinical use is taken from the patient's own body. However, it can be difficult to obtain enough skin sources for large-sized wounds and thus surgeons have started using commercial skin substitutes. The current commercial skin, which includes epidermis substitute, dermis substitute, and bilateral skin substitute, has been popularized in clinic. However, the application is limited by the occurrence of ischemia necrosis after transplantation. Recent studies suggest the use of pre-vascularized skin substitutes for wound healing is a promising area in the research field of skin tissue engineering. Pre-vascularization can be induced by changes in cultivation periods, exertion of mechanical stimuli, or coculture with endothelial cells and various factors. However, few methods could control the formation of vascular branches in engineering tissue in a self-assembly way. In this study, we use three-dimensional (3D) printing technology to confirm that a mechanical force can control the growth of blood vessels in the direction of mechanical stimulation with no branches, and that Yes-associated protein activity is involved in the regulatory progress. In vivo experiments verified that the blood vessels successfully function for blood circulation, and maintain the same direction. Results provide a theoretical basis for products of pre-vascularized skin tissues and other organs created by 3D bioprinting.

Bioinspired Design of Novel Microscaffolds for Fibroblast Guidance toward In Vitro Tissue Building

Porous microscaffolds (μ-scaffs) play a crucial role in modular tissue engineering as they control cell functions and guide hierarchical tissue formation toward building new functional tissue analogues. In the present study, we developed a new route to prepare porous polycaprolactone (PCL) μ-scaffs with a bioinspired trabecular structure that supported in vitro adhesion, growth, and biosynthesis of human dermal fibroblasts (HDFs). The method involved the use of poly(ethylene oxide) (PEO) as a biocompatible porogen and a fluidic emulsion/porogen leaching/particle coagulation process to obtain spherical μ-scaffs with controllable diameter and full pore interconnectivity. To achieve this objective, we investigated the effect of PEO concentration and the temperature of the coagulation bath on the μ-scaff architecture, while we modulated the μ-scaff diameter distribution by varying the PCL-PEO amount in the starting solution and changing the flow rate of the continuous phase (Q_CP). μ-Scaff morphology, pore architecture, and diameter distribution were assessed using scanning electron microscopy (SEM) analysis, microcomputed tomography (microCT), and Image analysis. We reported that the selection of 60 wt % PEO concentration, together with a 4 °C coagulation bath temperature and ultrasound postprocessing, allowed for the design and fabrication of μ-scaff with porosity up to 80% and fully interconnected pores on both the μ-scaff surface and the core. Furthermore, μ-scaff diameter distributions were finely tuned in the 100-600 μm range with the coefficient of variation lower than 5% by selecting the PCL-PEO concentration in the 1-10% w/v range and Q_CP of either 8 or 18 mL/min. Finally, we investigated the capability of the HDF-seeded PCL μ-scaff to form hybrid (biological/synthetic) tissue in vitro. Cell culture tests demonstrated that PCL μ-scaff enabled HDF adhesion, proliferation, colonization, and collagen biosynthesis within inter- and intraparticle spaces and guided the formation of a large (centimeter-sized) viable tissue construct.

Microvascular maturation by mesenchymal stem cells in vitro improves blood perfusion in implanted tissue constructs

Blood perfusion of grafted tissue constructs is a hindrance to the success of stem cell-based therapies by limiting cell survival and tissue regeneration. Implantation of a pre-vascularized network engineered in vitro has thus emerged as a promising strategy for promoting blood supply deep into the construct, relying on inosculation with the host vasculature. We aimed to fabricate in vitro tissue constructs with mature microvascular networks, displaying perivascular recruitment and basement membrane, taking advantage of the angiogenic properties of dental pulp stem cells and self-assembly of endothelial cells into capillaries. Using digital scanned light-sheet microscopy, we characterized the generation of dense microvascular networks in collagen hydrogels and established parameters for quantification of perivascular recruitment. We also performed original time-lapse analysis of stem cell recruitment. These experiments demonstrated that perivascular recruitment of dental pulp stem cells is driven by PDGF-BB. Recruited stem cells participated in deposition of vascular basement membrane and vessel maturation. Mature microvascular networks thus generated were then compared to those lacking perivascular coverage generated using stem cell conditioned medium. Implantation in athymic nude mice demonstrated that in vitro maturation of microvascular networks improved blood perfusion and cell survival within the construct. Taken together, these data demonstrate the strong potential of in vitro production of mature microvasculature for improving cell-based therapies.

Wound healing: cellular mechanisms and pathological outcomes

Wound healing is a complex, dynamic process supported by a myriad of cellular events that must be tightly coordinated to efficiently repair damaged tissue. Derangement in wound-linked cellular behaviours, as occurs with diabetes and ageing, can lead to healing impairment and the formation of chronic, non-healing wounds. These wounds are a significant socioeconomic burden due to their high prevalence and recurrence. Thus, there is an urgent requirement for the improved biological and clinical understanding of the mechanisms that underpin wound repair. Here, we review the cellular basis of tissue repair and discuss how current and emerging understanding of wound pathology could inform future development of efficacious wound therapies.

Intrinsic Abnormalities of Cystic Fibrosis Airway Connective Tissue Revealed by an In Vitro 3D Stromal Model

Cystic fibrosis is characterized by lung dysfunction involving mucus hypersecretion, bacterial infections, and inflammatory response. Inflammation triggers pro-fibrotic signals that compromise lung structure and function. At present, several in vitro cystic fibrosis models have been developed to study epithelial dysfunction but none of these focuses on stromal alterations. Here we show a new cystic fibrosis 3D stromal lung model made up of primary fibroblasts embedded in their own extracellular matrix and investigate its morphological and transcriptomic features. Cystic fibrosis fibroblasts showed a high proliferation rate and produced an abundant and chaotic matrix with increased protein content and elastic modulus. More interesting, they had enhanced pro-fibrotic markers and genes involved in epithelial function and inflammatory response. In conclusion, our study reveals that cystic fibrosis fibroblasts maintain in vitro an activated pro-fibrotic state. This abnormality may play in vivo a role in the modulation of epithelial and inflammatory cell behavior and lung remodeling. We argue that the proposed bioengineered model may provide new insights on epithelial/stromal/inflammatory cells crosstalk in cystic fibrosis, paving the way for novel therapeutic strategies.

Fabrication of a Microfluidic System Using Micromolded Alginate Gel as a Sacrificial Material for Tissues Engineering

We described a sacrificial molding for the formation of microfluidic networks. In this molding, the micromolded calcium alginate (Ca-Alg) is introduced as a sacrificial template. The basis of this procedure is fabricating a micromolded Ca-Alg hydrogel and encapsulating this model within a second gel and removing it by ion-exchange to leave a microchannel in the remaining gel. This microfluidic system can readily deliver solutes into the channels and even control the transport of solutes from channels into the bulk of the gels. Furthermore, the perfused vascular channels can sustain the metabolic activity of encapsulated cells, indicating the feasibility of this microfluidic system in the field of tissue engineering.

In Vitro Organotypic Systems to Model Tumor Microenvironment in Human Papillomavirus (HPV)-Related Cancers

Despite the well-known role of chronic human papillomavirus (HPV) infections in causing tumors (i.e., all cervical cancers and other human malignancies from the mucosal squamous epithelia, including anogenital and oropharyngeal cavity), its persistence is not sufficient for cancer development. Other co-factors contribute to the carcinogenesis process. Recently, the critical role of the underlying stroma during the HPV life cycle and HPV-induced disease have been investigated. The tumor stroma is a key component of the tumor microenvironment (TME), which is a specialized entity. The TME is dynamic, interactive, and constantly changing-able to trigger, support, and drive tumor initiation, progression, and metastasis. In previous years, in vitro organotypic raft cultures and in vivo genetically engineered mouse models have provided researchers with important information on the interactions between HPVs and the epithelium. Further development for an in-depth understanding of the interaction between HPV-infected tissue and the surrounding microenvironment is strongly required. In this review, we critically describe the HPV-related cancers modeled in vitro from the simplified 'raft culture' to complex three-dimensional (3D) organotypic models, focusing on HPV-associated cervical cancer disease platforms. In addition, we review the latest knowledge in the field of in vitro culture systems of HPV-associated malignancies of other mucosal squamous epithelia (anogenital and oropharynx), as well as rare cutaneous non-melanoma associated cancer.

Rapid Fabrication of Ready-to-Use Gelatin Scaffolds with Prevascular Networks Using Alginate Hollow Fibers as Sacrificial Templates

In this study, we developed a facile manufacturing method for interconnected prevascular networks using calcium chloride (CaCl₂) cross-linked alginate hollow fibers as sacrificial templates. The resulting network can be used to deliver oxygen and nutrients and remove waste for embedded cells in large-volume gelatin scaffolds during in vitro culturing. The sacrificial templates were printed by customized coaxial nozzles and embedded in scaffolds made of a mixture of gelatin, microbial transglutaminase (mTG), and sodium citrate. During the cross-linking of gelatin and mTG, the sacrificial templates started to dissolve from the scaffold-template interface due to the presence of the sodium citrate in the gelatin. The embedded sacrificial templates were completely dissolved without any postprocessing, and the designed prevascular networks successfully retained their geometries and dimensions. No residue of the template was observed at the scaffold-template interface after dissolution, which promoted cell adhesion. This manufacturing method has a high degree of freedom in templates' geometry, which was demonstrated by fabricating prevascular networks with various designs, including grid, branched, and dendritic networks. The effects of hollow fiber size and sodium citrate concentration on the dissolution time were analyzed. Human umbilical vein endothelial cells were injected into the aforementioned networks and formed a confluent endothelial cell monolayer with high viability during the culture process. The results suggest great promise to rapidly build large-scale ready-to-use gelatin scaffolds with prevascular networks for the applications in tissue engineering.

Vascularization strategies for skin tissue engineering

Lack of proper vascularization after skin trauma causes delayed wound healing. This has sparked the development of various tissue engineering strategies to improve vascularization.

Bioengineered Skin Substitutes: the Role of Extracellular Matrix and Vascularization in the Healing of Deep Wounds

The formation of severe scars still represents the result of the closure process of extended and deep skin wounds. To address this issue, different bioengineered skin substitutes have been developed but a general consensus regarding their effectiveness has not been achieved yet. It will be shown that bioengineered skin substitutes, although representing a valid alternative to autografting, induce skin cells in repairing the wound rather than guiding a regeneration process. Repaired skin differs from regenerated skin, showing high contracture, loss of sensitivity, impaired pigmentation and absence of cutaneous adnexa (i.e., hair follicles and sweat glands). This leads to significant mobility and aesthetic concerns, making the development of more effective bioengineered skin models a current need. The objective of this review is to determine the limitations of either commercially available or investigational bioengineered skin substitutes and how advanced skin tissue engineering strategies can be improved in order to completely restore skin functions after severe wounds.

Poly(lactic-co-glycolic acid)-bioactive glass composites as nanoporous scaffolds for bone tissue engineering: In vitro and in vivo studies

The aim of the present study was to investigate the feasibility of using composite scaffolds of poly(lactic-co-glycolic acid) (PLGA) and bioactive glass (BG) to repair bone defects. PLGA/BG composite scaffolds were prepared by thermally-induced phase separation. Scanning electron microscopy (SEM) was used to study the morphology, and liquid (absolute ethanol) replacement was used to calculate the porosity of the scaffold. The biocompatibility and degradation of the scaffold were determined using human osteosarcoma cell line MG-63 and animal experiments. SEM showed that the scaffold had a nanofibrous three-dimensional network structure with a fiber diameter of 160-320 nm, a pore size of 1-7 µm, and a porosity of 93.048±0.121%. The scaffold structure was conducive to cell adhesion and proliferation. It promoted cell osteogenesis and could be stably degraded in vivo.

Modular Strategies to Build Cell-Free and Cell-Laden Scaffolds towards Bioengineered Tissues and Organs

Engineering three-dimensional (3D) scaffolds for functional tissue and organ regeneration is a major challenge of the tissue engineering (TE) community. Great progress has been made in developing scaffolds to support cells in 3D, and to date, several implantable scaffolds are available for treating damaged and dysfunctional tissues, such as bone, osteochondral, cardiac and nerve. However, recapitulating the complex extracellular matrix (ECM) functions of native tissues is far from being achieved in synthetic scaffolds. Modular TE is an intriguing approach that aims to design and fabricate ECM-mimicking scaffolds by the bottom-up assembly of building blocks with specific composition, morphology and structural properties. This review provides an overview of the main strategies to build synthetic TE scaffolds through bioactive modules assembly and classifies them into two distinct schemes based on microparticles (µPs) or patterned layers. The µPs-based processes section starts describing novel techniques for creating polymeric µPs with desired composition, morphology, size and shape. Later, the discussion focuses on µPs-based scaffolds design principles and processes. In particular, starting from random µPs assembly, we will move to advanced µPs structuring processes, focusing our attention on technological and engineering aspects related to cell-free and cell-laden strategies. The second part of this review article illustrates layer-by-layer modular scaffolds fabrication based on discontinuous, where layers' fabrication and assembly are split, and continuous processes.

Induced Pluripotent Stem Cells as Vasculature Forming Entities

Tissue engineering (TE) pursues the ambitious goal to heal damaged tissues. One of the most successful TE approaches relies on the use of scaffolds specifically designed and fabricated to promote tissue growth. During regeneration the guidance of biological events may be essential to sustain vasculature neoformation inside the engineered scaffold. In this context, one of the most effective strategies includes the incorporation of vasculature forming cells, namely endothelial cells (EC), into engineered constructs. However, the most common EC sources currently available, intended as primary cells, are affected by several limitations that make them inappropriate to personalized medicine. Human induced Pluripotent Stem Cells (hiPSC), since the time of their discovery, represent an unprecedented opportunity for regenerative medicine applications. Unfortunately, human induced Pluripotent Stem Cells-Endothelial Cells (hiPSC-ECs) still display significant safety issues. In this work, we reviewed the most effective protocols to induce pluripotency, to generate cells displaying the endothelial phenotype and to perform an efficient and safe cell selection. We also provide noteworthy examples of both in vitro and in vivo applications of hiPSC-ECs in order to highlight their ability to form functional blood vessels. In conclusion, we propose hiPSC-ECs as the preferred source of endothelial cells currently available in the field of personalized regenerative medicine.

Biofabrication of thick vascularized neo-pedicle flaps for reconstructive surgery

Wound chronicity due to intrinsic and extrinsic factors perturbs adequate lesion closure and reestablishment of the protective skin barrier. Immediate and proper care of chronic wounds is necessary for a swift recovery and a reduction of patient vulnerability to infection. Advanced therapies supplemented with standard wound care procedures have been clinically implemented to restore aberrant tissue; however, these treatments are ineffective if local vasculature is too compromised to support minimally-invasive strategies. Autologous "flaps", which are tissues equipped with their own hierarchical vascular supply, can be harvested from one region of the patient and transplanted to the wound where it is reperfused upon microsurgical anastomosis to appropriate recipient vessels. Despite the success of autologous flap transfer, these procedures are extremely invasive, incur obligatory donor-site morbidity, and require sufficient donor-tissue availability, microsurgical expertise, and specialized equipment. 3D-bioprinting modalities, such as extrusion-based bioprinting, can be used to address the clinical constraints of autologous flap transfer, primarily addressing donor-site morbidity and tissue availability. This advancement in regenerative medicine allows the biofabrication of heterogeneous tissue structures with high shape fidelity and spatial resolution to generate biomimetic constructs with the anatomically-precise geometries of native tissue to ensure tissue-specific function. Yet, meaningful progress toward this clinical application has been limited by the lack of vascularization required to meet the nutrient and oxygen demands of clinically relevant tissue volumes. Thus, various criteria for the fabrication of functional tissues with hierarchical, patent vasculature must be considered when implementing 3D-bioprinting technologies for deep, chronic wounds.

Aligned fibrous decellularized cell derived matrices for mesenchymal stem cell amplification

Biochemical and biophysical stimuli of stem cell niches finely regulate the self-renewal/differentiation equilibrium. Replicating this in vitro is technically challenging, making the control of stem cell functions difficult. Cell derived matrices capture certain aspect of niches that influence fate decisions. Here, aligned fibrous matrices synthesized by MC3T3 cells were produced and the role of matrix orientation and stiffness on the maintenance of stem cell characteristics and adipo- or osteo-genic differentiation of murine mesenchymal stem cells (mMSCs) was investigated. Decellularized matrices promoted mMSC proliferation. Fibrillar alignment and matrix stiffness work in concert in defining cell fate. Soft matrices preserve stemness, whereas stiff ones, in presence of biochemical supplements, promptly induce differentiation. Matrix alignment impacts the homogeneity of the cell population, that is, soft aligned matrices ameliorate the spontaneous adipogenic differentiation, whereas stiff aligned matrices reduce cross-differentiation. We infer that mechanical signaling is a dominant factor in mMSC fate decision and the matrix alignment contributes to produce a more homogeneous environment, which results in a uniform response of cells to biophysical environment. Matrix thus produced can be obtained in vitro in a facile and consistent manner and can be used for homogeneous stem cell amplification or for mechanotransduction-related studies.

Direct-write and sacrifice-based techniques for vasculatures

Fabricating biomimetic vasculatures is considered one of the greatest challenges in tissue regeneration due to their complex structures across various length scales. Many strategies have been investigated on how to fabricate tissue-engineering vasculatures (TEVs), including vascular-like and vascularized structures that can replace their native counterparts. The advancement of additive manufacturing (AM) technologies has enabled a wide range of fabrication techniques that can directly-write TEVs with complex and delicate structures. Meanwhile, sacrifice-based techniques, which rely on the removal of encapsulated sacrificial templates to form desired cavity-like structures, have also been widely studied. This review will specifically focus on the two most promising methods in these recently developed technologies, which are the direct-write method and the sacrifice-based method. The performance, advantages, and shortcomings of each technique are analyzed and compared. In the discussion, we list current challenges in this field and present our vision of next-generation TEVs technologies. Perspectives on future research in this field are given at the end.