Prevascular structures promote vascularization in engineered human adipose tissue constructs upon implantation

Engineering Cell Microenvironment Using Nanopattern-Derived Multicellular Spheroids and Photo-Crosslinked Gelatin/Hyaluronan Hydrogels

Cell cultures of dispersed cells within hydrogels depict the interaction of the cell-extracellular matrix (ECM) in 3D, while the coculture of different cells within spheroids combines both the effects of cell-cell and cell-ECM interactions. In this study, the cell co-spheroids of human bone mesenchymal stem cells/human umbilical vein endothelial cells (HBMSC/HUVECs) are prepared with the assistance of a nanopattern, named colloidal self-assembled patterns (cSAPs), which is superior to low-adhesion surfaces. A phenol-modified gelatin/hyaluronan (Gel-Ph/HA-Ph) hydrogel is used to encapsulate the multicellular spheroids and the constructs are photo-crosslinked using blue light. The results show that Gel-Ph/HA-Ph hydrogels with a 5%-to-0.3% ratio have the best properties. Cells in HBMSC/HUVEC co-spheroids are more favorable for osteogenic differentiation (Runx2, ALP, Col1a1 and OPN) and vascular network formation (CD31+ cells) compared to HBMSC spheroids. In a subcutaneous nude mouse model, the HBMSC/HUVEC co-spheroids showed better performance than HBMSC spheroids in angiogenesis and the development of blood vessels. Overall, this study paves a new way for using nanopatterns, cell coculturing and hydrogel technology for the generation and application of multicellular spheroids.

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.

Vascularized Tissue Organoids

Tissue organoids hold enormous potential as tools for a variety of applications, including disease modeling and drug screening. To effectively mimic the native tissue environment, it is critical to integrate a microvasculature with the parenchyma and stroma. In addition to providing a means to physiologically perfuse the organoids, the microvasculature also contributes to the cellular dynamics of the tissue model via the cells of the perivascular niche, thereby further modulating tissue function. In this review, we discuss current and developing strategies for vascularizing organoids, consider tissue-specific vascularization approaches, discuss the importance of perfusion, and provide perspectives on the state of the field.

Construction of vascularized tissue-engineered breast with dual angiogenic and adipogenic micro-tissues

Hydrogel-based micro-tissue engineering technique, a bottom-up approach, is promising in constructing soft tissue of large size with homogeneous spatial distribution and superior regeneration capacity compared to the top-down approach. However, most of the studies employed micro-tissues with simple mesenchymal stem cells, which could hardly meet the growth of matrix and vessels. Therefore, we recommend a dual micro-tissues assembly strategy to construct vascularized tissue-engineered breast grafts (TEBGs). Adipose micro-tissues (AMs) and vessel micro-tissues (VMs) were fabricated by seeding adipose-derived stem cells (ADSCs) and human umbilical vein endothelial cells (HUVECs) on collagen microgels (COLs) with a uniform diameter of ∼250 ​μm, respectively. TEBGs were constructed by injecting the dual micro-tissues into 3D printed breast-like Thermoplastic Urethane (TPU) scaffolds, then implanted into the subcutaneous pockets on the back of nude mice. After 3 months of implantation, TEBGs based on dual micro-tissues performed larger volume of adipose tissue regeneration and neo-vessel formation compared to TEBGs based on single AMs. This study extends the application of micro-tissue engineering technique for the construction of soft grafts, and is expected to be useful for creating heterogeneous tissue constructs in the future.

Potency assays for human adipose-derived stem cells as a medicinal product toward wound healing

In pre-clinical studies, human adipose-derived stem cells (hASCs) have shown great promise as a treatment modality for healing of cutaneous wounds. The advantages of hASCs are that they are relatively easy to obtain in large numbers from basic liposuctions, they maintain their characteristics after long-term in vitro culture, and they possess low immunogenicity, which enables the use of hASCs from random donors. It has been hypothesized that hASCs exert their wound healing properties by reducing inflammation, inducing angiogenesis, and promoting fibroblast and keratinocyte growth. Due to the inherent variability associated with the donor-dependent nature of ASC-based products, it appears necessary that the quality of the different products is prospectively certified using a set of most relevant potency assays. In this review, we present an overview of the available methodologies to assess the Mode and the Mechanism of Action of hASCs, specifically in the wound healing scenario. In conclusion, we propose a panel of potential potency assays to include in the future production of ASC-based medicinal products.

Guiding cell migration in 3D with high-resolution photografting

Multi-photon lithography (MPL) has proven to be a suitable tool to precisely control the microenvironment of cells in terms of the biochemical and biophysical properties of the hydrogel matrix. In this work, we present a novel method, based on multi-photon photografting of 4,4′-diazido-2,2′-stilbenedisulfonic acid (DSSA), and its capabilities to induce cell alignment, directional cell migration and endothelial sprouting in a gelatin-based hydrogel matrix. DSSA-photografting allows for the fabrication of complex patterns at a high-resolution and is a biocompatible, universally applicable and straightforward process that is comparably fast. We have demonstrated the preferential orientation of human adipose-derived stem cells (hASCs) in response to a photografted pattern. Co-culture spheroids of hASCs and human umbilical vein endothelial cells (HUVECs) have been utilized to study the directional migration of hASCs into the modified regions. Subsequently, we have highlighted the dependence of endothelial sprouting on the presence of hASCs and demonstrated the potential of photografting to control the direction of the sprouts. MPL-induced DSSA-photografting has been established as a promising method to selectively alter the microenvironment of cells.

Methods for vascularization and perfusion of tissue organoids

Tissue organoids or "mini organs" can be invaluable tools for understanding health and disease biology, modeling tissue dynamics, or screening potential drug candidates. Effective vascularization of these models is critical for truly representing the in vivo tissue environment. Not only is the formation of a vascular network, and ultimately a microcirculation, essential for proper distribution and exchange of oxygen and nutrients throughout larger organoids, but vascular cells dynamically communicate with other cells to modulate overall tissue behavior. Additionally, interstitial fluid flow, mediated by a perfused microvasculature, can have profound influences on tissue biology. Thus, a truly functionally and biologically relevant organoid requires a vasculature. Here, we review existing strategies for fabricating and incorporating vascular elements and perfusion within tissue organoids.

Angiogenic efficacy of ASC spheroids filtrated on porous nanosheets for the treatment of a diabetic skin ulcer

Stem cell transplantation is expected to be an effective treatment for intractable skin ulcers by promoting angiogenesis; however, it is challenging to quickly realize a sufficient bloodstream for the ulcers. For this treatment, sheet-like materials with monolayer cells such as cell sheets have been investigated. However, they have a limitation of cell number that can be transplanted at one time due to the two-dimensional, monolayer cell structure, and sufficient secretion of growth factors cannot be expected. In this regard, cellular aggregates, such as spheroids, can reproduce three-dimensional cell-cell interactions that cause biological functions of living tissues more representative than monolayer cells, which is important to achieving efficient secretion of growth factors. In this study, we focused on free-standing porous polymer ultrathin films ("porous nanosheets") comprising poly(d,l-lactic acid) (PDLLA) and succeeded in developing a spheroid-covered nanosheet, on which more than 1000 spheroids from adipose-tissue derived stem cells (ASCs) were loaded. The porous structure with an average pore diameter of 4 μm allowed for facile filtration and carrying spheroids on the nanosheet, as well as sufficient oxygen and nutrients inflow to the cells. The spheroid-covered nanosheet achieved homogeneous transference of spheroids to a whole skin defect in diabetic model mice. Given the continuous release of vascular endothelial growth factor (VEGF) from the spheroids, the transplanted spheroids promoted healing with more accelerated angiogenesis than a nanosheet with a monolayer of cells. The spheroid-covered nanosheet may be a new regenerative material for promoting intractable skin ulcer healing.

In Vitro Strategies to Vascularize 3D Physiologically Relevant Models

Vascularization of 3D models represents a major challenge of tissue engineering and a key prerequisite for their clinical and industrial application. The use of prevascularized models built from dedicated materials could solve some of the actual limitations, such as suboptimal integration of the bioconstructs within the host tissue, and would provide more in vivo-like perfusable tissue and organ-specific platforms. In the last decade, the fabrication of vascularized physiologically relevant 3D constructs has been attempted by numerous tissue engineering strategies, which are classified here in microfluidic technology, 3D coculture models, namely, spheroids and organoids, and biofabrication. In this review, the recent advancements in prevascularization techniques and the increasing use of natural and synthetic materials to build physiological organ-specific models are discussed. Current drawbacks of each technology, future perspectives, and translation of vascularized tissue constructs toward clinics, pharmaceutical field, and industry are also presented. By combining complementary strategies, these models are envisioned to be successfully used for regenerative medicine and drug development in a near future.

Spheroid Coculture of Human Gingiva-Derived Progenitor Cells With Endothelial Cells in Modified Platelet Lysate Hydrogels

Cell coculture strategies can promote angiogenesis within tissue engineering constructs. This study aimed to test the angiogenic potential of human umbilical vein endothelial cells (HUVEC) cocultured with gingiva-derived progenitor cells (GPC) as spheroids in a xeno-free environment. Human platelet lysate (HPL) was used as a cell culture supplement and as a hydrogel matrix (HPLG) for spheroid encapsulation. HUVEC and HUVEC + GPC (1:1 or 5:1) spheroids were encapsulated in various HPLG formulations. Angiogenesis was assessed via in vitro sprouting and in vivo chick chorioallantoic membrane (CAM) assays. HUVEC revealed characteristic in vitro sprouting in HPL/HPLG and this was significantly enhanced in cocultures with GPC (p < 0.05). A trend for greater sprouting was observed in 5:1 vs 1:1 HUVEC + GPC spheroids and in certain HPLG formulations (p > 0.05). Both HUVEC and HUVEC + GPC spheroids in HPLG revealed abundant and comparable neoangiogenesis in the CAM assay (p > 0.05). Spheroid coculture of HUVEC + GPC in HPLG represents a promising strategy to promote angiogenesis.

3D Adipose Tissue Culture Links the Organotypic Microenvironment to Improved Adipogenesis

Obesity and type 2 diabetes are strongly associated with adipose tissue dysfunction and impaired adipogenesis. Understanding the molecular underpinnings that control adipogenesis is thus of fundamental importance for the development of novel therapeutics against metabolic disorders. However, translational approaches are hampered as current models do not accurately recapitulate adipogenesis. Here, a scaffold-free versatile 3D adipocyte culture platform with chemically defined conditions is presented in which primary human preadipocytes accurately recapitulate adipogenesis. Following differentiation, multi-omics profiling and functional tests demonstrate that 3D adipocyte cultures feature mature molecular and cellular phenotypes similar to freshly isolated mature adipocytes. Spheroids exhibit physiologically relevant gene expression signatures with 4704 differentially expressed genes compared to conventional 2D cultures (false discovery rate < 0.05), including the concerted expression of factors shaping the adipogenic niche. Furthermore, lipid profiles of >1000 lipid species closely resemble patterns of the corresponding isogenic mature adipocytes in vivo (R2 = 0.97). Integration of multi-omics signatures with analyses of the activity profiles of 503 transcription factors using global promoter motif inference reveals a complex signaling network, involving YAP, Hedgehog, and TGFβ signaling, that links the organotypic microenvironment in 3D culture to the activation and reinforcement of PPARγ and CEBP activity resulting in improved adipogenesis.

In vitro and ex vivo models of adipocytes

Adipocytes are specialized cells with pleiotropic roles in physiology and pathology. Several types of fat cells with distinct metabolic properties coexist in various anatomically defined fat depots in mammals. White, beige, and brown adipocytes differ in their handling of lipids and thermogenic capacity, promoting differences in size and morphology. Moreover, adipocytes release lipids and proteins with paracrine and endocrine functions. The intrinsic properties of adipocytes pose specific challenges in culture. Mature adipocytes float in suspension culture due to high triacylglycerol content and are fragile. Moreover, a fully differentiated state, notably acquirement of the unilocular lipid droplet of white adipocyte, has so far not been reached in two-dimensional culture. Cultures of mouse and human-differentiated preadipocyte cell lines and primary cells have been established to mimic white, beige, and brown adipocytes. Here, we survey various models of differentiated preadipocyte cells and primary mature adipocyte survival describing main characteristics, culture conditions, advantages, and limitations. An important development is the advent of three-dimensional culture, notably of adipose spheroids that recapitulate in vivo adipocyte function and morphology in fat depots. Challenges for the future include isolation and culture of adipose-derived stem cells from different anatomic location in animal models and humans differing in sex, age, fat mass, and pathophysiological conditions. Further understanding of fat cell physiology and dysfunction will be achieved through genetic manipulation, notably CRISPR-mediated gene editing. Capturing adipocyte heterogeneity at the single-cell level within a single fat depot will be key to understanding diversities in cardiometabolic parameters among lean and obese individuals.

Recent progress in extrusion 3D bioprinting of hydrogel biomaterials for tissue regeneration: a comprehensive review with focus on advanced fabrication techniques

Over the last decade, 3D bioprinting has received immense attention from research communities to bridge the divergence between artificially engineered tissue constructs and native tissues.

Strategies for re-vascularization and promotion of angiogenesis in trauma and disease

The maintenance of a healthy vascular system is essential to ensure the proper function of all organs of the human body. While macrovessels have the main role of blood transportation from the heart to all tissues, microvessels, in particular capillaries, are responsible for maintaining tissues' functionality by providing oxygen, nutrients and waste exchanges. Occlusion of blood vessels due to atherosclerotic plaque accumulation remains the leading cause of mortality across the world. Autologous vein and artery grafts bypassing are the current gold standard surgical procedures to substitute primarily obstructed vascular structures. Ischemic scenarios that condition blood supply in downstream tissues may arise from blockage phenomena, as well as from other disease or events leading to trauma. The (i) great demand for new vascular substitutes, arising from both the limited availability of healthy autologous vessels, as well as the shortcomings associated with small-diameter synthetic vascular grafts, and (ii) the challenging induction of the formation of adequate and stable microvasculature are current driving forces for the growing interest in the development of bioinspired strategies to ensure the proper function of vasculature in all its dimensional scales. Here, a critical review of well-established technologies and recent biotechnological advances to substitute or regenerate the vascular system is provided.

Cellular Based Strategies for Microvascular Engineering

Vascularization is a major hurdle in complex tissue and organ engineering. Tissues greater than 200 μm in diameter cannot rely on simple diffusion to obtain nutrients and remove waste. Therefore, an integrated vascular network is required for clinical translation of engineered tissues. Microvessels have been described as <150 μm in diameter, but clinically they are defined as <1 mm. With new advances in super microsurgery, vessels less than 1 mm can be anastomosed to the recipient circulation. However, this technical advancement still relies on the creation of a stable engineered microcirculation that is amenable to surgical manipulation and is readily perfusable. Microvascular engineering lays on the crossroads of microfabrication, microfluidics, and tissue engineering strategies that utilize various cellular constituents. Early research focused on vascularization by co-culture and cellular interactions, with the addition of angiogenic growth factors to promote vascular growth. Since then, multiple strategies have been utilized taking advantage of innovations in additive manufacturing, biomaterials, and cell biology. However, the anatomy and dynamics of native blood vessels has not been consistently replicated. Inconsistent results can be partially attributed to cell sourcing which remains an enigma for microvascular engineering. Variations of endothelial cells, endothelial progenitor cells, and stem cells have all been used for microvascular network fabrication along with various mural cells. As each source offers advantages and disadvantages, there continues to be a lack of consensus. Furthermore, discord may be attributed to incomplete understanding about cell isolation and characterization without considering the microvascular architecture of the desired tissue/organ.

Advances in the Research of Bioinks Based on Natural Collagen, Polysaccharide and Their Derivatives for Skin 3D Bioprinting

The skin plays an important role in protecting the human body, and wound healing must be set in motion immediately following injury or trauma to restore the normal structure and function of skin. The extracellular matrix component of the skin mainly consists of collagen, glycosaminoglycan (GAG), elastin and hyaluronic acid (HA). Recently, natural collagen, polysaccharide and their derivatives such as collagen, gelatin, alginate, chitosan and pectin have been selected as the matrix materials of bioink to construct a functional artificial skin due to their biocompatible and biodegradable properties by 3D bioprinting, which is a revolutionary technology with the potential to transform both research and medical therapeutics. In this review, we outline the current skin bioprinting technologies and the bioink components for skin bioprinting. We also summarize the bioink products practiced in research recently and current challenges to guide future research to develop in a promising direction. While there are challenges regarding currently available skin bioprinting, addressing these issues will facilitate the rapid advancement of 3D skin bioprinting and its ability to mimic the native anatomy and physiology of skin and surrounding tissues in the future.

Injectable Therapeutic Organoids Using Sacrificial Hydrogels

Organoids are becoming widespread in drug-screening technologies but have been used sparingly for cell therapy as current approaches for producing self-organized cell clusters lack scalability or reproducibility in size and cellular organization. We introduce a method of using hydrogels as sacrificial scaffolds, which allow cells to form self-organized clusters followed by gentle release, resulting in highly reproducible multicellular structures on a large scale. We demonstrated this strategy for endothelial cells and mesenchymal stem cells to self-organize into blood-vessel units, which were injected into mice, and rapidly formed perfusing vasculature. Moreover, in a mouse model of peripheral artery disease, intramuscular injections of blood-vessel units resulted in rapid restoration of vascular perfusion within seven days. As cell therapy transforms into a new class of therapeutic modality, this simple method—by making use of the dynamic nature of hydrogels—could offer high yields of self-organized multicellular aggregates with reproducible sizes and cellular architectures.

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Therapeutic, prevascularized organoids were formed in a sacrificial scaffold

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The organoids are highly reproducible and grown in a high-throughput manner

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The organoids rapidly formed perfusing vasculature in healthy mice

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Therapeutic potential was assessed in a mouse model of peripheral artery disease

A Straightforward Approach to Engineer Vascularized Adipose Tissue Using Microvascular Fragments

There is a need to overcome the donor-site morbidity and loss of volume over time that accompanies the current clinical approaches to treat soft tissue defects caused by disease and trauma. The development of bioactive constructs that can regenerate adipose tissue have made great progress toward addressing the limitations of current therapies, but their lack of vascularization and ability to meet the significant dimension requirements of tissue defects limit their clinical translatability. Microvascular fragments (MVFs) can form extensive vascular networks and contain resident cells that have the ability to differentiate into adipocytes. Therefore, the objective of this study was to determine if vascularized adipose tissue could be engineered using a fibrin-based hydrogel containing MVFs as the sole source of microvessels and adipocyte-forming cells. The potential for MVFs from different fat depots (epididymal, inguinal, and subcutaneous) to form microvascular networks and generate adipocytes when exposed to growth media (GM), adipogenic differentiation media (ADM), or when treated with GM before adipogenic induction (i.e., they were allowed to presprout before adipogenic induction) was evaluated. MVFs treated with adipogenic induction media, both with and without presprouting, contained lipid droplets, had an increase in expression levels of genes associated with adipogenesis (adiponectin and fatty acid synthase [FAS]), and had an increased rate of lipolysis. MVFs allowed to presprout before ADM treatment maintained their ability to form vascular networks while maintaining an elevated lipid content, adipogenic gene expression, and lipolysis rate. Collectively, these results support the contention that MVFs can serve as the sole source of biologic material for creating a vascularized adipose tissue scaffold. Impact statement Microvascular fragments have both the ability to form extensive vascular networks and function as a source of adipocytes. These phenomena were exploited as vascularized adipose tissue was generated by first allowing for a period of angiogenesis before the adipogenic induction. This strategy has the ability to provide a means of both improving soft tissue reconstruction while also serving as a model to better understand adipose tissue expansion.

Bone Tissue Regeneration in the Oral and Maxillofacial Region: A Review on the Application of Stem Cells and New Strategies to Improve Vascularization

Bone tissue engineering techniques are a promising alternative for the use of autologous bone grafts to reconstruct bone defects in the oral and maxillofacial region. However, for successful bone regeneration, adequate vascularization is a prerequisite. This review presents and discusses the application of stem cells and new strategies to improve vascularization, which may lead to feasible clinical applications. Multiple sources of stem cells have been investigated for bone tissue engineering. The stromal vascular fraction (SVF) of human adipose tissue is considered a promising single source for a heterogeneous population of essential cells with, amongst others, osteogenic and angiogenic potential. Enhanced vascularization of tissue-engineered grafts can be achieved by different mechanisms: vascular ingrowth directed from the surrounding host tissue to the implanted graft, vice versa, or concomitantly. Vascular ingrowth into the implanted graft can be enhanced by (i) optimizing the material properties of scaffolds and (ii) their bioactivation by incorporation of growth factors or cell seeding. Vascular ingrowth directed from the implanted graft towards the host tissue can be achieved by incorporating the graft with either (i) preformed microvascular networks or (ii) microvascular fragments (MF). The latter may have stimulating actions on both vascular ingrowth and outgrowth, since they contain angiogenic stem cells like SVF, as well as vascularized matrix fragments. Both adipose tissue-derived SVF and MF are cell sources with clinical feasibility due to their large quantities that can be harvested and applied in a one-step surgical procedure. During the past years, important advancements of stem cell application and vascularization in bone tissue regeneration have been made. The development of engineered in vitro 3D models mimicking the bone defect environment would facilitate new strategies in bone tissue engineering. Successful clinical application requires innovative future investigations enhancing vascularization.

Nipple Reconstruction: A Regenerative Medicine Approach Using 3D-Printed Tissue Scaffolds

IMPACT STATEMENT

This work provides a comprehensive overview and critique of nipple reconstruction techniques to date. It then explores different tissue engineering concepts and how these may improve clinical outcomes for patients undergoing nipple reconstruction. A novel technique is proposed, whereby a three-dimensional-printed tissue-engineered construct is used as an autologous graft to assist nipple reconstruction.

Advanced drug delivery systems and artificial skin grafts for skin wound healing

Cutaneous injuries, especially chronic wounds, burns, and skin wound infection, require painstakingly long-term treatment with an immense financial burden to healthcare systems worldwide. However, clinical management of chronic wounds remains unsatisfactory in many cases. Various strategies including growth factor and gene delivery as well as cell therapy have been used to enhance the healing of non-healing wounds. Drug delivery systems across the nano, micro, and macroscales can extend half-life, improve bioavailability, optimize pharmacokinetics, and decrease dosing frequency of drugs and genes. Replacement of the damaged skin tissue with substitutes comprising cell-laden scaffold can also restore the barrier and regulatory functions of skin at the wound site. This review covers comprehensively the advanced treatment strategies to improve the quality of wound healing.

Human adipose stromal-vascular fraction self-organizes to form vascularized adipose tissue in 3D cultures

Native human subcutaneous adipose tissue (AT) is well organized into unilocular adipocytes interspersed within dense vascularization. This structure is completely lost under standard culture conditions and may impair the comparison with native tissue. Here, we developed a 3-D model of human white AT reminiscent of the cellular architecture found in vivo. Starting with adipose progenitors derived from the stromal-vascular fraction of human subcutaneous white AT, we generated spheroids in which endogenous endothelial cells self-assembled to form highly organized endothelial networks among stromal cells. Using an optimized adipogenic differentiation medium to preserve endothelial cells, we obtained densely vascularized spheroids containing mature adipocytes with unilocular lipid vacuoles. In vivo study showed that when differentiated spheroids were transplanted in immune-deficient mice, endothelial cells within the spheroids connected to the recipient circulatory system, forming chimeric vessels. In addition, adipocytes of human origin were still observed in transplanted mice. We therefore have developed an in vitro model of vascularized human AT-like organoids that constitute an excellent tool and model for any study of human AT.

Ceria nanoparticles enhance endochondral ossification-based critical-sized bone defect regeneration by promoting the hypertrophic differentiation of BMSCs via DHX15 activation

Central ischemic necrosis is one of the biggest obstacles in the clinical application of traditional tissue-engineered bone (TEB) in critical-sized bone defect regeneration. Because of its ability to promote vascular invasion, endochondral ossification-based TEB has been applied for bone defect regeneration. However, inadequate chondrocyte hypertrophy can hinder vascular invasion and matrix mineralization during endochondral ossification. In light of recent studies suggesting that ceria nanoparticles (CNPs) improve the blood vessel distribution within TEB, we modified TEB scaffold surfaces with CNPs and investigated the effect and mechanism of CNPs on endochondral ossification-based bone regeneration. The CNPs used in this study were synthesized by the microemulsion method and modified with alendronate-anchored polyethylene glycol 600. We showed that CNPs accelerated new bone formation and enhanced endochondral ossification-based bone regeneration in both a subcutaneous ectopic osteogenesis model and a mouse model of critical-sized bone defects. Mechanistically, CNPs significantly promoted endochondral ossification-based bone regeneration by ensuring sufficient hypertrophic differentiation via the activation of the RNA helicase, DEAH (Asp-Glu-Ala-His) box helicase 15, and its downstream target, p38 MAPK. These results suggested that CNPs could be applied as a biomaterial to improve the efficacy of endochondral ossification-based bone regeneration in critical-sized bone defects.-Li, J., Kang, F., Gong, X., Bai, Y., Dai, J., Zhao, C., Dou, C., Cao, Z., Liang, M., Dong, R., Jiang, H., Yang, X., Dong, S. Ceria nanoparticles enhance endochondral ossification-based critical-sized bone defect regeneration by promoting the hypertrophic differentiation of BMSCs via DHX15 activation.

Association of Autologous Fat Injection in Facial Artery With Ophthalmological Complications: An Experimental Animal Study

Importance

Complications caused by autologous fat filling have been reported. Comprehensive knowledge of the possible adverse effects of autologous fat filling is needed.

Objective

To determine the association of autologous fat filling with ophthalmic function complications.

Design, Setting, and Participants

Four adult New Zealand white rabbits were killed for a facial anatomy study. Sixty-four adult New Zealand white rabbits underwent fat harvest using the Coleman technique. Autologous fat was minced or digested with collagenase 1 and centrifuged to separate fat lipid and fat granules. Either 0.2 mL or 0.4 mL of minced fat, fat granules, fat lipid, or saline (control) was retrogradely injected into the facial artery of rabbit models. Electroretinography and ophthalmic fundoscopy were performed to measure the retina and fundus artery occlusions 2 weeks after surgery.

Main Outcomes and Measures

Visual impairment, blindness, and death.

Results

Injection of 0.2 mL of fat granules, fat lipid, and saline resulted in 100% (8 of 8), 62.5% (5 of 8), and 0 ophthalmic complications, respectively; and 0.4 mL resulted in 87.5% (7 of 8), 12.5% (1 of 8), and 0 ophthalmic complications, respectively. Injection of 0.2 mL and 0.4 mL minced fat led to 100% (8 of 8) ophthalmic complications and death, respectively. The mortality rates were 37.5% (3 of 8), 12.5% (1 of 8), and 0 for 0.2 mL emboli injection, and 100% (8 of 8), 50% (4 of 8), and 0 for 0.4 mL, respectively.

Conclusions and Relevance

In this study, minced fat injection was associated with more ophthalmic complications than injection of fat granules and fat lipid. Increasing the injection volume of fat tissues could raise the incidence of morbidity and mortality.

Level of Evidence

NA.

Microvascular Networks From Endothelial Cells and Mesenchymal Stromal Cells From Adipose Tissue and Bone Marrow: A Comparison

A promising approach to overcome hypoxic conditions in tissue engineered constructs is to use the potential of endothelial cells (EC) to form networks in vitro when co-cultured with a supporting cell type in a 3D environment. Adipose tissue-derived stromal cells (ASC) as well as bone marrow-derived stromal cells (BMSC) have been shown to support vessel formation of EC in vitro, but only very few studies compared the angiogenic potential of both cell types using the same model. Here, we aimed at investigating the ability of ASC and BMSC to induce network formation of EC in a co-culture model in fibrin. While vascular structures of BMSC and EC remained stable over the course of 3 weeks, ASC-EC co-cultures developed more junctions and higher network density within the same time frame. Both co-cultures showed positive staining for neural glial antigen 2 (NG2) and basal lamina proteins. This indicates that vessels matured and were surrounded by perivascular cells as well as matrix molecules involved in stabilization. Gene expression analysis revealed a significant increase of vascular endothelial growth factor (VEGF) expression in ASC-EC co-culture compared to BMSC-EC co-culture. These observations were donor-independent and highlight the importance of organotypic cell sources for vascular tissue engineering.

Short-term post-implantation dynamics of in vitro engineered human microvascularized adipose tissues

Engineered adipose tissues are developed for their use as substitutes for tissue replacement in reconstructive surgery. To ensure a timely perfusion of the grafted substitutes, different strategies can be used such as the incorporation of an endothelial component. In this study, we engineered human adipose tissue substitutes comprising of functional adipocytes as well as a natural extracellular matrix using the self-assembly approach, without the use of exogenous scaffolding elements. Human microvascular endothelial cells (hMVECs) were incorporated during tissue production in vitro and we hypothesized that their presence would favor the early connection with the host vascular network translating into functional enhancement after implantation into nude mice in comparison to the substitutes that were not enriched in hMVECs. In vitro, no significant differences were observed between the substitutes in terms of histological aspects. After implantation, both groups presented numerous adipocytes and an abundant matrix in addition to the presence of host capillaries within the grafts. The substitutes thickness and volume were not significantly different between groups over the short-term time course of 14 days (d). For the microvascularized adipose tissues, human CD31 staining revealed a human capillary network connecting with the host microvasculature as early as 3 d after grafting. The detection of murine red blood cells within human CD31+ structures confirmed the functionality of the human capillary network. By analyzing the extent of the global vascularization achieved, a tendency towards increased total capillary network surface and volume was revealed for prevascularized tissues over 14 d. Therefore, applying this strategy on thicker reconstructed adipose tissues with rate-limiting oxygen diffusion might procure added benefits and prove useful to provide voluminous substitutes for patients suffering from adipose tissue loss or defects.

Induction of Expression of CD271 and CD34 in Mesenchymal Stromal Cells Cultured as Spheroids

Cultured mesenchymal stromal cells (MSCs) are cells that can be used for tissue engineering or cell therapies owing to their multipotency and ability to secrete immunomodulatory and trophic molecules. Several studies suggest that MSCs can become pericytes when cocultured with endothelial cells (ECs) but failed to use pericyte markers not already expressed by MSCs. We hypothesized ECs could instruct MSCs to express the molecules CD271 or CD34, which are expressed by pericytes in situ but not by MSCs. CD271 is a marker of especial interest because it is associated with multipotency, a characteristic that wanes in MSCs as they are culture expanded. Consequently, surface expression of CD271 and CD34 was detected in roughly half of the MSCs cocultured with ECs as spheroids in the presence of insulin-like growth factor 1 (IGF-1). Conversely, expression of CD271 and CD34 was detected in a similar proportion of MSCs cultured under these conditions without ECs, and expression of these markers was low or absent when no IGF-1 was added. These findings indicate that specific culture conditions including IGF-1 can endow cultured MSCs with expression of CD271 and CD34, which may enhance the multipotency of these cells when they are used for therapeutic purposes.

The Role of Vasculature Engineering in Dental Pulp Regeneration

Creating an optimal microenvironment that supports angiogenesis, cell-cell cross talk, cell migration, and differentiation is crucial for pulp/dentin regeneration. It was shown that dental stem cells being seeded onto a scaffold and transplanted in vivo could give rise to a new tissue similar to that of the native pulp. However, the unique structure of the tooth with a pulp space encased within hard dentin allows only a single blood supply from a small apical opening located at the apex of the root canals. Therefore, a further strategy that can address this limitation such as the incorporation of endothelial/endothelial progenitor cells or cells with high angiogenic potential into the transplant is required so that the added cells can contribute to the vascularization within the implant. However, the placement of 2 or more different cell types inside 3-dimensional porous scaffolds is technologically challenging. In contrast to the conventional scaffolding approach, self-assembly of monodispersed cells into 3-dimensional tissue mimics permits true physiological interactions between and among different types of cells without any influence from a secondary material. In this review, we discuss potential strategies that can be used in vasculature engineering in dental pulp regeneration with a specific emphasis on combining prevascularization and scaffold-based or scaffold-free approaches.

Skin bioprinting: a novel approach for creating artificial skin from synthetic and natural building blocks

Significant progress has been made over the past few decades in the development of in vitro-engineered substitutes that mimic human skin, either as grafts for the replacement of lost skin, or for the establishment of in vitro human skin models. Tissue engineering has been developing as a novel strategy by employing the recent advances in various fields such as polymer engineering, bioengineering, stem cell research and nanomedicine. Recently, an advancement of 3D printing technology referred as bioprinting was exploited to make cell loaded scaffolds to produce constructs which are more matching with the native tissue. Bioprinting facilitates the simultaneous and highly specific deposition of multiple types of skin cells and biomaterials, a process that is lacking in conventional skin tissue-engineering approaches. Bioprinted skin substitutes or equivalents containing dermal and epidermal components offer a promising approach in skin bioengineering. Various materials including synthetic and natural biopolymers and cells with or without signalling molecules like growth factors are being utilized to produce functional skin constructs. This technology emerging as a novel strategy to overcome the current bottle-necks in skin tissue engineering such as poor vascularization, absence of hair follicles and sweat glands in the construct.

The role of fibrinolysis inhibition in engineered vascular networks derived from endothelial cells and adipose-derived stem cells

Background

Co-cultures of endothelial cells with mesenchymal stem cells currently represent one of the most promising approaches in providing oxygen and nutrient supply for microvascular tissue engineering. Still, to translate this model into clinics several in vitro parameters including growth medium and scaffold degradation need to be fine-tuned.

Methods

We recently described the co-culture of adipose-derived stem cells with endothelial cells in fibrin, resulting in capillary formation in vitro as well as their perfusion in vivo. Here, we aimed to further characterise microvascular tube formation in fibrin by determining the role of scaffold degradation, thrombin concentration and culture conditions on vascularisation.

Results

We observed that inhibition of cell-mediated fibrin degradation by the commonly used inhibitor aprotinin resulted in impaired vascular network formation. Aprotinin had no effect on laminin and collagen type IV deposition or formation of tube-like structures in scaffold-free co-culture, indicating that poor vascularisation of fibrin clots is primarily caused by inhibition of plasminogen-driven fibrinolysis. Co-culture in plasminogen- and factor XIII-depleted fibrin did not result in different vascular network density compared to controls. Furthermore, we demonstrate that thrombin negatively affects vascular network density at high concentrations. However, only transient activation of incorporated endothelial cells by thrombin could be observed, thus excluding a long-term inflammatory response in tissue-engineered micro-capillaries. Finally, we show that vascularisation of fibrin scaffolds in basal medium is undermined because of increased fibrinolytic activity leading to scaffold destabilisation without aprotinin.

Conclusions

Taken together, our data reveal a critical role of fibrinolysis inhibition in in vitro cell-mediated vascularisation of fibrin scaffolds.

Electronic supplementary material

The online version of this article (10.1186/s13287-017-0764-2) contains supplementary material, which is available to authorized users.

Scaffold-free generation of uniform adipose spheroids for metabolism research and drug discovery

Adipose tissue dysfunction is critical to the development of type II diabetes and other metabolic diseases. While monolayer cell culture has been useful for studying fat biology, 2D culture often does not reflect the complexity of fat tissue. Animal models are also problematic in that they are expensive, time consuming, and may not completely recapitulate human biology because of species variation. To address these problems, we have developed a scaffold-free method to generate 3D adipose spheroids from primary or immortal human or mouse pre-adipocytes. Pre-adipocytes self-organize into spheroids in hanging drops and upon transfer to low attachment plates, can be maintained in long-term cultures. Upon exposure to differentiation cues, the cells mature into adipocytes, accumulating large lipid droplets that expand with time. The 3D spheroids express and secrete higher levels of adiponectin compared to 2D culture and respond to stress, either culture-related or toxin-associated, by secreting pro-inflammatory adipokines. In addition, 3D spheroids derived from brown adipose tissue (BAT) retain expression of BAT markers better than 2D cultures derived from the same tissue. Thus, this model can be used to study both the maturation of pre-adipocytes or the function of mature adipocytes in a 3D culture environment.

αvβ3 and α5β1 integrin-specific ligands: From tumor angiogenesis inhibitors to vascularization promoters in regenerative medicine?

Integrins are cell adhesion receptors predominantly important during normal and tumor angiogenesis. A sequence present on several extracellular matrix proteins composed of Arg-Gly-Asp (RGD) has attracted attention due to its role in cell adhesion mediated by integrins. The development of ligands that can bind to integrins involved in tumor angiogenesis and brake disease progression has resulted in new investigational drug entities reaching the clinical trial phase in humans. The use of integrin-specific ligands can be useful for the vascularization of regenerative medicine constructs, which remains a major limitation for translation into clinical practice. In order to enhance vascularization, immobilization of integrin-specific RGD peptidomimetics within constructs is a recommended approach, due to their high specificity and selectivity towards certain desired integrins. This review endeavours to address the potential of peptidomimetic-coated biomaterials as vascular network promoters for regenerative medicine purposes. Clinical studies involving molecules tracking active integrins in cancer angiogenesis and reasons for their failure are also addressed.

Engineering Blood and Lymphatic Microvascular Networks in Fibrin Matrices

Vascular network engineering is essential for nutrient delivery to tissue-engineered constructs and, consequently, their survival. In addition, the functionality of tissues also depends on tissue drainage and immune cell accessibility, which are the main functions of the lymphatic system. Engineering both the blood and lymphatic microvasculature would advance the survival and functionality of tissue-engineered constructs. The aim of this study was to isolate pure populations of lymphatic endothelial cells (LEC) and blood vascular endothelial cells (BEC) from human dermal microvascular endothelial cells and to study their network formation in our previously described coculture model with adipose-derived stromal cells (ASC) in fibrin scaffolds. We could follow the network development over a period of 4 weeks by fluorescently labeling the cells. We show that LEC and BEC form separate networks, which are morphologically distinguishable and sustainable over several weeks. In addition, lymphatic network development was dependent on vascular endothelial growth factor (VEGF)-C, resulting in denser networks with increasing VEGF-C concentration. Finally, we confirm the necessity of cell–cell contact between endothelial cells and ASC for the formation of both blood and lymphatic microvascular networks. This model represents a valuable platform for in vitro drug testing and for the future in vivo studies on lymphatic and blood microvascularization.

Prevascularization in tissue engineering: Current concepts and future directions

The survival of engineered tissue constructs during the initial phase after their implantation depends on the rapid development of an adequate vascularization. This, in turn, is a major prerequisite for the constructs' long-term function. 'Prevascularization' has emerged as a promising concept in tissue engineering, aiming at the generation of a preformed microvasculature in tissue constructs prior to their implantation. This should shorten the time period during which the constructs are avascular and suffer hypoxic conditions. Herein, we provide an overview of current strategies for the generation of preformed microvascular networks within tissue constructs. In vitro approaches use cell seeding, spheroid formation or cell sheet technologies. In situ approaches use the body as a natural bioreactor to induce vascularization by angiogenic ingrowth or flap and arteriovenous (AV)-loop techniques. In future, these strategies may be supplemented by the transplantation of adipose tissue-derived microvascular fragments or the in vitro generation of highly organized microvascular networks by means of sophisticated microscale technologies and microfluidic systems. The further advancement of these prevascularization concepts and their adaptation to individual therapeutic interventions will markedly contribute to a broad implementation of tissue engineering applications into clinical practice.

Electrospun polycaprolactone 3D nanofibrous scaffold with interconnected and hierarchically structured pores for bone tissue engineering

For the first time, electrospun polycaprolactone (PCL) 3D nanofibrous scaffold has been developed by an innovative and convenient approach (i.e., thermally induced nanofiber self-agglomeration followed by freeze drying), and the scaffold possesses interconnected and hierarchically structured pores including macropores with sizes up to ≈300 μm. The novel PCL 3D scaffold is soft and elastic with very high porosity of ≈96.4%, thus it is morphologically/structurally similar to natural extracellular matrix and well suited for cell functions and tissue formation. The in vitro studies reveal that the scaffold can lead to high cell viability; more importantly, it is able to promote more potent BMP2-induced chondrogenic than osteogenic differentiation of mouse bone marrow mesenchymal stem cells. Consistent to the in vitro findings, the in vivo results indicate that the electrospun PCL 3D scaffold acts as a favorable synthetic extracellular matrix for functional bone regeneration through the physiological endochondral ossification process.

Vascularization mediated by mesenchymal stem cells from bone marrow and adipose tissue: a comparison

Tissue-engineered constructs are promising to overcome shortage of organ donors and to reconstruct at least parts of injured or diseased tissues or organs. However, oxygen and nutrient supply are limiting factors in many tissues, especially after implantation into the host. Therefore, the development of a vascular system prior to implantation appears crucial. To develop a functional vascular system, different cell types that interact with each other need to be co-cultured to simulate a physiological environment in vitro. This review provides an overview and a comparison of the current knowledge of co-cultures of human endothelial cells (ECs) with human adipose tissue-derived stem/stromal cells (ASCs) or bone marrow-mesenchymal stem cells (BMSCs) in three dimensional (3D) hydrogel matrices. Mesenchymal stem cells (MSCs), BMSCs or ASCs, have been shown to enhance vascular tube formation of ECs and to provide a stabilizing function in addition to growth factor delivery and permeability control for ECs. Although phenotypically similar, MSCs from different tissues promote tubulogenesis through distinct mechanisms. In this report, we describe differences and similarities regarding molecular interactions in order to investigate which of these two cell types displays more favorable characteristics to be used in clinical applications. Our comparative study shows that ASCs as well as BMSCs are both promising cell types to induce vascularization with ECs in vitro and consequently are promising candidates to support in vivo vascularization.

Endothelial Differentiation of Human Adipose-Derived Stem Cells on Polyglycolic Acid/Polylactic Acid Mesh

Adipose-derived stem cell (ADSC) is considered as a cell source potentially useful for angiogenesis in tissue engineering and regenerative medicine. This study investigated the growth and endothelial differentiation of human ADSCs on polyglycolic acid/polylactic acid (PGA/PLA) mesh compared to 2D plastic. Cell adhesion, viability, and distribution of hADSCs on PGA/PLA mesh were observed by CM-Dil labeling, live/dead staining, and SEM examination while endothelial differentiation was evaluated by flow cytometry, Ac-LDL/UEA-1 uptake assay, immunofluorescence stainings, and gene expression analysis of endothelial related markers. Results showed hADSCs gained a mature endothelial phenotype with a positive ratio of 21.4 ± 3.7% for CD31+/CD34- when induced in 3D mesh after 21 days, which was further verified by the expressions of a comprehensive range of endothelial related markers, whereas hADSCs in 2D induced and 2D/3D noninduced groups all failed to differentiate into endothelial cells. Moreover, compared to 2D groups, the expression for α-SMA was markedly suppressed in 3D cultured hADSCs. This study first demonstrated the endothelial differentiation of hADSCs on the PGA/PLA mesh and pointed out the synergistic effect of PGA/PLA 3D culture and growth factors on the acquisition of mature characteristic endothelial phenotype. We believed this study would be the initial step towards the generation of prevascularized tissue engineered constructs.

In vitro analysis of scaffold-free prevascularized microtissue spheroids containing human dental pulp cells and endothelial cells

INTRODUCTION

Scaffolds often fail to mimic essential functions of the physiologic extracellular matrix (ECM) that regulates cell-cell communication in tissue microenvironments. The development of scaffold-free microtissues containing stem cell-derived ECM may serve as a successful alternative to the use of artificial scaffolds. The current study aimed to fabricate 3-dimensional microtissue spheroids of dental pulp cells (DPCs) prevascularized by human umbilical vein endothelial cells (HUVECs) and to characterize these scaffold-free spheroids for the in vitro formation of pulplike tissue constructs.

METHODS

Three-dimensional microtissue spheroids of DPC alone and DPC-HUVEC co-cultures were fabricated using agarose micro-molds. Cellular organization within the spheroids and cell viability (live/dead assay) were assessed at days 1, 7, and 14. Microtissue spheroids were allowed to self-assemble into macrotissues, induced for odontogenic differentiation (21 days), and examined for expression levels of osteo/odontogenic markers: alkaline phosphatase, bone sialoprotein and RUNX2 (Real-time PCR), mineralization (von-Kossa), and prevascularisation (immunohistochemistry for CD31).

RESULTS

The DPC microtissue microenvironment supported HUVEC survival and capillary network formation in the absence of a scaffolding material and external angiogenic stimulation. Immunohistochemical staining for CD31 showed the capillary network formed by HUVECs did sustain-for a prolonged period-even after the microtissues transformed into a macrotissue. Induced, prevascularized macrotissues showed enhanced differentiation capacity compared with DPC alone macrotissues, as shown by higher osteo/odontogenic gene expression levels and mineralization.

CONCLUSIONS

These findings provide insight into the complex intercellular cross talk occurring between DPCs and HUVECs in the context of angiogenesis and pulp regeneration and highlight the significance of developing a favorable 3-dimensional microenvironment that can, in turn, contribute toward successful pulp regeneration strategies.

Creating capillary networks within human engineered tissues: impact of adipocytes and their secretory products

The development of tissue-engineered substitutes of substantial volume is closely associated with the need to ensure rapid vascularization upon grafting. Strategies promoting angiogenesis include the in vitro formation of capillary-like networks within engineered substitutes. We generated both connective and adipose tissues based on a cell sheet technology using human adipose-derived stromal cells. This study evaluates the morphology and extent of the capillary networks that developed upon seeding of human microvascular endothelial cells during tissue production. We posited that adipocyte presence/secretory products could modulate the resulting capillary network when compared to connective substitutes. Analyses including confocal imaging of CD31-labeled capillary-like networks indicated slight differences in their morphological appearance. However, the total volume occupied by the networks as well as the frequency distribution of the structure's volumes were similar between connective and adipose tissues. The average diameter of the capillary structures tended to be 20% higher in reconstructed adipose tissues. Quantification of pro-angiogenic molecules in conditioned media showed greater amounts of leptin (15×), angiopoietin-1 (3.4×) and HGF (1.7×) secreted from adipose than connective tissues at the time of endothelial cell seeding. However, this difference was attenuated during the following coculture period in endothelial cell-containing media, correlating with the minor differences noted between the networks. Taken together, we developed a protocol allowing reconstruction of both connective and adipose tissues featuring well-developed capillary networks in vitro. We performed a detailed characterization of the network architecture within engineered tissues that is relevant for graft assessment before implantation as well as for in vitro screening of angiogenic modulators using three-dimensional models.

Salidroside exerts angiogenic and cytoprotective effects on human bone marrow-derived endothelial progenitor cells via Akt/mTOR/p70S6K and MAPK signalling pathways

BACKGROUND AND PURPOSE

With the increase of age, increased susceptibility to apoptosis and senescence may contribute to proliferative and functional impairment of endothelial progenitor cells (EPCs). The aim of this study was to investigate whether salidroside (SAL) can induce angiogenic differentiation and inhibit oxidative stress-induced apoptosis in bone marrow-derived EPCs (BM-EPCs), and if so, through what mechanism.

EXPERIMENTAL APPROACH

BM-EPCs were isolated and treated with different concentrations of SAL for up to 4 days. Cell proliferation, migration and tube formation ability were detected by DNA content quantification, transwell assay and Matrigel-based angiogenesis assay. Gene and protein expression were assessed by qRT-PCR and Western blot respectively.

KEY RESULTS

Treatment with SAL promoted cellular proliferation and angiogenic differentiation of BM-EPCs, and increased VEGF and NO secretion, which in turn mediated the enhanced angiogenic differentiation of BM-EPCs. Furthermore, SAL significantly attenuated hydrogen peroxide (H₂O₂)-induced cell apoptosis, reduced the intracellular level of reactive oxygen species and restored the mitochondrial membrane potential of BM-EPCs. Moreover, SAL stimulated the phosphorylation of Akt, mammalian target of rapamycin and p70 S6 kinase, as well as ERK1/2, which is associated with cell migration and capillary tube formation. Additionally, SAL reversed the phosphorylation of JNK and p38 MAPK induced by H₂O₂ and suppressed the changes in the Bax/Bcl-xL ratio observed after stimulation with H₂O₂.

CONCLUSIONS AND IMPLICATIONS

These findings identify novel mechanisms that regulate EPC function and suggest that SAL has therapeutic potential as a new agent to enhance vasculogenesis as well as protect against oxidative endothelial injury.

Experimental and clinical methods used for fat volume maintenance after autologous fat grafting

Management of soft tissue deficits resulting from congenital abnormalities, trauma, systemic disease, and tumors is a particularly challenging field of plastic and reconstructive surgery. Fat grafting, a technique traditionally used in the correction of facial asymmetry, is commonly seen in aesthetic procedures which use the grafted fat for soft tissue augmentation and recontouring. Despite its widespread use in reconstruction and aesthetic surgery, therapeutic modalities applied in fat grafting are crude and the results of this intervention are unpredictable. The aim of this review was to present the most recent evidence regarding experimental studies and designs which confirmed or disproved fat volume expansion or fat maintenance after autologous fat grafting.

Tuning graft- and host-derived vascularization in modular tissue constructs: a potential role of HIF1 activation

A better understanding of the factors governing the vascularization of engineered tissues is crucial for their advancement as therapeutic platforms. Here, we studied the effect of implant volume and cell densities on the in vivo vascularization of modular engineered tissue constructs. Sub-millimeter collagen modules containing adipose-derived mesenchymal stromal cells (adMSC) and enveloped by human umbilical vein endothelial cells (HUVEC) were subcutaneously implanted in severe-combined immunodeficient mice with a beige-mutation (SCID-bg) mice. Implant volume and cell density was varied relative to a base case, defined as a 0.01 mL implant containing 1.5×10(7) adMSC/mL and 3.9×10(6) HUVEC/mL. At 7 and 14 days post-transplantation, the constructs were harvested for immunohistochemical analysis of total (CD31(+)) and graft-derived (UEA1(+)) vessel formation, hypoxia-inducible factor 1-alpha (HIF1α) expression, infiltration of host-derived leukocytes (CD45), and macrophages (F4/80). Implant volume and cell density affected the relative contributions of host- versus graft-derived vascularization, highlighting that different mechanisms underlie the two processes. Graft-derived vessel formation was most rapid and robust in implants with high HIF1α expression, namely large volume implants and implants with high adMSC and HUVEC density (p<0.01 compared to base case at day 7). Many HIF1α(+) cells were vessel-lining HUVEC, suggesting that HIF1 activation may be key to vessel assembly in the graft. Host vessel ingrowth, however, dominated the vascularization of small volume implants (of high and low adMSC density alike), which showed low HIF1α expression at day 7. Host vessels were sustained to day 14 when adMSC density alone was increased, presumably due to increased paracrine secretions. This study points to a potential role of HIF1 activation in the vascularization of tissue constructs, which may be harnessed to engineer robust vessels for therapeutic applications.

Scaffold-free Prevascularized Microtissue Spheroids for Pulp Regeneration

Creating an optimal microenvironment that mimics the extracellular matrix (ECM) of natural pulp and securing an adequate blood supply for the survival of cell transplants are major hurdles that need to be overcome in dental pulp regeneration. However, many currently available scaffolds fail to mimic essential functions of natural ECM. The present study investigated a novel approach involving the use of scaffold-free microtissue spheroids of dental pulp stem cells (DPSCs) prevascularized by human umbilical vein endothelial cells (HUVECs) in pulp regeneration. In vitro-fabricated microtissue spheroids were inserted into the canal space of tooth-root slices and were implanted subcutaneously into immunodeficient mice. Histological examination revealed that, after four-week implantation, tooth-root slices containing microtissue spheroids resulted in well-vascularized and cellular pulp-like tissues, compared with empty tooth-root slices, which were filled with only subcutaneous fat tissue. Immunohistochemical staining indicated that the tissue found in the tooth-root slices was of human origin, as characterized by the expression of human mitochondria, and contained odontoblast-like cells organized along the dentin, as assessed by immunostaining for nestin and dentin sialoprotein (DSP). Vascular structures formed by HUVECs in vitro were successfully anastomosed with the host vasculature upon transplantation in vivo, as shown by immunostaining for human CD31. Collectively, these findings demonstrate that prevascularized, scaffold-free, microtissue spheroids can successfully regenerate vascular dental pulp-like tissue and also highlight the significance of the microtissue microenvironment as an optimal environment for successful pulp-regeneration strategies.

Tissue engineering for plastic surgeons: a primer

A central tenet of reconstructive surgery is the principle of "replacing like with like." However, due to limitations in the availability of autologous tissue or because of the complications that may ensue from harvesting it, autologous reconstruction may be impractical to perform or too costly in terms of patient donor-site morbidity. The field of tissue engineering has long held promise to alleviate these shortcomings. Scaffolds are the structural building blocks of tissue-engineered constructs, akin to the extracellular matrix within native tissues. Commonly used scaffolds include allogenic or xenogenic decellularized tissue, synthetic or naturally derived hydrogels, and synthetic biodegradable nonhydrogel polymeric scaffolds. Embryonic, induced pluripotent, and mesenchymal stem cells also hold immense potential for regenerative purposes. Chemical signals including growth factors and cytokines may be harnessed to augment wound healing and tissue regeneration. Tissue engineering is already clinically prevalent in the fields of breast augmentation and reconstruction, skin substitutes, wound healing, auricular reconstruction, and bone, cartilage, and nerve grafting. Future directions for tissue engineering in plastic surgery include the development of prevascularized constructs and rationally designed scaffolds, the use of stem cells to regenerate organs and tissues, and gene therapy.

Vascular smooth muscle cell optimization of vasculogenesis within naturally derived, biodegradable, hybrid hydrogel scaffolds

BACKGROUND

As vascularization represents the rate-limiting step in permanent incorporation of hydrogel-based tissue-regeneration templates, the authors sought to identify the material chemistry that would optimize endothelial cell adhesion and invasion into custom hydrogel constructs. The authors further investigated induction of endothelial tubule formation by growth factor supplementation and paracrine stimulation.

METHODS

Hydrogel scaffolds consisting of combinations of alginate, collagen type I, and chitosan were seeded with human umbilical vein endothelial cells and maintained under standard conditions for 14 days. Cell density and invasion were then evaluated. Tubule formation was evaluated following basic fibroblast growth factor addition or co-culture with human aortic smooth muscle cells.

RESULTS

Human umbilical vein endothelial cells demonstrated greatest cell-surface density and invasion volumes with alginate and collagen (10:1 weight/weight) scaffolds (p < 0.05). Supplementation with basic fibroblast growth factor increased surface density but neither invasion nor tubule formation. A significant increase in tubule content/organization was observed with increasing human aortic smooth muscle cell-to-human umbilical vein endothelial cell ratio co-culture.

CONCLUSIONS

Alginate and collagen 10:1 scaffolds allow for maximal cellularization compared with other combinations studied. Growth factor supplementation did not affect human umbilical vein endothelial cell invasion or morphology. Paracrine signaling by means of co-culture with human umbilical vein endothelial cells stimulated endothelial tubule formation and vascular protonetwork organization. These findings serve to guide future endeavors toward fabrication of prevascularized tissue constructs.

Controlling stem cell-mediated bone regeneration through tailored mechanical properties of collagen scaffolds

Mechanical properties of the extracellular matrix (ECM) play an essential role in cell fate determination. To study the role of mechanical properties of ECM in stem cell-mediated bone regeneration, we used a 3D in vivo ossicle model that recapitulates endochondral bone formation. Three-dimensional gelatin scaffolds with distinct stiffness were developed using 1-Ethyl-3-[3-dimethylaminopropyl] carbodiimide hydrochloride (EDC) mediated zero-length crosslinking. The mechanical strength of the scaffolds was significantly increased by EDC treatment, while the microstructure of the scaffold was preserved. Cell behavior on the scaffolds with different mechanical properties was evaluated in vitro and in vivo. EDC-treated scaffolds promoted early chondrogenic differentiation, while it promoted both chondrogenic and osteogenic differentiation at later time points. Both micro-computed tomography and histologic data demonstrated that EDC-treatment significantly increased trabecular bone formation by transplanted cells transduced with AdBMP. Moreover, significantly increased chondrogenesis was observed in the EDC-treated scaffolds. Based on both in vitro and in vivo data, we conclude that the high mechanical strength of 3D scaffolds promoted stem cell mediated bone regeneration by promoting endochondral ossification. These data suggest a new method for harnessing stem cells for bone regeneration in vivo by tailoring the mechanical properties of 3D scaffolds.

Update on vascularization in tissue engineering

Vascularization of engineered tissues is critical for success. Adequate and physiologically regulated blood supply is important for viability of the implanted tissue but even more important for the proper function of parenchymal cells, which is the desired clinical outcome for most applications in regenerative medicine. Several methods are being developed to stimulate revascularization of engineered tissue. Prevascularized scaffolds with a hierarchical vascular pattern, allowing surgical hook-up of the inflow and outflow tracts, that are already preseeded and cultured with primary vascular cells or precursors will be required for larger tissues or tissues with an immediate high metabolism, such as myocardium. The preimplantation presence of a mature vasculature will improve differentiation and maturation of the parenchyma, thus meeting the functional demands of the host. This may also be true for smaller or metabolically less-active tissues, yet for viability and immediate function they may rely on facilitated postimplantation ingrowth of the host vasculature.