Adipose Tissue Engineering Based on the Controlled Release of Fibroblast Growth Factor-2 in a Collagen Matrix

Biofabrication of vascularized adipose tissues and their biomedical applications

Recent advances in adipose tissue engineering and cell biology have led to the development of innovative therapeutic strategies in regenerative medicine for adipose tissue reconstruction. To date, the many in vitro and in vivo models developed for vascularized adipose tissue engineering cover a wide range of research areas, including studies with cells of various origins and types, polymeric scaffolds of natural and synthetic derivation, models presented using decellularized tissues, and scaffold-free approaches. In this review, studies on adipose tissue types with different functions, characteristics and body locations have been summarized with 3D in vitro fabrication approaches. The reason for the particular focus on vascularized adipose tissue models is that current liposuction and fat transplantation methods are unsuitable for adipose tissue reconstruction as the lack of blood vessels results in inadequate nutrient and oxygen delivery, leading to necrosis in situ. In the first part of this paper, current studies and applications of white and brown adipose tissues are presented according to the polymeric materials used, focusing on the studies which could show vasculature in vitro and after in vivo implantation, and then the research on adipose tissue fabrication and applications are explained.

Heparan Sulfate: A Regulator of White Adipocyte Differentiation and of Vascular/Adipocyte Interactions

White adipose tissues are major endocrine organs that release factors, termed adipokines, which affect other major organ systems. The development and functions of adipose tissues depend largely upon the glycosaminoglycan heparan sulfate. Heparan sulfate proteoglycans (HSPGs) surround both adipocytes and vascular structures and facilitate the communication between these two components. This communication mediates the continued export of adipokines from adipose tissues. Heparan sulfates regulate cellular physiology and communication through a sulfation code that ionically interacts with heparan-binding regions on a select set of proteins. Many of these proteins are growth factors and chemokines that regulate tissue function and inflammation. Cells regulate heparan sulfate sulfation through the release of heparanases and sulfatases. It is now possible to tissue engineer vascularized adipose tissues that express heparan sulfate proteoglycans. This makes it possible to use these tissue constructs to study the role of heparan sulfates in the regulation of adipokine production and release. It is possible to regulate the production of heparanases and sulfatases in order to fine-tune experimental studies.

Application of Advanced Nanomaterials for Kidney Failure Treatment and Regeneration

The implementation of nanomedicine not only provides enhanced drug solubility and reduced off-target adverse effects, but also offers novel theranostic approaches in clinical practice. The increasing number of studies on the application of nanomaterials in kidney therapies has provided hope in a more efficient strategy for the treatment of renal diseases. The combination of biotechnology, material science and nanotechnology has rapidly gained momentum in the realm of therapeutic medicine. The establishment of the bedrock of this emerging field has been initiated and an exponential progress is observed which might significantly improve the quality of human life. In this context, several approaches based on nanomaterials have been applied in the treatment and regeneration of renal tissue. The presented review article in detail describes novel strategies for renal failure treatment with the use of various nanomaterials (including carbon nanotubes, nanofibrous membranes), mesenchymal stem cells-derived nanovesicles, and nanomaterial-based adsorbents and membranes that are used in wearable blood purification systems and synthetic kidneys.

Elastin-Collagen Based Hydrogels as Model Scaffolds to Induce Three-Dimensional Adipocyte Culture from Adipose Derived Stem Cells

This study aimed to probe the effect of formulation of scaffolds prepared using collagen and elastin-like polypeptide (ELP) and their resulting physico-chemical and mechanical properties on the adipogenic differentiation of human adipose derived stem cells (hASCs). Six different ELP-collagen scaffolds were prepared by varying the collagen concentration (2 and 6 mg/mL), ELP addition (6 mg/mL), or crosslinking of the scaffolds. FTIR spectroscopy indicated secondary bonding interactions between collagen and ELP, while scanning electron microscopy revealed a porous structure for all scaffolds. Increased collagen concentration, ELP addition, and presence of crosslinking decreased swelling ratio and increased elastic modulus and compressive strength of the scaffolds. The scaffold characteristics influenced cell morphology, wherein the hASCs seeded in the softer, non-crosslinked scaffolds displayed a spread morphology. We determined that stiffer and/or crosslinked elastin-collagen based scaffolds constricted the spreading of hASCs, leading to a spheroid morphology and yielded an enhanced adipogenic differentiation as indicated by Oil Red O staining. Overall, this study underscored the importance of spheroid morphology in adipogenic differentiation, which will allow researchers to create more physiologically-relevant three-dimensional, in vitro culture models.

Release of the Non-Steroidal Anti-Inflammatory Drug Flufenamic Acid by Multiparticulate Delivery Systems Promotes Adipogenic Differentiation of Adipose-Derived Stem Cells

Engineered tissue-like structures often instigate an inflammatory response in the host that can inhibit wound healing and ultimately lead to the rejection of the implant. In our previous study, we have characterized the properties and biocompatibility of novel multiparticulate drug delivery systems (MDDS), based on collagen matrix with gradual release of anti-inflammatory drug flufenamic acid, we evaluated their anti-inflammatory potential and demonstrated their efficiency against burns and soft tissue lesions. In addition to these results, FA was previously described as a stimulant for adipogenesis, therefore we hypothesized that MDDS might also be appropriate for adipose tissue engineering. After the cell-scaffold constructs were obtained, cell morphology, adhesion and spreading on the systems were highlighted by scanning electron microscopy, immunostaining and confocal microscopy. The effect of FA-enriched materials on adipogenesis was evaluated at gene and protein level, by RT-qPCR, confocal microscopy and immunohistochemistry. Our current work indicates that flufenamic acid plays a beneficial role in adipocyte differentiation, with a direct effect upon the gene and protein expression of important early and late markers of adipogenesis, such as PPARγ2 and perilipin.

Bioprinting predifferentiated adipose-derived mesenchymal stem cell spheroids with methacrylated gelatin ink for adipose tissue engineering

The increasing number of mastectomies results in a greater demand for breast reconstruction characterized by simplicity and a low complication profile. Reconstructive surgeons are investigating tissue engineering (TE) strategies to overcome the current surgical drawbacks. 3D bioprinting is the rising technique for the fabrication of large tissue constructs which provides a potential solution for unmet clinical needs in breast reconstruction building on decades of experience in autologous fat grafting, adipose-derived mesenchymal stem cell (ASC) biology and TE. A scaffold was bioprinted using encapsulated ASC spheroids in methacrylated gelatin ink (GelMA). Uniform ASC spheroids with an ideal geometry and diameter for bioprinting were formed, using a high-throughput non-adhesive agarose microwell system. ASC spheroids in adipogenic differentiation medium (ADM) were evaluated through live/dead staining, histology (HE, Oil Red O), TEM and RT-qPCR. Viable spheroids were obtained for up to 14 days post-printing and showed multilocular microvacuoles and successful differentiation toward mature adipocytes shown by gene expression analysis. Moreover, spheroids were able to assemble at random in GelMA, creating a macrotissue. Combining the advantage of microtissues to self-assemble and the controlled organization by bioprinting technologies, these ASC spheroids can be useful as building blocks for the engineering of soft tissue implants.

ColXV Aggravates Adipocyte Apoptosis by Facilitating Abnormal Extracellular Matrix Remodeling in Mice

The extracellular matrix (ECM) is a highly dynamic structural network and plays an essential role in cell behavior and regulation during metabolic homeostasis and obesity progression. Abnormal ECM remodeling impairs adipocyte plasticity required for diverse cellular functions. Collagen XV (ColXV) is a proteoglycan localized to the outermost layer of basement membranes (BMs) and forms a bridge between the BMs and the fibrillar collagen matrix. Nevertheless, how ColXV affects ECM composition and the reason for subsequent adipocyte apoptosis is still unclear. This report found, through RNA-seq data, that ColXV is linked to cell growth and ECM remodeling. Findings show that, in response to excessive expression of extracellular ColXV, the AMPK/mTORC1 pathway is strongly activated and triggers a cascade of mitochondrial apoptosis. This is the first study to make use of ECM three-dimensional reconstruction, based on decellularization in the adipose tissues and the study reveals that ColXV is an activation factor that alters ECM remodeling in adipose tissues. It was also demonstrated that the fibroblast growth factor 2 (FGF2)/fibroblast growth factor receptor 1 (FGFR1) axis involved in ECM remodeling is suppressed by ColXV due to reduction of FGF2 translocation to FGFR1. Furthermore, ColXV induced remodeling of ECM preceding apoptosis and continued to induce apoptosis in adipocytes. Collectively, our findings establish ColXV as a basement membrane collagen with homology to ColXVIII, indicating that it is one of the positive regulators for inducing ECM remodeling and further promoting adipocyte apoptosis.

Halloysite Nanoclay/Biopolymers Composite Materials in Tissue Engineering

Biocompatible materials for the fabrication of tissue substitutes are crucially important in the advancement of modern medicinal biotechnology. These materials, to serve their function, should be similar in physical, chemical, biological, and structural properties to native tissues which they are aimed to mimic. The porosity of artificial scaffolds is essential for normal nutrient transmission to cells, gas diffusion, and cell attachment and proliferation. Nanoscale inorganic additives and dopants are widely used to improve the functional properties of the polymer materials for tissue engineering. Among these inorganic dopants, halloysite nanotubes are arguably the most perspective candidates because of their biocompatibility and functional properties allowing to enhance significantly the mechanical and chemical stability of tissue engineering scaffolds. Here, this vibrant field of biotechnology for regenerative medicine is overviewed.

Tissue Engineering: New Paradigm of Biomedicine

Tissue engineering is a multidisciplinary field of biomedicine that is being used to develop a new tissue or restore the function of diseased tissue/organ. The main objective of tissue engineering is to overcome the shortage of donor organs. Tissue engineering is mainly based on three components i.e. cells, scaffold and growth factors. Among these three components, scaffold is a primary influencing factor that provides the structural support to the cells and helps to deliver the growth factors which stimulate the proliferation and differentiation of cells to regenerate a new tissue. The properties of a scaffold mainly depend upon types of biomaterial and fabrication techniques that are used to fabricate the scaffold. Biofabrication facilitates the construction of three-dimensional complex of living (cells) and non-living (signaling molecules and extracellular matrices polymers etc.) components. Biofabrication has potential application especially in skin and bone tissue regeneration due to its accuracy, reproducibility and customization of scaffolds as well as cell and signaling molecule delivery. In this review article, different types of biomaterials and fabrication techniques have been discussed to fabricate of a nanofibrous scaffold along with different types of cells and growth factor which are used for tissue engineering applications to regenerate a new tissue. Among different techniques to fabricate a scaffold, electrospinning is simple and cost effective technique that has been mainly focused in the review to produce nanofibous scaffold. On the other hand, a tissue might be repair itself and restore to its normal function inside the body by applying the principle of regenerative medicine.

Tissue Engineering of the Microvasculature

The ability to generate new microvessels in desired numbers and at desired locations has been a long-sought goal in vascular medicine, engineering, and biology. Historically, the need to revascularize ischemic tissues nonsurgically (so-called therapeutic vascularization) served as the main driving force for the development of new methods of vascular growth. More recently, vascularization of engineered tissues and the generation of vascularized microphysiological systems have provided additional targets for these methods, and have required adaptation of therapeutic vascularization to biomaterial scaffolds and to microscale devices. Three complementary strategies have been investigated to engineer microvasculature: angiogenesis (the sprouting of existing vessels), vasculogenesis (the coalescence of adult or progenitor cells into vessels), and microfluidics (the vascularization of scaffolds that possess the open geometry of microvascular networks). Over the past several decades, vascularization techniques have grown tremendously in sophistication, from the crude implantation of arteries into myocardial tunnels by Vineberg in the 1940s, to the current use of micropatterning techniques to control the exact shape and placement of vessels within a scaffold. This review provides a broad historical view of methods to engineer the microvasculature, and offers a common framework for organizing and analyzing the numerous studies in this area of tissue engineering and regenerative medicine. © 2019 American Physiological Society. Compr Physiol 9:1155-1212, 2019.

Current Therapeutic Strategies for Adipose Tissue Defects/Repair Using Engineered Biomaterials and Biomolecule Formulations

Tissue engineered scaffolds for adipose restoration/repair has significantly evolved in recent years. Patients requiring soft tissue reconstruction, caused by defects or pathology, require biomaterials that will restore void volume with new functional tissue. The gold standard of autologous fat grafting (AFG) is not a reliable option. This review focuses on the latest therapeutic strategies for the treatment of adipose tissue defects using biomolecule formulations and delivery, and specifically engineered biomaterials. Additionally, the clinical need for reliable off-the-shelf therapies, animal models, and challenges facing current technologies are discussed.

The Future of Fat Grafting

Fat grafting has emerged as an important tool in the armamentarium of plastic surgeons. Its applications in aesthetic and reconstructive surgery continue to expand. However, its limitations (such as inconsistent survival) remain problematic. Recent research efforts have focused on tacking these issues with the use of platelet rich plasma, hypoxic preconditioning, hyperbaric oxygen, and the tissue engineering of scaffolds. In the future, as these laboratory discoveries are translated to clinical application, the beneficial uses of fat grafting will continue to increase.

3D Fabrication of Polymeric Scaffolds for Regenerative Therapy

Recent advances in bioprinting technology have been used to precisely dispense cell-laden biomaterials for the construction of complex 3D functional living tissues or artificial organs. Organ printing and biofabrication provides great potential for the freeform fabrication of 3D living organs using cellular spheroids, biocomposite nanofibers, or bioinks as building blocks for regenerative therapy. Vascularization is often identified as a main technological barrier for building 3D organs in tissue engineering. 3D printing of living tissues starts with potential support of biomaterials to maintain structural integrity and degradation of certain time periods after printing of the scaffolds. Biofabrication is the production of complex living and nonliving biological products from raw materials such as cells, molecules, ECM, and biomaterials. Generally, two basic methods are used for the fabrication of scaffolds such as conventional/traditional fabrication processes and advance fabrication processes for engineering organs. A wide range of polymers and biomaterials are used for the fabrication of scaffolds in tissue engineering applications. 3D additive manufacturing is advancing day-by-day; however, there are various critical challenging factors used for fabricating 3D scaffolds. This review is aimed at understanding the various scaffold fabrication techniques, types of polymers and biomaterials used for the fabrication processes, various fields of applications, and different challenges faced in their fabrication of scaffolds in regenerative therapy.

Polycaprolactone nanofibrous mesh reduces foreign body reaction and induces adipose flap expansion in tissue engineering chamber

Tissue engineering chamber technique can be used to generate engineered adipose tissue, showing the potential for the reconstruction of soft tissue defects. However, the consequent foreign body reaction induced by the exogenous chamber implantation causes thick capsule formation on the surface of the adipose flap following capsule contracture, which may limit the internal tissue expansion. The nanotopographical property and architecture of nanofibrous scaffold may serve as a promising method for minimizing the foreign body reaction. Accordingly, electrospinning porous polycaprolactone (PCL) nanofibrous mesh, a biocompatible synthetic polymer, was attached to the internal surface of the chamber for the reducing local foreign body reaction. Adipose flap volume, level of inflammation, collagen quantification, capsule thickness, and adipose tissue-specific gene expression in chamber after implantation were evaluated at different time points. The in vivo study revealed that the engineered adipose flaps in the PCL group had a structure similar to that in the controls and normal adipose tissue structure but with a larger flap volume. Interleukin (IL)-1β, IL-6, and transforming growth factor-β expression decreased significantly in the PCL group compared with the control. Moreover, the control group had much more collagen deposition and thicker capsule than that observed in the PCL group. These results indicate that the unique nanotopographical effect of electrospinning PCL nanofiber can reduce foreign body reaction in a tissue engineering chamber, which maybe a promising new method for generating a larger volume of mature, vascularized, and stable adipose tissue.

A bio-inspired hybrid nanosack for graft vascularization at the omentum

For three-dimensional tissue engineering scaffolds, the major challenges of hydrogels are poor mechanical integrity and difficulty in handling during implantation. In contrast, electrospun scaffolds provide tunable mechanical properties and high porosity; but, are limited in cell encapsulation. To overcome these limitations, we developed a "hybrid nanosack" by combination of a peptide amphiphile (PA) nanomatrix gel and an electrospun poly (ε-caprolactone) (ePCL) nanofiber sheet with porous crater-like structures. This hybrid nanosack design synergistically possessed the characteristics of both approaches. In this study, the hybrid nanosack was applied to enhance local angiogenesis in the omentum, which is required of tissue engineering scaffolds for graft survival. The ePCL sheet with porous crater-like structures improved cell and blood vessel penetration through the hybrid nanosack. The hybrid nanosack also provided multi-stage fibroblast growth factor-2 (FGF-2) release kinetics for stimulating local angiogenesis. The hybrid nanosack was implanted into rat omentum for 14days and vascularization was analyzed by micro-CT and immunohistochemistry; the data clearly demonstrated that both FGF-2 delivery and porous crater-like structures work synergistically to enhance blood vessel formation within the hybrid nanosack. Therefore, the hybrid nanosack will provide a new strategy for engineering scaffolds to achieve graft survival in the omentum by stimulating local vascularization, thus overcoming the limitations of current strategies.

STATEMENT OF SIGNIFICANCE

For three-dimensional tissue engineering scaffolds, the major challenges of hydrogels are poor mechanical integrity and difficulty in handling during implantation. In contrast, electrospun scaffolds provide tunable mechanical properties and high porosity; but, are limited in cell encapsulation. To overcome these limitations, we developed a "hybrid nanosack" by combination of a peptide amphiphile (PA) nanomatrix gel and an electrospun poly (ε-caprolactone) (ePCL) nanofiber sheet with porous crater-like structures. This design synergistically possessed the characteristics of both approaches. In this study, the hybrid nanosack was applied to enhance local angiogenesis in the omentum, which is required of tissue engineering scaffolds for graft survival. The hybrid nanosack was implanted into rat omentum for 14days and vascularization was analyzed by micro-CT and immunohistochemistry. We demonstrate that both FGF-2 delivery and porous crater-like structures work synergistically to enhance blood vessel formation within the hybrid nanosack. Therefore, the hybrid nanosack will provide a new strategy for engineering scaffolds to achieve graft survival in the omentum by stimulating local vascularization, thus overcoming the limitations of current strategies.

Clinical translation of controlled protein delivery systems for tissue engineering

Strategies that utilize controlled release of drugs and proteins for tissue engineering have enormous potential to regenerate damaged organs and tissues. The multiple advantages of controlled release strategies merit overcoming the significant challenges to translation, including high costs and long, difficult regulatory pathways. This review highlights the potential of controlled release of proteins for tissue engineering and regenerative medicine. We specifically discuss treatment modalities that have reached preclinical and clinical trials, with emphasis on controlled release systems for bone tissue engineering, the most advanced application with several products already in clinic. Possible strategies to address translational and regulatory concerns are also discussed.

In situ-forming click-crosslinked gelatin based hydrogels for 3D culture of thymic epithelial cells

In situ-forming gelatin based hydrogels, which are crosslinked using an efficient nitrile oxide-norbornene click reaction, provide a suitable 3D culture environment for thymic epithelial cells.

Using a 3D Culture System to Differentiate Visceral Adipocytes In Vitro

It has long been recognized that body fat distribution and regional adiposity play a major role in the control of metabolic homeostasis. However, the ability to study and compare the cell autonomous regulation and response of adipocytes from different fat depots has been hampered by the difficulty of inducing preadipocytes isolated from the visceral depot to differentiate into mature adipocytes in culture. Here, we present an easily created 3-dimensional (3D) culture system that can be used to differentiate preadipocytes from the visceral depot as robustly as those from the sc depot. The cells differentiated in these 3D collagen gels are mature adipocytes that retain depot-specific characteristics, as determined by imaging, gene expression, and functional assays. This 3D culture system therefore allows for study of the development and function of adipocytes from both depots in vitro and may ultimately lead to a greater understanding of site-specific functional differences of adipose tissues to metabolic dysregulation.

Injectable gelatin derivative hydrogels with sustained vascular endothelial growth factor release for induced angiogenesis

Injectable biomaterials are attractive for soft tissue regeneration because they are handled in a minimally invasive manner and can easily adapt to complex defects. However, inadequate vascularization of the injectable constructs has long been a barrier, leading to necrosis or volume reduction after implantation. In this work, we developed a three-step process to synthesize injectable gelatin-derived hydrogels that are capable of controlling growth factor delivery to induce angiogenesis. In our approach, tyramine was first introduced into gelatin chains to provide enzymatic crosslinking points for gel formation after injection. Next, heparin, a polysaccharide with binding domains to many growth factors, was covalently linked to the tyramine-modified gelatin. Finally, vascular endothelial growth factor (VEGF) was incorporated into the gelatin derivative by binding with the heparin in the gelatin derivative, and an injectable gel with controlled VEGF release was formed by an enzymatic catalytic reaction with hydrogen peroxide (H2O2) and horseradish peroxidase (HRP). The gelation time, mechanical properties and degradation of the gel was readily tailored by the gelatin concentration and the ratio of H2O2/HRP. Binding VEGF to heparin stabilizes this growth factor, protects it from denaturation and proteolytic degradation and subsequently prolongs the sustained release. An in vitro release study and bioactivity assay indicated that the VEGF was released in a sustained manner with high bioactivity for over 3 weeks. Furthermore, a chicken chorioallantoic membrane (CAM) assay and animal experiments were performed to evaluate in vivo bioactivity of the VEGF released from the hydrogels. After 5 days of incubation on CAM, the number of blood vessels surrounding the heparin-modified hydrogels was increased by 2.4-fold compared with that of the control group. Deeper and denser cell infiltration and angiogenesis in the heparin-modified gelatin/VEGF gels were observed compared to the controls after being subcutaneously injected in the dorsal side of the mice for 2 weeks. Interestingly, even without the incorporation of VEGF, the heparin-modified gelatin derivative still had the capability to induce angiogenesis to a certain degree. Our results suggest that the gelatin derivative/VEGF is an excellent injectable delivery system for induced angiogenesis of soft tissue regeneration.

Increased adipogenesis of human adipose-derived stem cells on polycaprolactone fiber matrices

With accelerating rates of obesity and type 2 diabetes world-wide, interest in studying the adipocyte and adipose tissue is increasing. Human adipose derived stem cells--differentiated to adipocytes in vitro--are frequently used as a model system for white adipocytes, as most of their pathways and functions resemble mature adipocytes in vivo. However, these cells are not completely like in vivo mature adipocytes. Hosting the cells in a more physiologically relevant environment compared to conventional two-dimensional cell culturing on plastic surfaces, can produce spatial cues that drive the cells towards a more mature state. We investigated the adipogenesis of adipose derived stem cells on electro spun polycaprolactone matrices and compared functionality to conventional two-dimensional cultures as well as to human primary mature adipocytes. To assess the degree of adipogenesis we measured cellular glucose-uptake and lipolysis and used a range of different methods to evaluate lipid accumulation. We compared the averaged results from a whole population with the single cell characteristics--studied by coherent anti-Stokes Raman scattering microscopy--to gain a comprehensive picture of the cell phenotypes. In adipose derived stem cells differentiated on a polycaprolactone-fiber matrix; an increased sensitivity in insulin-stimulated glucose uptake was detected when cells were grown on either aligned or random matrices. Furthermore, comparing differentiation of adipose derived stem cells on aligned polycaprolactone-fiber matrixes, to those differentiated in two-dimensional cultures showed, an increase in the cellular lipid accumulation, and hormone sensitive lipase content. In conclusion, we propose an adipocyte cell model created by differentiation of adipose derived stem cells on aligned polycaprolactone-fiber matrices which demonstrates increased maturity, compared to 2D cultured cells.

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.

Equine model for soft-tissue regeneration

Soft-tissue regeneration methods currently yield suboptimal clinical outcomes due to loss of tissue volume and a lack of functional tissue regeneration. Grafted tissues and natural biomaterials often degrade or resorb too quickly, while most synthetic materials do not degrade. In previous research we demonstrated that soft-tissue regeneration can be supported using silk porous biomaterials for at least 18 months in vivo in a rodent model. In the present study, we scaled the system to a survival study using a large animal model and demonstrated the feasibility of these biomaterials for soft-tissue regeneration in adult horses. Both slow and rapidly degrading silk matrices were evaluated in subcutaneous pocket and intramuscular defect depots. We showed that we can effectively employ an equine model over 6 months to simultaneously evaluate many different implants, reducing the number of animals needed. Furthermore, we were able to tailor matrix degradation by varying the initial format of the implanted silk. Finally, we demonstrate ultrasound imaging of implants to be an effective means for tracking tissue regeneration and implant degradation.

Porous PLGA microspheres tailored for dual delivery of biomolecules via layer-by-layer assembly

Tissue engineering is a complex and dynamic process that requires varied biomolecular cues to promote optimal tissue growth. Consequently, the development of delivery systems capable of sequestering more than one biomolecule with controllable release profiles is a key step in the advancement of this field. This study develops multilayered polyelectrolyte films incorporating alpha-melanocyte stimulating hormone (α-MSH), an anti-inflammatory molecule, and basic fibroblast growth factor (bFGF). The layers were successfully formed on macroporous poly lactic-co-glycolic acid microspheres produced using a combined inkjet and thermally induced phase separation technique. Release profiles could be varied by altering layer properties including the number of layers and concentrations of layering molecules. α-MSH and bFGF were released in a sustained manner and the bioactivity of α-MSH was shown to be preserved using an activated macrophage cell assay in vitro. The system performance was also tested in vivo subcutaneously in rats. The multilayered microspheres reduced the inflammatory response induced by a carrageenan stimulus 6 weeks after implantation compared to the non-layered microspheres without the anti-inflammatory and growth factors, demonstrating the potential of such multilayered constructs for the controlled delivery of bioactive molecules.

Tissue engineering chamber promotes adipose tissue regeneration in adipose tissue engineering models through induced aseptic inflammation

Tissue engineering chamber (TEC) makes it possible to generate significant amounts of mature, vascularized, stable, and transferable adipose tissue. However, little is known about the role of the chamber in tissue engineering. Therefore, to investigate the role of inflammatory response and the change in mechanotransduction started by TEC after implantation, we placed a unique TEC model on the surface of the groin fat pads in rats to study the expression of cytokines and tissue development in the TEC. The number of infiltrating cells was counted, and vascular endothelial growth factor (VEGF) and monocyte chemotactic protein-1 (MCP-1) expression levels in the chamber at multiple time points postimplantation were analyzed by enzyme-linked immunosorbent assay. Tissue samples were collected at various time points and labeled for specific cell populations. The result showed that new adipose tissue formed in the chamber at day 60. Also, the expression of MCP-1 and VEGF in the chamber decreased slightly from an early stage as well as the number of the infiltrating cells. A large number of CD34+/perilipin- perivascular cells could be detected at day 30. Also, the CD34+/perilipin+ adipose precursor cell numbers increased sharply by day 45 and then decreased by day 60. CD34-/perilipin+ mature adipocytes were hard to detect in the chamber content at day 30, but their number increased and then peaked at day 60. Ki67-positive cells could be found near blood vessels and their number decreased sharply over time. Masson's trichrome showed that collagen was the dominant component of the chamber content at early stage and was replaced by newly formed small adipocytes over time. Our findings suggested that the TEC implantation could promote the proliferation of adipose precursor cells derived from local adipose tissue, increase angiogenesis, and finally lead to spontaneous adipogenesis by inducing aseptic inflammation and changing local mechanotransduction.

Delivery of basic fibroblast growth factors from heparinized decellularized adipose tissue stimulates potent de novo adipogenesis

Scaffolds based on decellularized adipose tissue (DAT) are gaining popularity in adipose tissue engineering due to their high biocompatibility and adipogenic inductive property. However, previous studies involving DAT-derived scaffolds have not fully revealed their potentials for in vivo adipose tissue construction. With the aim of developing a more efficient adipose tissue engineering technique based on DAT, in this study, we investigated the in vivo adipogenic potential of a basic fibroblast growth factor (bFGF) delivery system based on heparinized DAT (Hep-DAT). To generate this system, heparins were cross-linked to mouse DATs by using 1-ethyl-3-[3-dimethylaminopropyl]carbodiimide and N-Hydroxysuccinimide. The bFGF-binding Hep-DATs were first tested for controlled release ability in vitro and then transplanted subcutaneously. Highly vascularized adipose tissues were formed 6weeks after transplantation. Histology and gene expression analysis revealed that majority of the Hep-DAT scaffolds were infiltrated with host-derived adipose tissues that possessed similar adipogenic and inflammatory gene expression as endogenous adipose tissues. Additionally, strong de novo adipogenesis could also be induced when bFGF-binding Hep-DATs were thoroughly minced and injected subcutaneously. In conclusion, our study demonstrated that bFGF-binding Hep-DAT could be an efficient, biocompatible and injectable adipogenic system for in vivo adipose tissue engineering.

Adipose and mammary epithelial tissue engineering

Breast reconstruction is a type of surgery for women who have had a mastectomy, and involves using autologous tissue or prosthetic material to construct a natural-looking breast. Adipose tissue is the major contributor to the volume of the breast, whereas epithelial cells comprise the functional unit of the mammary gland. Adipose-derived stem cells (ASCs) can differentiate into both adipocytes and epithelial cells and can be acquired from autologous sources. ASCs are therefore an attractive candidate for clinical applications to repair or regenerate the breast. Here we review the current state of adipose tissue engineering methods, including the biomaterials used for adipose tissue engineering and the application of these techniques for mammary epithelial tissue engineering. Adipose tissue engineering combined with microfabrication approaches to engineer the epithelium represents a promising avenue to replicate the native structure of the breast.

Sustainable three-dimensional tissue model of human adipose tissue

The need for physiologically relevant sustainable human adipose tissue models is crucial for understanding tissue development, disease progression, in vitro drug development and soft tissue regeneration. The coculture of adipocytes differentiated from human adipose-derived stem cells, with endothelial cells, on porous silk protein matrices for at least 6 months is reported, while maintaining adipose-like outcomes. Cultures were assessed for structure and morphology (Oil Red O content and CD31 expression), metabolic functions (leptin, glycerol production, gene expression for GLUT4, and PPARγ) and cell replication (DNA content). The cocultures maintained size and shape over this extended period in static cultures, while increasing in diameter by 12.5% in spinner flask culture. Spinner flask cultures yielded improved adipose tissue outcomes overall, based on structure and function, when compared to the static cultures. This work establishes a tissue model system that can be applied to the development of chronic metabolic dysfunction systems associated with human adipose tissue, such as obesity and diabetes, due to the long term sustainable functions demonstrated here.

An adipoinductive role of inflammation in adipose tissue engineering: key factors in the early development of engineered soft tissues

Tissue engineering and cell implantation therapies are gaining popularity because of their potential to repair and regenerate tissues and organs. To investigate the role of inflammatory cytokines in new tissue development in engineered tissues, we have characterized the nature and timing of cell populations forming new adipose tissue in a mouse tissue engineering chamber (TEC) and characterized the gene and protein expression of cytokines in the newly developing tissues. EGFP-labeled bone marrow transplant mice and MacGreen mice were implanted with TEC for periods ranging from 0.5 days to 6 weeks. Tissues were collected at various time points and assessed for cytokine expression through ELISA and mRNA analysis or labeled for specific cell populations in the TEC. Macrophage-derived factors, such as monocyte chemotactic protein-1 (MCP-1), appear to induce adipogenesis by recruiting macrophages and bone marrow-derived precursor cells to the TEC at early time points, with a second wave of nonbone marrow-derived progenitors. Gene expression analysis suggests that TNFα, LCN-2, and Interleukin 1β are important in early stages of neo-adipogenesis. Increasing platelet-derived growth factor and vascular endothelial cell growth factor expression at early time points correlates with preadipocyte proliferation and induction of angiogenesis. This study provides new information about key elements that are involved in early development of new adipose tissue.

Adipose tissue regeneration: a state of the art

Adipose tissue pathologies and defects have always represented a reconstructive challenge for plastic surgeons. In more recent years, several allogenic and alloplastic materials have been developed and used as fillers for soft tissue defects. However, their clinical use has been limited by further documented complications, such as foreign-body reactions potentially affecting function, degradation over time, and the risk for immunogenicity. Tissue-engineering strategies are thus being investigated to develop methods for generating adipose tissue. This paper will discuss the current state of the art in adipose tissue engineering techniques, exploring the biomaterials used, stem cells application, culture strategies, and current regulatory framework that are in use are here described and discussed.

Role of hyaluronan in angiogenesis and its utility to angiogenic tissue engineering

Angiogenesis represents the outgrowth of new blood vessels from existing ones, a physiologic process that is vital to supply nourishment to newly forming tissues during development and tissue remodeling and repair (wound healing). Regulation of angiogenesis in the healthy body occurs through a fine balance of angiogenesis-stimulating factors and angiogenesis inhibitors. When this balance is disturbed, excessive or deficient angiogenesis can result and contribute to development of a wide variety of pathological conditions. The therapeutic stimulation or suppression of angiogenesis could be the key to abrogating these diseases. In recent years, tissue engineering has emerged as a promising technology for regenerating tissues or organs that are diseased beyond repair. Among the critical challenges that deter the practical realization of the vision of regenerating functional tissues for clinical implantation, is how tissues of finite size can be regenerated and maintained viable in the long-term. Since the diffusion of nutrients and essential gases to cells, and removal of metabolic wastes is typically limited to a depth of 150-250 microm from a capillary (3-10 cells thick), tissue constructs must mandatorily permit in-growth of a blood capillary network to nourish and sustain the viability of cells within. The purpose of this article is to provide an overview of the role and significance of hyaluronan (HA), a glycosaminoglycan (GAG) component of connective tissues, in physiologic and pathological angiogenesis, its applicability as a therapeutic to stimulate or suppress angiogenesis in situ within necrotic tissues in vivo, and the factors determining its potential utility as a pro-angiogenic stimulus that will enable tissue engineering of neo-vascularized and functional tissue constructs for clinical use.

Adipose tissue engineering with cells in engineered matrices

Tissue engineering has shown promise for the development of constructs to facilitate large volume soft tissue augmentation in reconstructive and cosmetic plastic surgery. This article reviews the key progress to date in the field of adipose tissue engineering. In order to effectively design a soft tissue substitute, it is critical to understand the native tissue environment and function. As such, the basic physiology of adipose tissue is described and the process of adipogenesis is discussed. In this article, we have focused on tissue engineering using a cell-seeded scaffold approach, where engineered extracellular matrix substitutes are seeded with exogenous cells that may contribute to the regenerative response. The strengths and limitations of each of the possible cell sources for adipose tissue engineering, including adipose-derived stem cells, are detailed. We briefly highlight some of the results from the major studies to date, involving a range of synthetic and naturally derived scaffolds. While these studies have shown that adipose tissue regeneration is possible, more research is required to develop optimized constructs that will facilitate safe, predictable and long-term augmentation in clinical applications.

Layer-shaped alginate hydrogels enhance the biological performance of human adipose-derived stem cells

BACKGROUND

The reconstruction of adipose tissue defects is often challenged by the complications that may occur following plastic and reconstructive surgery, including donor-site morbidity, implant migration and foreign body reaction. To overcome these problems, adipose tissue engineering (ATE) using stem cell-based regeneration strategies has been widely explored in the last years. Mounting evidence has shown that adipose-derived stem cells (ADSCs) represent a promising cell source for ATE. In the context of a small number of reports concerning adipose tissue regeneration using three-dimensional (3-D) systems, the present study was designed to evaluate the biological performance of a novel alginate matrix that incorporates human ADSCs (hADSCs).

RESULTS

Culture-expanded cells isolated from the stromal vascular fraction (SVF), corresponding to the third passage which showed the expression of mesenchymal stem cell (MSC) markers, were used in the 3-D culture systems. The latter represented a calcium alginate hydrogel, obtained by the diffusion of calcium gluconate (CGH matrix), and shaped as discoid-thin layer. For comparative purposes, a similar hADSC-laden alginate hydrogel cross-linked with calcium chloride was considered as reference hydrogel (RH matrix). Both hydrogels showed a porous structure under scanning electron microscopy (SEM) and the hADSCs embedded displayed normal spherical morphologies, some of them showing signs of mitosis. More than 85% of the entrapped cells survived throughout the incubation period of 7 days. The percentage of viable cells was significantly higher within CGH matrix at 2 days post-seeding, and approximately similar within both hydrogels after 7 days of culture. Moreover, both alginate-based hydrogels stimulated cell proliferation. The number of hADSC within hydrogels has increased during the incubation period of 7 days and was higher in the case of CGH matrix. Cells grown under adipogenic conditions for 21 days showed that both analyzed 3-D culture systems support adipogenic differentiation in terms of neutral lipid accumulation and perillipin expression. Furthermore, the cells encapsulated in CGH matrix displayed a more differentiated phenotype.

CONCLUSIONS

The results of this study suggest that both CGH and RH matrices successfully support the survival and adipogenesis of hADSC. An enhancement of biological performance was detected in the case of CGH matrix, suggesting its promising application in ATE.

Revascularization and tissue regeneration of an empty root canal space is enhanced by a direct blood supply and stem cells

BACKGROUND

Regenerative endodontics is an innovative treatment concept aiming to regenerate pulp, dentin and root structures. In the diseased or necrotic tooth, the limitation in vascular supply renders successful tissue regeneration/generation in a whole tooth challenging. The aim of this study is to evaluate the ability of vascularized tissue to develop within a pulpless tooth using tissue engineering techniques.

MATERIALS AND METHODS

A pulpless tooth chamber, filled with collagen I gel containing isolated rat dental pulp cells (DPC) and angiogenic growth factors, was placed into a hole created in the femoral cortex or into its own tooth socket, respectively. The gross, histological and biochemical characteristics of the de novo tissue were evaluated at 4 and 8 weeks post-transplantation.

RESULTS

Tooth revascularization and tissue generation was observed only in the femur group, confirming the important role of vascular supply in tissue regeneration. The addition of cells and growth factors significantly promoted connective tissue production in the tooth chamber.

CONCLUSION

Successful revascularization and tissue regeneration in this model demonstrate the importance of a direct vascular supply and the advantages of a stem cell approach.

Sustained release of adiponectin improves osteogenesis around hydroxyapatite implants by suppressing osteoclast activity in ovariectomized rabbits

Lack of estrogen could lead to decreased bone mass and increased risk for osteoporosis, which has a negative influence on biomaterial implantation. Adiponectin (APN), an adipose-derived hormone, has been shown to increase bone density by inhibiting osteoclast formation and promoting the formation of osteoblasts. This study was designed to investigate the direct effects of APN released from the Matrigel controlled-release system on the activity of rabbit mature osteoclasts and osteoclast precursor RAW264.7 cells in vitro, and to determine its effects by improving osteogenesis around the hydroxyapatite (HA) implant in ovariectomized (OVX) rabbits. APN+Matrigel+HA, APN+HA, Matrigel+HA and HA were implanted into mandibular defects of OVX rabbits. At 4 weeks after implantation, the mandibles were examined by histology, microcomputed tomography and biomechanical testing. The results demonstrated that Matrigel extended the length of APN released to achieve long-term persistence. The sustained release of APN suppressed the osteoclastic activity both in vitro and in vivo, and improved the peri-implant osteogenesis in OVX rabbits, while the short-term APN treatment did not. Sustained release of APN may be an effective strategy to improve the restoration of bone defects by the use of HA materials under osteoporotic conditions in which osteoclasts are highly activated.

Drug delivery in soft tissue engineering

INTRODUCTION

Tissue defects, sustained through disease or trauma, present enormous challenges in regenerative medicine. Modern tissue engineering (TE) aims at replacing or repairing these defects through a combined approach of biodegradable scaffolds, suitable cell sources and appropriate environmental cues, such as biomolecules presented on scaffold surfaces or sustainably released from within.

AREAS COVERED

This review provides a brief overview of the various drugs and bioactive molecules of interest to TE, as well as a selection of materials that have been proposed for TE scaffolds and matrices in the past. It then proceeds to discuss encapsulation, immobilization and controlled release strategies for bioactive proteins, before discussing recent advances in this area with a special focus on soft TE.

EXPERT OPINION

Overall, minimal clinical success has been achieved so far in using growth factor, morphogen, or adhesion factor modified scaffolds and matrices; only one growth factor delivery system (Regranex Gel), has been approved by the FDA for clinical use, with only a handful of other growth factors being approved for human use so far. However, many more growth factors are currently in clinical Phase I - II or preclinical trials and many delivery systems utilize materials already approved by the FDA for other purposes. With respect to drug delivery in soft TE, a combination of increased research efforts in hydrogel and support material development as well as growth factor development is needed before clinical success is realized.

Adipose tissue engineering for soft tissue regeneration

Current treatment modalities for soft tissue defects caused by various pathologies and trauma include autologous grafting and commercially available fillers. However, these treatment methods present a number of challenges and limitations, such as donor-site morbidity and volume loss over time. As such, improved therapeutic modalities need to be developed. Tissue engineering techniques offer novel solutions to these problems through development of bioactive tissue constructs that can regenerate adipose tissue in both structure and function. Recently, a number of studies have been designed to explore various methods to engineer human adipose tissue. This review will focus on these developments in the area of adipose tissue engineering for soft tissue replacement. The physiology of adipose tissue and current surgical therapies used to replace lost tissue volume, specifically in breast tissue, are introduced, and current biomaterials, cell sources, and tissue culture strategies are discussed. We discuss future areas of study in adipose tissue engineering.

Dermis-derived hydrogels support adipogenesis in vivo

Biomaterials that support adipogenesis could contribute to tissue engineering therapies to be used as alternatives to traditional methods of tissue reconstruction and regeneration. We have recently shown that hydrogels comprised of urea soluble proteins and polysaccharides extracted from adipose tissue promote preadipocyte differentiation in vitro and adipogenesis in vivo. However, it is not clear if these findings result from the adipose tissue source of the extracts or if the technique isolates adipogenic factors from other tissues. The present study investigates whether the application of this technique to dermis samples would provide adipogenic hydrogels. Extracts from dermis assembled into hydrogels by either temperature or pH mechanisms. Both formulations supported preadipocyte differentiation in vitro and vascularized adipose formation in vivo. The temperature formulation of the gels induced more rapid adipose formation than the pH formulation in vivo. Interestingly, in comparison to our previous studies the dermis derived hydrogels had comparable adipogenic properties to adipose gels in vivo but not in vitro. Further study of these materials could lead to insight of the role of specific matrix properties on adipogenesis.

Culture effects of epidermal growth factor (EGF) and basic fibroblast growth factor (bFGF) on cryopreserved human adipose-derived stromal/stem cell proliferation and adipogenesis

Previous studies have demonstrated that EGF and bFGF maintain the stem cell properties of proliferating human adipose-derived stromal/stem cells (hASCs) in vitro. While the expansion and cryogenic preservation of isolated hASCs are routine, these manipulations can impact their proliferative and differentiation potential. This study examined cryogenically preserved hASCs (n = 4 donors), with respect to these functions, after culture with basic fibroblast growth factor (bFGF) and epidermal growth factor (EGF) at varying concentrations (0-10 ng/ml). Relative to the control, cells supplemented with EGF and bFGF significantly increased proliferation by up to three-fold over 7-8 days. Furthermore, cryopreserved hASCs expanded in the presence of EGF and bFGF displayed increased oil red O staining following adipogenic induction. This was accompanied by significantly increased levels of several adipogenesis-related mRNAs: aP2, C/EBPalpha, lipoprotein lipase (LPL), PPARgamma and PPARgamma co-activator-1 (PGC1). Adipocytes derived from EGF- and bFGF-cultured hASCs exhibited more robust functionality based on insulin-stimulated glucose uptake and atrial natriuretic peptide (ANP)-stimulated lipolysis. These findings indicate that bFGF and EGF can be used as culture supplements to optimize the proliferative capacity of cryopreserved human ASCs and their adipogenic differentiation potential.

The effect of FGF-1 loaded alginate microbeads on neovascularization and adipogenesis in a vascular pedicle model of adipose tissue engineering

Engineered vascularized adipose tissue could serve as an alternative to traditional tissue reconstruction procedures. Adipose formation occurs in a coordinated fashion with neovascularization. Previous studies have shown that extracellular matrix-based materials supplemented with factors that stimulate neovascularization promote adipogenesis in a number of animal models. The present study examines the ability of fibroblast growth factor (FGF-1) delivered from alginate microbeads to induce neovascularization and adipogenesis in type I collagen gels in a vascular pedicle model of adipose tissue engineering. FGF-1 loaded microbeads stimulated greater vascular network formation in an in vitro 3D co-culture model than a single bolus of FGF-1. In in vivo studies, FGF-1 loaded beads suspended in collagen and implanted in a chamber surrounding the exposed femoral pedicle of a rat resulted in a significant increase in vascular density at 1 and 6 weeks in comparison to bolus administration of FGF-1. Staining for smooth muscle actin showed that over 48% of vessels had associated mural cells. While an increase in neovascularization was achieved, there was less than 3% adipose under any condition. These results show that delivery of FGF-1 from alginate beads stimulated a more persistent neovascularization response than bolus FGF-1 both in vitro and in vivo. However, unlike previous studies, this increased neovascularization did not result in adipogenesis. Future studies need to provide a better understanding of the relationship between neovascularization and adipogenesis in order to design advanced tissue engineering therapies.

Materials for engineering vascularized adipose tissue

Loss of adipose tissue can occur due to congenital and acquired lipoatrophies, trauma, tumor resection, and chronic disease. Clinically, it is difficult to regenerate or reconstruct adipose tissue. The extensive microvsacular network present in adipose, and the sensitivity of adipocytes to hypoxia, hinder the success of typical tissue transfer procedures. Materials that promote the formation of vascularized adipose tissue may offer alternatives to current clinical treatment options. A number of synthetic and natural biomaterials common in tissue engineering have been investigated as scaffolds for adipose regeneration. While these materials have shown some promise they do not account for the unique extracellular microenvironment of adipose. Adipose derived hydrogels more closely approximate the physical and chemical microenvironment of adipose tissue, promote preadipocyte differentiation and vessel assembly in vitro, and stimulate vascularized adipose formation in vivo. The combination of these materials with techniques that promote rapid and stable vascularization could lead to new techniques for engineering stable, vascularized adipose tissue for clinical application. In this review we discuss materials used for adipose tissue engineering and strategies for vascularization of these scaffolds.

CLINICAL RELEVANCE

Materials that promote formation of vascularized adipose tissue have the potential to serve as alternatives or supplements to existing treatment options, for adipose defects or deficiencies resulting from chronic disease, lipoatrophies, trauma, and tumor resection.

In vitro 3D model for human vascularized adipose tissue

The clinical need for both three-dimensional (3D) soft tissue replacements and in vitro adipose tissue models continues to grow. In this study, we evaluated structural and functional characteristics of an in vitro 3D coculture model of vascularized adipose tissue. Tomato red-infected human adipose tissue-derived mesenchymal stem cells (hASCs) and green fluorescence protein-infected human umbilical vein endothelial cells were cocultured on 3D aqueous-derived silk scaffolds for 2 weeks. Confocal microscopy images demonstrated viability of cocultures and organization of both cell types over time. Endothelial cells aligned with time, and further histological analyses revealed continuous endothelial lumen formation in both differentiated and undifferentiated cocultures. Differentiated adipose cocultures secreted significantly higher levels of leptin than undifferentiated cocultures at 1 and 2 weeks. Additionally, lipid accumulation was demonstrated with Oil Red O staining, where positive staining was higher in the differentiated cocultures. A promising in vitro approach for the vascularization of tissue-engineered adipose tissue, and the ability to vascularize a construct containing hASCs was demonstrated. The strategy outlined provides a basis for the formation of other in vitro vascularized tissues as well as a path forward for the sustainable formation of soft tissue due to the use of slowly degrading silk scaffolds.