FGF-2 enhances vascularization for adipose tissue engineering

Fatter Is Better: Boosting the Vascularization of Adipose Tissue Grafts

Adipose tissue resorption after fat grafting is a major drawback in plastic and reconstructive surgery, which is primarily caused by the insufficient blood perfusion of the grafts in the initial phase after transplantation. To overcome this problem, several promising strategies to boost the vascularization and, thus, increase survival rates of fat grafts have been developed in preclinical studies in recent years. These include the angiogenic stimulation of the grafts by growth factors and botulinum neurotoxin A, biologically active gels, and cellular enrichment, as well as the physical and pharmacological stimulation of the transplantation site. To transfer these approaches into future clinical practice, it will be necessary to establish standardized procedures for their safe application in humans. If this succeeds, the surgical outcomes of fat grafting may be markedly improved, resulting in a significant reduction of the physical and psychological stress for the patients.

In situ Adipogenesis in Biomaterials Without Cell Seeds: Current Status and Perspectives

For cosmetic and reconstructive purposes in the setting of small-volume adipose tissue damage due to aging, traumatic defects, oncological resections, and degenerative diseases, the current strategies for soft tissue replacement involve autologous fat grafts and tissue fillers with synthetic, bioactive, or tissue-engineered materials. However, they all have drawbacks such as volume shrinkage and foreign-body responses. Aiming to regenerate bioactive vascularized adipose tissue on biomaterial scaffolds, adipose tissue engineering (ATE) has emerged as a suitable substitute for soft tissue repair. The essential components of ATE include scaffolds as support, cells as raw materials for fat formation, and a tolerant local environment to allow regeneration to occur. The commonly loaded seeding cells are adipose-derived stem cells (ASCs), which are expected to induce stable and predictable adipose tissue formation. However, defects in stem cell enrichment, such as donor-site sacrifice, limit their wide application. As a promising alternative approach, cell-free bioactive scaffolds recruit endogenous cells for adipogenesis. In biomaterials without cell seeds, the key to sufficient adipogenesis relies on the recruitment of endogenous host cells and continuous induction of cell homing to scaffolds. Regeneration, rather than repair, is the fundamental dominance of an optimal mature product. To induce in situ adipogenesis, many researchers have focused on the mechanical and biochemical properties of scaffolds. In addition, efforts to regulate an angiogenic and adipogenic microenvironment in cell-free settings involve integrating growth factors or extracellular matrix (ECM) proteins onto bioactive scaffolds. Despite the theoretical feasibility and encouraging results in animal models, few of the reported cell-free biomaterials have been tested in humans, and failures of decellularized adipose tissues in adipogenesis have also been reported. In these cases, the most likely reason was the lack of supporting vasculature. This review summarizes the current status of biomaterials without cell seeds. Related mechanisms and influencing factors of in situ adipogenesis in cell-free biomaterials, dilemma in the development of biomaterials, and future perspectives are also addressed.

Fat Graft Enrichment Strategies: A Systematic Review

BACKGROUND

Autologous fat grafting is a dynamic modality used in plastic surgery as an adjunct to improve functional and aesthetic form. However, current practices in fat grafting for soft-tissue augmentation are plagued by tremendous variability in long-term graft retention, resulting in suboptimal outcomes and repetitive procedures. This systematic review identifies and critically appraises the evidence for various enrichment strategies that can be used to augment and improve the viability of fat grafts.

METHODS

A comprehensive literature search of the Medline and PubMed databases was conducted for animal and human studies published through October of 2017 with multiple search terms related to adipose graft enrichment agents encompassing growth factors, platelet-rich plasma, adipose-derived and bone marrow stem cells, gene therapy, tissue engineering, and other strategies. Data on level of evidence, techniques, complications, and outcomes were collected.

RESULTS

A total of 1382 articles were identified, of which 147 met inclusion criteria. The majority of enrichment strategies demonstrated positive benefit for fat graft survival, particularly with growth factors and adipose-derived stem cell enrichment. Platelet-rich plasma and adipose-derived stem cells had the strongest evidence to support efficacy in human studies and may demonstrate a dose-dependent effect.

CONCLUSIONS

Improved understanding of enrichment strategies contributing to fat graft survival can help to optimize safety and outcomes. Controlled clinical studies are lacking, and future studies should examine factors influencing graft survival through controlled clinical trials in order to establish safety and to obtain consistent outcomes.

Evaluation of different commercial hyaluronic acids as a vehicle for injection of human adipose-derived mesenchymal stem cells

Objective

The main purpose of this study is to evaluate, in vitro, the cytotoxicity of different commercial brands of hyaluronic acids to be used as a vehicle for injection of human adipose-derived mesenchymal stem cells (AD-MSCs).

Methods

AD-MSCs were divided into seven groups: one control group where AD-MSCs were cultivated with phosphate-buffered saline (PBS) and six other groups where AD-MSCs were cultivated with different commercial brands of hyaluronic acid. AD-MSC viability analysis was performed after 4, 24, and 48 h in contact with each treatment, using the trypan staining method on a Countess automated cell counter (Thermo Fisher Scientific).

Results

The results clearly demonstrated a significant difference in cell viability when AD-MSCs were exposed to different hyaluronic acids when compared with the control group.

Conclusion

These data suggest that hyaluronic acid can be used as a vehicle for injection of human AD-MSCs, but caution is needed to choose the best product, aiming at its future therapeutic application.

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.

Adipogenic Differentiation of Human Adipose-Derived Stem Cells Grown as Spheroids

Understanding the process of adipogenesis is critical if suitable therapeutics for obesity and related metabolic diseases are to be found. The current study presents proof of feasibility of creating a 3-D spheroid model using human adipose-derived stem cells (hASCs) and their subsequent adipogenic differentiation. hASC spheroids were formed atop an elastin-like polypeptide-polyethyleneimine (ELP-PEI) surface and differentiated using an adipogenic cocktail. Spheroids were matured in the presence of dietary fatty acids (linoleic or oleic acid) and evaluated based on functional markers including intracellular protein, CD36 expression, triglyceride accumulation, and PPAR-γ gene expression. Spheroid size was found to increase as the hASCs matured in the adipocyte maintenance medium, though the fatty acid treatment generally resulted in smaller spheroids compared to control. A stable protein content over the 10-day maturation period indicated contact-inhibited proliferation as well as minimal loss of spheroids during culture. Spheroids treated with fatty acids showed greater amounts of intracellular triglyceride content and greater expression of the key adipogenic gene, PPAR-γ. We also demonstrated that 3-D spheroids outperformed 2-D monolayer cultures in adipogenesis. We then compared the adipogenesis of hASC spheroids to that in 3T3-L1 spheroids and found that the triglyceride accumulation was less profound in hASC spheroids than that in 3T3-L1 adipocytes, correlated with smaller average spheroids, suggesting a relatively slower differentiation process. Taken together, we have shown the feasibility of adipogenic differentiation of patient-derived hASC spheroids, which with further development, may help elucidate key features in the adipogenesis process.

Computational Model-Based Analysis of Strategies to Enhance Scaffold Vascularization

Stable and extensive blood vessel networks are required for cell function and survival in engineered tissues. A number of different strategies are currently being investigated to enhance biomaterial vascularization with screening primarily through extensive in vitro and in vivo experiments. In this article, we describe an agent-based model (ABM) developed to evaluate various strategies in silico, including design of optimal biomaterial structure, delivery of angiogenic factors, and application of prevascularized biomaterials. The model predictions are evaluated using experimental data. The ABM developed provides insight into different strategies currently applied for scaffold vascularization and will enable researchers to rapidly screen new hypotheses and explore alternative strategies for enhancing vascularization.

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.

Adipose-derived stem cell differentiation as a basic tool for vascularized adipose tissue engineering

The development of in vitro adipose tissue constructs is highly desired to cope with the increased demand for substitutes to replace damaged soft tissue after high graded burns, deformities or tumor removal. To achieve clinically relevant dimensions, vascularization of soft tissue constructs becomes inevitable but still poses a challenge. Adipose-derived stem cells (ASCs) represent a promising cell source for the setup of vascularized fatty tissue constructs as they can be differentiated into adipocytes and endothelial cells in vitro and are thereby available in sufficiently high cell numbers. This review summarizes the currently known characteristics of ASCs and achievements in adipogenic and endothelial differentiation in vitro. Further, the interdependency of adipogenesis and angiogenesis based on the crosstalk of endothelial cells, stem cells and adipocytes is addressed at the molecular level. Finally, achievements and limitations of current co-culture conditions for the construction of vascularized adipose tissue are evaluated.

Strategies for the chemical and biological functionalization of scaffolds for cardiac tissue engineering: a review

The development of biomaterials for cardiac tissue engineering (CTE) is challenging, primarily owing to the requirement of achieving a surface with favourable characteristics that enhances cell attachment and maturation. The biomaterial surface plays a crucial role as it forms the interface between the scaffold (or cardiac patch) and the cells. In the field of CTE, synthetic polymers (polyglycerol sebacate, polyethylene glycol, polyglycolic acid, poly-l-lactide, polyvinyl alcohol, polycaprolactone, polyurethanes and poly(N-isopropylacrylamide)) have been proven to exhibit suitable biodegradable and mechanical properties. Despite the fact that they show the required biocompatible behaviour, most synthetic polymers exhibit poor cell attachment capability. These synthetic polymers are mostly hydrophobic and lack cell recognition sites, limiting their application. Therefore, biofunctionalization of these biomaterials to enhance cell attachment and cell material interaction is being widely investigated. There are numerous approaches for functionalizing a material, which can be classified as mechanical, physical, chemical and biological. In this review, recent studies reported in the literature to functionalize scaffolds in the context of CTE, are discussed. Surface, morphological, chemical and biological modifications are introduced and the results of novel promising strategies and techniques are discussed.

Fat grafting: a citation analysis of the seminal articles

Background:

There has been substantial rise in the volume of published works on fat transfer in the medical literature in the past 25 years, and this is indicative of its growing popularity. However, many unanswered questions remain, and there is no consensus as to the optimum technique. Consequently, the scientific and clinical research on fat grafting continues to increase rapidly. The purpose of our study was to perform a bibliometric analysis of the most-cited articles in fat transfer.

Methods:

Through the Web of Science, all articles relating to fat grafting were identified in the plastic and reconstructive literature. The 100 most-cited articles were identified and analyzed individually.

Results:

Total citations ranged from 35 to 363 and the most-cited paper by Sidney Coleman was published in Plastic and Reconstructive Surgery. The United States produced 46% of the most-cited papers, and the University of California was the most prolific institution. Twenty-one articles focused on lipofilling to the face while 14 articles looked at fat grafting to the breast.

Conclusions:

The scientific relevance of a published work is reflected in the number of citations from peers that it receives. Therefore, the 100 most-cited papers in fat grafting have been the most influential articles on this field, and they are likely to be the ones that are remembered most.

Improvement of the survival of human autologous fat transplantation by adipose-derived stem-cells-assisted lipotransfer combined with bFGF

Adipose-derived stem cells (ASCs) transplanted along with autologous adipose tissue may improve fat graft survival; however, the efficacy of ASCs has been diluted by low vascularization. This study was designed to test the hypothesis that basic fibroblast growth factor (bFGF) may improve the effects of ASCs because it owns the property to boost angiogenesis. In the present study, human fat tissues were mixed with ASCs, ASCs plus 100 U bFGF, or medium as the control and then injected subcutaneously into immunologically compromised nude mice for 12 weeks. Our findings demonstrated that mixture with the ASCs significantly increased the weight and volume of the fat grafts compared to control grafts, and histological analysis revealed that both ASCs and ASCs plus bFGF grafts consisted predominantly of adipose tissue and had significantly less fibrosis but greater microvascular density compared with control and also grafts mixed with ASCs had a high expression of angiogenic factors. More importantly, the bFGF treated fat grafts shown elevate in survival, vascularization, and angiogenic factors expression when compared with the grafts that received ASCs alone. These results indicated that bFGF together with ASCs can enhance the efficacy of autologous fat transplantation and increase blood vessel generation involved in the benefits from bFGF.

Adipose-derived stem cells: Implications in tissue regeneration

Adipose-derived stem cells (ASCs) are mesenchymal stem cells (MSCs) that are obtained from abundant adipose tissue, adherent on plastic culture flasks, can be expanded in vitro, and have the capacity to differentiate into multiple cell lineages. Unlike bone marrow-derived MSCs, ASCs can be obtained from abundant adipose tissue by a minimally invasive procedure, which results in a high number of cells. Therefore, ASCs are promising for regenerating tissues and organs damaged by injury and diseases. This article reviews the implications of ASCs in tissue regeneration.

Effect of an inductive hydrogel composed of urinary bladder matrix upon functional recovery following traumatic brain injury

Traumatic brain injury (TBI) is a major public health problem with no effective clinical treatment. Use of bioactive scaffold materials has been shown to be a promising strategy for tissue regeneration and repair in a number of injury models. Of these scaffold materials, urinary bladder matrix (UBM) derived from porcine bladder tissue, has demonstrated desirable properties for supporting and promoting the growth of neural cells in vitro, suggesting its potential as a scaffold for brain tissue repair in the treatment of TBI. Herein we evaluate the biocompatibility of UBM within brain tissue and the effects of UBM delivery upon functional outcome following TBI. A hydrogel form of UBM was injected into healthy rat brains for 1, 3, and 21 days to examine the tissue response to UBM. Multiple measures of tissue injury, including reactive astrocytosis, microglial activation, and neuron degeneration showed that UBM had no deleterious effects on normal brain. Following TBI, the brains were evaluated histologically and behaviorally between sham-operated controls and UBM- and vehicle-treated groups. Application of UBM reduced lesion volume and attenuated trauma-induced myelin disruption. Importantly, UBM treatment resulted in significant neurobehavioral recovery following TBI as demonstrated by improvements in vestibulomotor function; however, no differences in cognitive recovery were observed between the UBM- and vehicle-treated groups. The present study demonstrated that UBM is not only biocompatible within the brain tissue, but also can exert protective effects upon injured brain.

Recellularization of acellular human small intestine using bone marrow stem cells

We aimed to produce an acellular human tissue scaffold with a view to test the possibility of recellularization with bone marrow stem cells to produce a tissue-engineered small intestine (TESI). Human small-bowel specimens (n = 5) were obtained from cadaveric organ donors and treated sequentially with 6% dimethyl sulfoxide in hypotonic buffer, 1% Triton X-100, and DNase. Each small intestine (SI) piece (6 cm) was recellularized with EPCAM+ and CD133+ allogeneic bone marrow stem cells. Histological and molecular analysis demonstrated that after decellularization, all cellular components and nuclear material were removed. Our analysis also showed that the decellularized human SI tissue retained its histoarchitecture with intact villi and major structural proteins. Protein films of common extracellular matrix constituents (collagen I, laminin, and fibronectin) were found in abundance. Furthermore, several residual angiogenic factors were found in the decellularized SI. Following recellularization, we found viable mucin-positive goblet cells, CK18+ epithelial cells in villi adjacent to a muscularis mucosa with α-actin+ smooth muscle cells, and a high repopulation of blood vessels with CD31+ endothelial cells. Our results show that in the future, such a TESI would be ideal for clinical purposes, because it can be derived from the recipient's own immunocompatible bone marrow cells, thus avoiding the use of immunosuppression.

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.

Tissue engraftment of hypoxic-preconditioned adipose-derived stem cells improves flap viability

Adipose-derived stem cells (ASCs) have the ability to release multiple growth factors in response to hypoxia. In this study, we investigated the potential of ASCs to prevent tissue ischemia. We found conditioned media from hypoxic ASCs had increased levels of vascular endothelial growth factor (VEGF) and enhanced endothelial cell tubule formation. To investigate the effect of injecting rat ASCs into ischemic flaps, 21 Lewis rats were divided into three groups: control, normal oxygen ASCs (10(6) cells), and hypoxic preconditioned ASCs (10(6) cells). At the time of flap elevation, the distal third of the flap was injected with the treatment group. At 7 days post flap elevation, flap viability was significantly improved with injection of hypoxic preconditioned ASCs. Cluster of differentiation-31-positive cells were more abundant along the margins of flaps injected with ASCs. Fluorescent labeled ASCs localized aside blood vessels or throughout the tissue, dependent on oxygen preconditioning status. Next, we evaluated the effect of hypoxic preconditioning on ASC migration and chemotaxis. Hypoxia did not affect ASC migration on scratch assay or chemotaxis to collagen and laminin. Thus, hypoxic preconditioning of injected ASCs improves flap viability likely through the effects of VEGF release. These effects are modest and represent the limitations of cellular and growth factor-induced angiogenesis in the acute setting of ischemia.

Characterizing bovine adipocyte distribution and its relationship with carcass and meat characteristics using a finite mixture model

The appreciation of adipose tissue complexity has initiated a new era of multifaceted investigations that continue to provide findings in adipocyte biology, but quantitative descriptions of adipocyte distribution are lacking. The first objective was to develop a finite mixture model to model adipocyte bimodal distribution and to correlate these estimates with carcass and meat characteristics. A secondary objective was to demonstrate within-animal observed variability in adipocyte cellularity. Steers were finished on a high-grain diet (n = 14) or grass (n = 16). One 12-cm thick LM steak from each steer was collected during harvest. A probability density function was developed that partitioned the cell diameter population into small and large populations and described the relative proportions of cells for each animal in these 2 distinct populations. Five parameters were estimated through the finite mixture model: the means (μ(1) and μ(2)) and SD (σ(1) and σ(2)) for the small and large adipocyte populations, respectively, and a proportion parameter (p) describing the proportion of the distribution of the smaller adipocyte populations. The proportion parameter for all animals tended to be different (P = 0.07) between groups with the grain presenting a p of 22.5 ± 12.5% and grass 16.2 ± 4.7%. The μ(2) was correlated with yield grade (YG, P = 0.04), and σ(2) with final BW, HCW, dressing percentage, YG, and quality grade score (P = 0.01). When correlating these parameters with the sensory data, μ(2) and σ(2) were correlated with tenderness (P ≤ 0.05), σ(1) and p with juiciness (P ≤ 0.05), and p with overall palatability (P = 0.01). Adipocyte cellularity variability was measured by examining the results from 5 randomly chosen steers from each group (grain and grass). In this subset, the μ(1) and p ranged from 32.1 to 46.1 μm and 1 to 27% for grass-finished steers, and ranged from 33.7 to 41.0 μm and 10 to 48% for grain-finished steers. The μ(2) and (1 - p) ranged from 75.0 to 105.1 μm and 73 to 99% for grass-finished steers, respectively, and ranged from 84.8 to 124.0 μm and 52 to 90% for grain-finished steers, respectively. The finite mixture model provides a quantitative description of the distribution of adipocytes and contributes to explaining adipocyte biology. Adipocyte cellularity variability among samples within an animal is a topic that should be further evaluated, as well as its correlation with other factors, such as gene expression and hormone secretion.

Injectable biomaterials for adipose tissue engineering

Adipose tissue engineering has recently gained significant attention from materials scientists as a result of the exponential growth of soft tissue filler procedures being performed within the clinic. While several injectable materials are currently being marketed for filling subcutaneous voids, they often face limited longevity due to rapid resorption. Their inability to encourage natural adipose formation or ingrowth necessitates repeated injections for a prolonged effect and thus classifies them as temporary fillers. As a result, a significant need for injectable materials that not only act as fillers but also promote in vivo adipogenesis is beginning to be realized. This paper will discuss the advantages and disadvantages of commercially available soft tissue fillers. It will then summarize the current state of research using injectable synthetic materials, biopolymers and extracellular matrix-derived materials for adipose tissue engineering. Furthermore, the successful attributes observed across each of these materials will be outlined along with a discussion of the current difficulties and future directions for adipose tissue engineering.

Adipose-derived mesenchymal stem cells: biology and potential applications

Adipose tissue is derived from the mesoderm during embryonic development and is present in every mammalian species, located throughout the body. Adipose tissue serves as an endocrine organ, functioning to maintain energy metabolism through the storage of lipids. While two types of adipose tissue exist (brown and white), white adipose yields the commonly studied adipose-derived stem cells (ASCs). Adipose-derived stem cells provide a promising future in the field of tissue engineering and regenerative medicine. Due to their wide availability and ability to differentiate into other tissue types of the mesoderm-including bone, cartilage, muscle, and adipose-ASCs may serve a wide variety of applications. Adipose stem cells have been utilized in studies addressing osteoarthritis, diabetes mellitus, heart disease, and soft tissue regeneration and reconstruction after mastectomy and facial repair. Various delivery systems and scaffolds to incorporate adipose stem cells have also been established. Adipose stem cells have been studied in vitro and in vivo. Much information in vitro has been obtained on adipose stem cell potency and biology as a function of donor gender, body mass index, and anatomical location. Further in vitro studies have examined the various cell populations within the heterogeneous population within the stromal vascular fraction (SVF) from which ASCs are obtained. While many animal models are used to investigate adipose tissue, preclinical in vivo experiments are most widely conducted in the mouse model. Common analyses of animal studies utilizing ASCs include pre-labeling cells and immunostaining cells.

Three-dimensional spatial patterning of proteins in hydrogels

The ability to create three-dimensional biochemical environments that mimic those in vivo is valuable for the elucidation of fundamental biological phenomena and pathways. To this end, we designed a system in which proteins can be photochemically patterned in three dimensions within hydrogels under physiological conditions. Fibroblast growth factor-2 (FGF2) was immobilized within agarose hydrogels that were modified with two-photon labile 6-bromo-7-hydroxycoumarin-protected thiols. Two different methods were developed for FGF2 immobilization. The first procedure relies on the protein containing free cysteines for the formation of disulfide bonds with photoexposed agarose thiols. The second procedure takes advantage of the femtomolar binding partners, human serum albumin (HSA) and albumin binding domain (ABD), which have K(D) values of ~10(-14) M. Here HSA-maleimide was chemically bound to photoexposed agarose thiols, and then the FGF2-ABD fusion protein was added to form a stable complex with the immobilized HSA. The use of orthogonal, physical binding pairs allows protein immobilization under mild conditions and can be broadly applied to any protein expressed as an ABD fusion.

Long-term stability of adipose tissue generated from a vascularized pedicled fat flap inside a chamber

BACKGROUND

Numerous studies demonstrate the generation and short-term survival of adipose tissue; however, long-term persistence remains elusive. This study evaluates long-term survival and transferability of de novo adipose constructs based on a ligated vascular pedicle and tissue engineering chamber combination.

METHODS

Defined adipose tissue flaps were implanted into rats in either intact or perforated domed chambers. In half of the groups, the chambers were removed after 10 weeks and the constructs transferred on their vascular pedicle to a new site, where they were observed for a further 10 weeks. In the remaining groups, the tissue construct was observed for 20 weeks inside the chamber. Tissue volume was assessed using magnetic resonance imaging and histologic measures, and constructs were assessed for stability and necrosis. Sections were assessed histologically and for proliferation using Ki-67.

RESULTS

At 20 weeks, volume analysis revealed an increase in adipose volume from 0.04 ± 0.001 ml at the time of insertion into the chambers to 0.27 ± 0.004 ml in the closed and 0.44 ± 0.014 ml in the perforated chambers. There was an additional increase of approximately 10 to 15 percent in tissue volume in flaps that remained in chambers for 20 weeks, whereas the volume of the transferred tissue not in chambers remained unaltered. Histomorphometric assessment of the tissues documented no signs of hypertrophy, fat necrosis, or atypical changes of the newly generated tissue.

CONCLUSION

This study presents a promising new method of generating significant amounts of mature, vascularized, stable, and transferable adipose tissue for permanent autologous soft-tissue replacement.

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.

In vivo growth of a bioengineered internal anal sphincter: comparison of growth factors for optimization of growth and survival

PURPOSE

Our laboratory has developed and implanted a novel bioengineered internal anal sphincter (IAS) to treat anal incontinence. Fibroblast growth factor-2 (FGF-2) has been used in mice; however, the optimal growth factor for successful IAS implantation is unclear. This study compares several growth factors in order to optimize IAS viability and functionality.

METHODS

Bioengineered IAS rings were implanted subcutaneously into the dorsum of wildtype C57Bl/6 mice, with an osmotic pump dispensing FGF-2, vascular endothelial growth factor (VEGF), or platelet-derived growth factor (PDGF) (n = 4 per group). Control mice received IAS implants but no growth factor. The IAS was harvested approximately 25 days post-implantation. Tissue was subjected to physiologic testing, then histologically analyzed. Muscle phenotype was confirmed by immunofluorescence.

RESULTS

All implants supplemented with growth factors maintained smooth muscle phenotype. Histological scores, blood vessel density and muscle fiber thickness were all markedly better with growth factors. Neovascularization was comparable between the three growth factors. Basal tonic force of the constructs was highest with VEGF or PDGF.

CONCLUSION

All growth factors demonstrated excellent performance. As our ultimate goal is clinical implantation, our strong results with PDGF, a drug approved for use in the United States and the European Union, pave the way for translating bioengineered IAS implantation to the clinical realm.

Adipose-derived stem cells for clinical applications: a review

The use of stem cells derived from adipose tissue as an autologous and self-replenishing source for a variety of differentiated cell phenotypes, provides a great deal of promise for reconstructive surgery. In this article, we review available literature encompassing methods of extraction of pluripotent adipose stem cells (ASCs) from lipoaspirate locations, their storage, options for culture, growth and differentiation, cryopreservation and its effect on stem cell survival and proliferation, and new technologies involving biomaterials and scaffolds. We will conclude by assessing potential avenues for developing this incredibly promising field.

The effects of processing methods upon mechanical and biologic properties of porcine dermal extracellular matrix scaffolds

Biologic materials from various species and tissues are commonly used as surgical meshes or scaffolds for tissue reconstruction. Extracellular matrix (ECM) represents the secreted product of the cells comprising each tissue and organ, and therefore provides a unique biologic material for selected regenerative medicine applications. Minimal disruption of ECM ultrastructure and content during tissue processing is typically desirable. The objective of this study was to systematically evaluate effects of commonly used tissue processing steps upon porcine dermal ECM scaffold composition, mechanical properties, and cytocompatibility. Processing steps evaluated included liming and hot water sanitation, trypsin/SDS/TritonX-100 decellularization, and trypsin/TritonX-100 decellularization. Liming decreased the growth factor and glycosaminoglycan content, the mechanical strength, and the ability of the ECM to support in vitro cell growth (p ≤ 0.05 for all). Hot water sanitation treatment decreased only the growth factor content of the ECM (p ≤ 0.05). Trypsin/SDS/TritonX-100 decellularization decreased the growth factor content and the ability of the ECM to support in vitro cell growth (p ≤ 0.05 for both). Trypsin/Triton X-100 decellularization also decreased the growth factor content of the ECM but increased the ability of the ECM to support in vitro cell growth (p ≤ 0.05 for both). We conclude that processing steps evaluated in the present study affect content, mechanical strength, and/or cytocompatibility of the resultant porcine dermal ECM, and therefore care must be taken in choosing appropriate processing steps to maintain the beneficial effects of ECM in biologic scaffolds.

Slow and sustained release of active cytokines from self-assembling peptide scaffolds

Controlling the cellular microenvironment is thought to be critical for the successful application of biomaterials for regenerative medicine strategies. Self-assembling peptides are proving to be a promising platform for a variety of regenerative medicine applications. Specifically, RADA16-I self-assembling peptides have been successfully used for 3D cell culture, accelerated wound healing, and nerve-repair. Understanding the fundamental mechanisms for protein mobility within, and ultimately release from, this nanostructured system is a critical aspect for controlling cellular activity; studies which are largely lacking within the literature. Herein, we report that designer self-assembling peptide scaffolds facilitate slow and sustained release of active cytokines that are extremely relevant to many areas of regenerative medicine. In addition, multiple diffusive mechanisms are observed to exist for human betaFGF, VEGF and BDNF within RADA16-I and two different RADA16-I nanofiber forming peptides with net positive or negative charges located at the C-terminus. In some cases, two populations of diffusing molecules are observed at the molecular level: one diffusing fully within the solvent, and another that exhibits hindered mobility. Results suggest that protein mobility is inhibited by both physical hinderances and charge induced interactions between the protein and peptide nanofibers. Moreover, assays using adult neural stem cells (NSCs) are employed to assess the functional release of active cytokine (betaFGF) up to three weeks. Our results not only provide evidence for long-term molecular release from self-assembling peptide scaffolds but also inspiration for a plethora of slow molecular release strategies for clinical applications.

Novel multiarm PEG-based hydrogels for tissue engineering

Injectable scaffolds are promising substrates for regenerative medicine applications. In this study, multiarm amino-terminated poly(ethylene glycol) (PEG) hydrogels were crosslinked with genipin, a compound naturally derived from the gardenia fruit. Four- and eight-arm amino-terminated PEG hydrogels crosslinked with varying concentrations of genipin were characterized. Both surface and cross-sectional structures of PEG-based hydrogels were observed by scanning electron microscopy. In vitro gelation time, water uptake, swelling, and weight loss of PEG hydrogels in phosphate buffered saline at 37 degrees C were studied. The results showed that the eight-arm PEG demonstrated a much slower gelation time compared with the four-arm PEG, which may be due to the differing structures of the multiarm PEG hydrogels, which in turn affects the ability of genipin to react with the amine groups. Human adipose-derived stem cells were seeded onto the four- and eight-arm PEG hydrogels in vitro to assess the biological performance and applicability of the gels as cell carriers. The four-arm PEG hydrogel resulted in enhanced cell adhesion when compared with the eight-arm PEG hydrogel. Overall, these characteristics provide a potential opportunity for multiarm PEG hydrogels as injectable scaffolds in a variety of tissue engineering applications.

Fat depot-specific differences in angiogenic growth factor gene expression and its relation to adipocyte size in cattle

Adipocytes derived from different anatomical sites vary in the expression of adipocytokines and growth factor genes. Adipogenesis is tightly associated with angiogenesis, although the regional variation of angiogenic growth factor gene expression in adipose tissues remains unclear. In this experiment, we studied the fat depot-specific differences (subcutaneous, intramuscular, intermuscular, renal, and mesenteric) in the expression of angiogenic growth factor mRNA [vascular endothelial growth factor (VEGF), fibroblast growth factor-2 (FGF-2), fibroblast growth factor-10 (FGF-10), hepatocyte growth factor (HGF), and leptin], as well as the relationship between angiogenic growth factor mRNA level and adipocyte size in bovine adipose tissues. Intermuscular, renal, and mesenteric adipose tissues expressed significantly higher VEGF, FGF-2, and leptin mRNA levels than did subcutaneous and intramuscular adipose tissues. Mesenteric adipose tissue also expressed higher FGF-10 mRNA levels than did subcutaneous and intramuscular adipose tissues. There was no significant difference in the expression of HGF mRNA among adipose tissue depots. A significant correlation existed between adipocyte size and VEGF, FGF-2, FGF-10, and leptin mRNA levels. These results indicate that fat depot-specific difference in angiogenic growth factor gene expression results from the difference in adipocyte size.

Injectable, Biodegradable Hydrogels for Tissue Engineering Applications

Hydrogels have many different applications in the field of regenerative medicine. Biodegradable, injectable hydrogels could be utilized as delivery systems, cell carriers, and scaffolds for tissue engineering. Injectable hydrogels are an appealing scaffold because they are structurally similar to the extracellular matrix of many tissues, can often be processed under relatively mild conditions, and may be delivered in a minimally invasive manner. This review will discuss recent advances in the field of injectable hydrogels, including both synthetic and native polymeric materials, which can be potentially used in cartilage and soft tissue engineering applications.

Surgical implantation of a bioengineered internal anal sphincter

PURPOSE

Fecal incontinence is a common disorder that can have devastating social and psychologic consequences. However, there are no long-term ideal solutions for such patients. Although loss of continence is multifactorial, the integrity of the internal anal sphincter (IAS) has particular significance. We previously described the development of 3-dimensional bioengineered constructs using isolated smooth muscle tissue from donor C57BL/6 IAS. We hypothesized that the bioengineered ring constructs would retain cellular viability and promote neovascularization upon implantation into a recipient mouse.

METHODS

Internal anal sphincter ring constructs were surgically implanted into the subcutaneous tissue of syngeneic C57BL/6 mice and treated with either fibroblastic growth factor 2 (0.26 microg daily) or saline controls using a microosmotic pump. Internal anal sphincter constructs were harvested after 25 days (range, 23-26 days) and assessed morphologically and for tissue viability.

RESULT

Gross morphology showed that there was no rejection. Rings showed muscle attachment to the back of the mouse with no sign of inflammation. Fibroblastic growth factor 2 infusion resulted in a significantly improved histologic score and muscle viability compared with the control group.

CONCLUSIONS

Three-dimensional bioengineered IAS rings can be successfully implanted into the subcutaneous tissue of recipient mice. The addition of fibroblastic growth factor 2 led to improved muscle viability, vascularity, and survival. This approach may become a feasible option for patients with fecal incontinence.

Epimorphic regeneration approach to tissue replacement in adult mammals

Urodeles and fetal mammals are capable of impressive epimorphic regeneration in a variety of tissues, whereas the typical default response to injury in adult mammals consists of inflammation and scar tissue formation. One component of epimorphic regeneration is the recruitment of resident progenitor and stem cells to a site of injury. Bioactive molecules resulting from degradation of extracellular matrix (ECM) have been shown to recruit a variety of progenitor and stem cells in vitro in adult mammals. The ability to recruit multipotential cells to the site of injury by in vivo administration of chemotactic ECM degradation products in a mammalian model of digit amputation was investigated in the present study. Adult, 6- to 8-week-old C57/BL6 mice were subjected to midsecond phalanx amputation of the third digit of the right hind foot and either treated with chemotactic ECM degradation products or left untreated. At 14 days after amputation, mice treated with ECM degradation products showed an accumulation of heterogeneous cells that expressed markers of multipotency, including Sox2, Sca1, and Rex1 (Zfp42). Cells isolated from the site of amputation were capable of differentiation along neuroectodermal and mesodermal lineages, whereas cells isolated from control mice were capable of differentiation along only mesodermal lineages. The present findings demonstrate the recruitment of endogenous stem cells to a site of injury, and/or their generation/proliferation therein, in response to ECM degradation products.

Vascularization strategies for tissue engineering

Tissue engineering is currently limited by the inability to adequately vascularize tissues in vitro or in vivo. Issues of nutrient perfusion and mass transport limitations, especially oxygen diffusion, restrict construct development to smaller than clinically relevant dimensions and limit the ability for in vivo integration. There is much interest in the field as researchers have undertaken a variety of approaches to vascularization, including material functionalization, scaffold design, microfabrication, bioreactor development, endothelial cell seeding, modular assembly, and in vivo systems. Efforts to model and measure oxygen diffusion and consumption within these engineered tissues have sought to quantitatively assess and improve these design strategies. This review assesses the current state of the field by outlining the prevailing approaches taken toward producing vascularized tissues and highlighting their strengths and weaknesses.

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.

Thermosensitive injectable hyaluronic acid hydrogel for adipose tissue engineering

A series of thermosensitive copolymer hydrogels, aminated hyaluronic acid-g-poly(N-isopropylacrylamide) (AHA-g-PNIPAAm), were synthesized by coupling carboxylic end-capped PNIPAAm (PNIPAAm-COOH) to AHA through amide bond linkages. AHA was prepared by grafting adipic dihydrazide to the HA backbone and PNIPAAm-COOH copolymer was synthesized via a facile thermo-radical polymerization technique by polymerization of NIPAAm using 4,4'-azobis(4-cyanovaleric acid) as an initiator, respectively. The structure of AHA and AHA-g-PNIPAAm copolymer was determined by (1)H NMR. Two AHA-g-PNIPAAm copolymers with different weight ratios of PNIPAAm on the applicability of injectable hydrogels were characterized. The lower critical solution temperature (LCST) of AHA-g-PNIPAAm copolymers in PBS were measured as approximately 30 degrees C by rheological analysis, regardless of the grafting degrees. Enzymatic resistance of AHA-g-PNIPAAm hydrogels with 28% and 53% of PNIPAAm in 100U/mL hyaluronidase/PBS at 37 degrees C was 12.3% and 37.6% over 28 days, respectively. Equilibrium swelling ratios of AHA-g-PNIPAAm hydrogels with 28% of PNIPAAm were 21.5, and significantly decreased to 13.3 with 53% of PNIPAAm in PBS at 37 degrees C. Results from SEM observations confirm a porous 3D AHA-g-PNIPAAm hydrogel structure with interconnected pores after freeze-drying and the pore diameter depends on the weight ratios of PNIPAAm. Encapsulation of human adipose-derived stem cells (ASCs) within hydrogels showed the AHA-g-PNIPAAm copolymers were noncytotoxic and preserved the viability of the entrapped cells. A preliminary in vivo study demonstrated the usefulness of the AHA-g-PNIPAAm copolymer as an injectable hydrogel for adipose tissue engineering. This newly described thermoresponsive AHA-g-PNIPAAm copolymer demonstrated attractive properties to serve as cell or pharmaceutical delivery vehicles for a variety of tissue engineering applications.

Injectable biomaterials for regenerating complex craniofacial tissues

Engineering complex tissues requires a precisely formulated combination of cells, spatiotemporally released bioactive factors, and a specialized scaffold support system. Injectable materials, particularly those delivered in aqueous solution, are considered ideal delivery vehicles for cells and bioactive factors and can also be delivered through minimally invasive methods and fill complex 3D shapes. In this review, we examine injectable materials that form scaffolds or networks capable of both replacing tissue function early after delivery and supporting tissue regeneration over a time period of weeks to months. The use of these materials for tissue engineering within the craniofacial complex is challenging but ideal as many highly specialized and functional tissues reside within a small volume in the craniofacial structures and the need for minimally invasive interventions is desirable due to aesthetic considerations. Current biomaterials and strategies used to treat craniofacial defects are examined, followed by a review of craniofacial tissue engineering, and finally an examination of current technologies used for injectable scaffold development and drug and cell delivery using these materials.

The possible use of stem cells in regenerative medicine: dream or reality?

Stem cells are one of the most fascinating areas in regenerative medicine today. They play a crucial role in the development and regeneration of human life and are defined as cells that continuously reproduce themselves while maintaining the ability to differentiate into various cell types. Stem cells are found at all developmental stages, from embryonic stem cells that differentiate into all cell types found in the human body to adult stem cells that are responsible for tissue regeneration. The general opinion postulates that clinical therapies based on the properties of stem cells may have the potential to change the treatment of degenerative diseases or important traumatic injuries in the "near" future. We here briefly review the literature in particularly for the liver, heart, kidney, cartilage, and bone regeneration.