In vitro 3D model for human vascularized adipose tissue

Vascularization of prevascularized and non-prevascularized fibrin-based human adipose tissue constructs after implantation in nude mice

Adipose regeneration strategies have been hampered by the inability to supply an adequate vascular supply following implantation. Vascularization in vitro, also called prevascularization, is a promising method that could promote the vascularization of engineered adipose tissue constructs upon implantation. In this study we compared the ability of prevascularized-to-non-prevascularized fibrin-based human adipose tissue to promote vascularization. Human adipose tissue-derived stromal cells (ASCs) and different mixtures (1:1, 1:2 and 1:5) of ASCs with human umbilical vein endothelial cells (HUVECs) were cultured in fibrin at two different densities (1.0 × 10(6) and 10 × 10(6) cells/ml) for 7 days. Histological analysis revealed that prevascular structures formed in 1:5 ASC/HUVEC fibrin-based constructs seeded with a total of 10 × 10(6) cells/ml. These constructs and ASC-only constructs were implanted subcutaneously in athymic mice for 7 days and generated lipid-containing grafts. The numbers and densities of blood vessels within the ASC/HUVEC constructs were similar to those of ASC-only constructs. Furthermore, immunostaining studies demonstrated human-derived vasculature within a few of the ASC/HUVEC and ASC-only constructs. A subset of this human-derived vasculature contained erythrocytes, indicating integration with the host vasculature. In conclusion, our study indicated no difference in the rate of vascularization of prevascularized ASC/HUVEC and non-prevascularized ASC-only fibrin-based constructs, suggesting that prevascularization of these fibrin-based constructs does not promote vascularization. Our results further indicated that not only endothelial cells, but also ASCs may contribute to the formation of vascular lumina upon implantation. This finding is interesting, since it demonstrates the possibility of vascularized adipose tissue engineering from a single cell source.

Effects of hyperinsulinemia on lipolytic function of three-dimensional adipocyte/endothelial co-cultures

The increased incidence of both type 2 diabetes mellitus and obesity has prompted the need to develop physiologically relevant adipose tissue models for controlled study of both normal and diseased adipose functions. Insulin resistance, characteristic of both type 2 diabetes mellitus and obesity, is often preceded by hyperinsulinemia. We propose here a three-dimensional (3D) co-culture adipose tissue model to study the effects of high insulin exposure, which enabled the study of physiological cell responses to hyperinsulinemic conditions. Two-dimensional adipocyte studies were initially conducted to establish a baseline control in which insulin levels were established. Adipocytes and endothelial cells were subsequently co-cultured on 3D porous silk fibroin scaffolds in normal or high insulin concentrations, and their physiological responses were assessed with respect to lipogenesis and lipolysis. High insulin levels stimulated both an increase in triglyceride accumulation and a decrease in lipolysis levels compared to that of normal insulin conditions. In contrast, adipocyte monocultures did not exhibit any differences between insulin levels. The ability of this 3D system to elicit physiological responses to hyperinsulinemia in co-culture serves as a significant step forward in adipose tissue engineering. The development of physiologically relevant 3D in vitro adipose tissue models presents promise for the study of disease mechanisms as well as in assessing therapeutic treatments.

Adipogenic differentiation of human adipose-derived stem cells on 3D silk scaffolds

Current treatment modalities for soft tissue defects due to various pathologies and trauma include autologous grafting and the use of commercially available fillers. However, these treatment methods are associated with a number of limitations, such as donor site morbidity and volume loss over time. As such, improved therapeutic options are needed. Tissue engineering techniques offer novel solutions to these problems through development of bioactive tissue constructs that can regenerate adipose tissue with an appropriate structure and function. The recent advances in the derivation and characterization of hASCs have led to numerous studies of soft tissue reconstruction. In this chapter, we discuss methods in which our laboratory has used hASCs and silk scaffolds for adipose tissue engineering. The use of naturally occurring and clinically acceptable materials such as silk protein for tissue-engineering applications poses advantages with respect to biocompatibility and mechanical and biological properties.

Adipose tissue as a stem cell source for musculoskeletal regeneration

Adipose tissue is an abundant, easily accessible, and reproducible cell source for musculo-skeletal regenerative medicine applications. Initial derivation steps yield a heterogeneous population of cells of stromal vascular fraction (SVF) cells. Subsequent adherent selection of the SVF results in a relatively homogeneous population of adipose-derived stromal/stem cells (ASCs) capable of adipogenic, chondrogenic, myogenic, and osteogenic differentiation in vitro on scaffolds in bioreactors and in vivo in pre-clinical animal models. Unlike hematopoietic cells, ASCs do not elicit a robust lymphocyte reaction and instead release immunosuppressive factors, such as prostaglandin E2. These immunomodulatory features suggest that allogeneic and autologous ASCs will engraft successfully for tissue regeneration purposes. The differentiation and expansion potential of ASCs can be modified by growth factors, bio-inductive scaffolds, and bioreactors providing environmental control and biophysical stimulation. Gene therapy approaches using lentiviral transduction can be used to direct differentiation of ASCs to particular lineages. We discuss the utility of ASCs for musculo-skeletal tissue repair and some of the technologies that can be implemented to unlock the full regenerative potential of these highly valuable cells.

Synthetic adipose tissue models for studying mammary gland development and breast tissue engineering

The mammary gland is a dynamic organ that continually changes its architecture and function. Reciprocal interactions between epithelium and adipocyte-containing stroma exert profound effects on all stages of its development, even though the details of these events are not fully understood. To address this issue, enormous potential exists in the utilization of synthetic adipose tissue model systems to uncover the properties and functions of adipocytes in the mammary gland. The first part of this review focuses on mammary adipose tissue (or adipocyte)-related model systems developed in recent years and their utility in investigating adipose-epithelial interactions, mammary gland morphogenesis, development and tumorigenesis. The second part shifts to the field of adipose-based breast tissue engineering, focusing on how these synthetic adipose tissue models are being constructed in vitro or in vivo for regeneration of the mammary gland, and their potentials in adipose tissue engineering also are discussed.

Cervical tissue engineering using silk scaffolds and human cervical cells

Spontaneous preterm birth is a frequent complication of pregnancy and a common cause of morbidity in childhood. Obstetricians suspect abnormalities of the cervix are implicated in a significant number of preterm births. The cervix is composed of fibrous connective tissue and undergoes significant remodeling in preparation for birth. We hypothesized that a tissue engineering strategy could be used to develop three-dimensional cervical-like tissue constructs that would be suitable for investigating cervical remodeling. Cervical cells were isolated from two premenopausal women undergoing hysterectomy for a benign gynecological condition, and the cells were seeded on porous silk scaffolds in the presence or absence of dynamic culture and with 10% or 20% serum. Morphological, biochemical, and mechanical properties were measured during the 8-week culture period. Cervical cells proliferated in three-dimensions and synthesized an extracellular matrix with biochemical constituents and morphology similar to native tissue. Compared to static culture, dynamic culture was associated with significantly increased collagen deposition (p < 0.05), sulfated glycosaminoglycan synthesis (p < 0.05), and mechanical stiffness (p < 0.05). Serum concentration did not affect measured variables. Relevant human tissue-engineered cervical-like constructs constitute a novel model system for a range of fundamental and applied studies related to cervical remodeling.

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

Vascularization is still one of the most important limitations for the survival of engineered tissues after implantation. In this study, we aim to improve the in vivo vascularization of engineered adipose tissue by preforming vascular structures within in vitro-engineered adipose tissue constructs that can integrate with the host vascular system upon implantation. Different cell culture media were tested and different amounts of human adipose tissue-derived mesenchymal stromal cells (ASC) and human umbilical vein endothelial cells (HUVEC) were combined in spheroid cocultures to obtain optimal conditions for the generation of prevascularized adipose tissue constructs. Immunohistochemistry revealed that prevascular structures were formed in the constructs only when 20% ASC and 80% HUVEC were combined and cultured in a 1:1 mixture of endothelial cell medium and adipogenic medium. Moreover, the ASC in these constructs accumulated lipid and expressed the adipocyte-specific gene fatty acid binding protein-4. Implantation of prevascularized ASC/HUVEC constructs in nude mice resulted in a significantly higher amount of vessels (37 ± 17 vessels/mm(2)) within the constructs compared to non-prevascularized constructs composed only of ASC (3 ± 4 vessels/mm(2)). Moreover, a subset of the preformed human vascular structures (3.6 ± 4.2 structures/mm(2)) anastomosed with the mouse vasculature as indicated by the presence of intravascular red blood cells. Our results indicate that preformed vascular structures within in vitro-engineered adipose tissue constructs can integrate with the host vascular system and improve the vascularization upon implantation.

The use of decellularized adipose tissue to provide an inductive microenvironment for the adipogenic differentiation of human adipose-derived stem cells

The development of an engineered adipose tissue substitute, capable of supporting reliable, predictable, and complete fat tissue formation, would be of significant value in the fields of plastic and reconstructive surgery. Towards the goal of engineering an optimized microenvironment for adipogenesis, a decellularization strategy was developed for adipose tissue, which yielded 3-D scaffolds with preserved extracellular matrix architecture. A significant volume of scaffolding material could be obtained from a human tissue source that is commonly discarded. Histology, immunohistochemistry, and scanning electron microscopy confirmed the efficacy and reproducibility of the approach, and also indicated that the basement membrane was conserved in the processed matrix, including laminin and collagen type IV. Seeding experiments with human adipose-derived stem cells indicated that the decellularized adipose tissue (DAT) provided an inductive microenvironment for adipogenesis, supporting the expression of the master regulators PPARgamma and CEBPalpha, without the need for exogenous differentiation factors. High levels of adipogenic gene expression and glycerol-3-phosphate dehydrogenase activity were observed in the induced DAT scaffolds, as compared to cells grown in monolayer or cell aggregate culture. The protein data emphasized the importance of the cell donor source in the development of tissue-engineering strategies for large-volume soft tissue regeneration.