Human hepatocyte isolation and transplantation into an athymic rat, using prevascularized cell polymer constructs

Subcutaneous Transplantation of Islets into Streptozocin-Induced Diabetic Rats

Pancreatic islet transplantation into type 1 diabetic patients is currently being performed by intraportal infusion. This method, albeit reproducible, has some disadvantages including potential development of portal hypertension, hemorrhage, and an inability to retrieve or detect the transplanted tissue. Other transplant sites have been examined in animal models including the omentum, peritoneal cavity, and the spleen. A transplant site that has not been successful in supporting functional islet tissue transplantation in humans is the subcutaneous space due primarily to the lack of a well-defined vascular bed. This site has many favorable characteristics such as ease of access for transplantation and potential for removal of the transplanted tissue with a minimally invasive surgical procedure. This report addresses the evaluation of a subcutaneously placed device for the support of rat syngeneic islet transplantation in a streptozocin-induced diabetic model. The data generated support the use of this device for islet engraftment. In addition, beta cell function in this device compared favorably with the function of islets transplanted to the renal subcapsular space as well as islets within the native pancreas.

Hepatocyte isolation from resected benign tissues: Results of a 5-year experience

AIM

To analyze retrospectively a 5-year experience of human hepatocyte isolation from resected liver tissues with benign disease.

METHODS

We established a method of modified four-step retrograde perfusion to isolate primary human hepatocytes. Samples were collected from the resected livers of patients with intrahepatic duct calculi (n = 7) and liver hemangioma (n = 17). Only the samples weighing ≥ 15 g were considered suitable for hepatocyte isolation. By using the standard trypan blue exclusion technique, hepatocyte viability and yield were immediately determined after isolation.

RESULTS

Twenty-four liver specimens, weighing 15-42 g, were immediately taken from the margin of the removed samples and transferred to the laboratory for hepatocyte isolation. Warm ischemia time was 5-35 min and cold ischemia time was 15-45 min. For the 7 samples of intrahepatic duct calculi, the method resulted in a hepatocyte yield of 3.49 ± 2.31 × 10⁶ hepatocytes/g liver, with 76.4% ± 10.7% viability. The 17 samples of liver hemangioma had significantly higher yield of cells (5.4 ± 1.71 × 10⁶ cells/g vs 3.49 ± 2.31 × 10⁶ cells/g, P < 0.05) than the samples of intrahepatic duct calculi. However, there seems to be no clear difference in cell viability (80.3% ± 9.67% vs 76.4% ± 10.7%, P > 0.05). We obtained a cell yield of 5.31 ± 1.87 × 10⁶ hepatocytes/g liver when the samples weighed > 20 g. However, for the tissues weighing ≤ 20 g, a reduction in yield was found (3.08 ± 1.86 × 10⁶ cells/g vs 5.31 ± 1.87 × 10⁶ cells/g, P < 0.05).

CONCLUSION

Benign diseased livers are valuable sources for large-number hepatocyte isolation. Our study represents the largest number of primary human hepatocytes isolated from resected specimens from patients with benign liver disease. We evaluated the effect of donor liver characteristics on cell isolation, and we found that samples of liver hemangioma can provide better results than intrahepatic duct calculi, in terms of cell yield. Furthermore, the size of the tissues can affect the outcome of hepatocyte isolation.

In situ vascularization of injectable fibrin/poly(ethylene glycol) hydrogels by human amniotic fluid-derived stem cells

One of the greatest challenges in regenerative medicine is generating clinically relevant engineered tissues with functional blood vessels. Vascularization is a key hurdle faced in designing tissue constructs larger than the in vivo limit of oxygen diffusion. In this study, we utilized fibrin-based hydrogels to serve as a foundation for vascular formation, poly(ethylene glycol) (PEG) to modify fibrinogen and increase scaffold longevity, and human amniotic fluid-derived stem cells (AFSC) as a source of vascular cell types (AFSC-EC). AFSC hold great potential for use in regenerative medicine strategies, especially those involving autologous congenital applications, and we have shown previously that AFSC-seeded fibrin-PEG hydrogels have the potential to form three-dimensional vascular-like networks in vitro. We hypothesized that subcutaneously injecting these hydrogels in immunodeficient mice would both induce a fibrin-driven angiogenic host response and promote in situ AFSC-derived neovascularization. Two weeks postinjection, hydrogels were sectioned, and the following was demonstrated: the average maximum invasion distance of host murine cells into the subcutaneous fibrin/PEG scaffold was 147 ± 90 µm after 1 week and 395 ± 138 µm after 2 weeks; the average number of cell-lined lumen per square millimeter was significantly higher in hydrogels seeded with stem cells or cocultures containing stem cells (MSC, 36.5 ± 11.4; AFSC, 47.0 ± 18.9; AFSC/AFSC-EC, 32.8 ± 11.6; and MSC/HUVEC, 43.1 ± 25.1) versus endothelial cell types alone (AFSC-EC, 9.7 ± 6.1; HUVEC, 14.2 ± 8.8); and a subset of these lumen were characterized by the presence of red blood cells. Select areas of cell-seeded hydrogels contained CD31(+) lumen surrounded by α-smooth muscle cell support cells, whereas control hydrogels with no cells only showed infiltration of α-smooth muscle cell-positive host cells.

The effect of hypoxia on in vitro prevascularization of a thick soft tissue

Prevascularizing an implantable tissue is one strategy to improve oxygen (O(2)) transport throughout larger tissues upon implantation. This study examined the role of hypoxia both during (i.e., as a stimulus) and after (i.e., mimicking implant conditions) vascularization of an implantable tissue. Tissues consisted of microcarrier beads coated with human umbilical vein endothelial cells embedded in fibrin. The fibrin was covered with a monolayer of normal human lung fibroblasts (NHLFs), or exposed to conditioned media from NHLFs. Capillary networks developed at 20% or 1% O(2) tension for 8 days. In some experiments, tissues were supplemented with vascular endothelial growth factor (VEGF) and basic fibroblast growth factor, whereas in others the tissues prevascularized at 20% O(2) were transferred to 1% O(2) for 8 additional days. Maximal capillary formation occurred in media conditioned by NHLFs at 20% O(2), supplemented with VEGF (concentration >10 pM). Hypoxia (1% O(2)) did not stimulate basic fibroblast growth factor production and decreased in vitro angiogenesis, despite an increase in endogenous VEGF production. Hypoxia also degraded a preformed capillary network within 4 days. Hence, strategies to prevascularize implantable tissues may not require the physical presence of stromal cells, but will likely require fibroblast-derived growth factors in addition to VEGF to maintain capillary growth.

Homogeneous differentiation of hepatocyte-like cells from embryonic stem cells: applications for the treatment of liver failure

One of the major hurdles of cellular therapies for the treatment of liver failure is the low availability of functional human hepatocytes. While embryonic stem (ES) cells represent a potential cell source for therapy, current methods for differentiation result in mixed cell populations or low yields of the cells of interest. Here we describe a rapid, direct differentiation method that yields a homogeneous population of endoderm-like cells with 95% purity. Mouse ES cells cultured on top of collagen-sandwiched hepatocytes differentiated and proliferated into a uniform and homogeneous cell population of endoderm-like cells. The endoderm-like cell population was positive for Foxa2, Sox17, and AFP and could be further differentiated into hepatocyte-like cells, demonstrating hepatic morphology, functionality, and gene and protein expression. Incorporating the hepatocyte-like cells into a bioartificial liver device to treat fulminant hepatic failure improved animal survival, thereby underscoring the therapeutic potential of these cells.

Long-bone critical-size defects treated with tissue-engineered grafts: a study on sheep

Standardized particulate bone constructs, obtained by expanding autologous mesenchymal stem cells (MSCs) onto coral granules in vitro, were transplanted into long-bone, critical-size defects in sheep. Control experiments were also performed in which autologous bone grafts were implanted. Defect cavities were lined with a preformed vascularized membrane (induced by temporarily inserting a cement spacer for 6 weeks prior to bone construct implantation), which served as a mold keeping the engineered bone granules in place. Radiographic, histological, and computed tomographic tests performed 6 months later showed that the osteogenic abilities of the engineered construct and autograft were significantly greater than those of coral scaffold alone. No significant differences were found between the amount of newly formed bone in defects filled with coral/MSCs and those filled with autograft, yet radiological scores differed significantly between the two groups (21% and 100% healed cortices, respectively). The present study on a clinically relevant animal model provides the first evidence that standardized particulate bone constructs can be used to repair large bone defects and that their osteogenic ability approaches that of bone autograft, the bone repair benchmark. By proving feasibility, the present study makes possible the treatment of segmental bone losses with bone constructs engineered from granules, a process which is much simpler than preparing customized massive constructs using computer-assisted techniques. Important parameters, such as the rate of scaffold resorption and the number of MSCs to be seeded on the scaffolds, need to be optimized before reaching pertinent definitive conclusions.

Co-culture of Bone Marrow Fibroblasts and Endothelial Cells on Modified Polycaprolactone Substrates for Enhanced Potentials in Bone Tissue Engineering

The creation of a vascularized bed makes the survival of seeded cells on 3-dimensional scaffolds much more likely. However, relying purely on random capillary ingrowth into the porous scaffolds from the host may compromise vascularization of a scaffold. One solution is to transplant cells capable of differentiating into new blood vessels into the scaffolds to accelerate the creation of a vascularized scaffold. Because endothelial cells are the key cells involved in blood vessel formation, the present study was designed to investigate the development of a biomaterial surface that supports endothelial cell attachment and proliferation. The subsequent effects of the material surface modifications on the differentiation and proliferation of human bone marrow-derived fibroblasts (HBMFs) when grown in co-culture with a human bone marrow endothelial cell line (HBMEC-60) were studied. Endothelialization studies showed that the gelatin-coated and hydroxyapatite-coated substrates were superior for HBMEC-60 attachment and proliferation to hydrolyzed-only or untreated polycaprolactone substrates. Co-culture studies showed that the presence of the HBMEC-60 specifically enhanced HBMF cell proliferation and differentiation and that this effect was not observed with co-culture with skin fibroblasts. It is concluded that the co-culture of endothelial cells with HBMFs could be a promising culture system for bone tissue- engineering applications.

Analysis of neovascularization of PEGT/PBT-copolymer dermis substitutes in balb/c-mice

INTRODUCTION

A fundamental prerequisite for using degradable synthetic biopolymers as composite skin substitutes is the ability to establish vascular tissue. PEGT/PBT block-copolymer matrices have previously been shown as a favorable dermal substitute. In this study, quantitative data on neovascularization of PEGT/PBT block-copolymer matrices are presented.

MATERIALS AND METHODS

PEGT/PBT-block-copolymer discs of three different pore diameters (1: < 75 microm, 2: 75-212 microm, 3: 250-300 microm) were implanted into dorsal skinfold chambers of balb/c mice. Histological sections were evaluated 7, 14, and 21 days post implantation by light and scanning electron microscopy. Blood vessel analysis was performed by means of digital image analysis (n = 288) of hematoxylin/eosin stained sections within apical (AOF) and basal (BOF) observation fields of the matrices.

RESULTS

Twenty-one days after implantation the density of blood vessels within the BOF of the scaffolds with a pore size of 75-212 and 250-300 microm were 4.6 +/- 0.45 and 5.8 +/- 0.62 (mean +/- S.E.M.; blood vessel profiles (BVF)), respectively. In <75 microm scaffolds, smaller numbers of BVF were found (4.2 +/- 0.39). In contrast, the evaluation within the AOF revealed significantly higher numbers of BVF in 75-212 microm group (3.5 +/- 0.49) and 250-300 microm group (4.5 +/- 0.66) as compared to the < 75 microm group (2.3 +/- 0.48).

CONCLUSION

There is evidence that the three-dimensional structure of PEGT/PBT-block-copolymer (pore size structure) influences neovascularization. The porous structures of copolymer matrices with adequate interconnection of pores (pore sizes of 75-212 and 250-300 microm) are characterized by faster ingrowth of vascular tissue.

Generation of a vascularized organoid using skeletal muscle as the inductive source

The technology required for creating an in vivo microenvironment and a neovasculature that can grow with and service new tissue is lacking, precluding the possibility of engineering complex three-dimensional organs. We have shown that when an arterio-venous (AV) loop is constructed in vivo in the rat groin, and placed inside a semisealed chamber, an extensive functional vasculature is generated. To test whether this unusually angiogenic environment supports the survival and growth of implanted tissue or cells, we inserted various preparations of rat and human skeletal muscle. We show that after 6 weeks incubation of muscle tissue, the chamber filled with predominantly well-vascularized recipient-derived adipose tissue, but some new donor-derived skeletal muscle and connective tissue were also evident. When primary cultured myoblasts were inserted into the chamber with the AV loop, they converted to mature striated muscle fibers. Furthermore, we identify novel adipogenesis-inducing properties of skeletal muscle. This represents the first report of a specific three-dimensional tissue grown on its own vascular supply.

In vitro culture of rat neuromicrovascular endothelial cells on polymeric scaffolds

Polyphosphazenes are polymers possessing a skeleton composed of alternating phosphorous and nitrogen atoms, and two side-moieties linked to each phosphorous atom. Polyphosphazenes with amino acid esters as side-moieties are biocompatible and biodegradable polymers. Two polyphosphazenes, poly[bis(ethyl alanate) phosphazene] and poly[(ethyl phenylalanate)0.8(ethyl alanate)0.8(ethyl glycinate)0.4 phosphazene] (PPAGP) were synthesized, and processed to form small fibers. Their ability to support rat neuromicrovascular endothelial cell (EC) adhesion and growth has been studied, using poly(D,L-lactic acid) as reference compound. Scanning electron microscopy revealed that both poly[bis(ethyl alanate) phosphazene] and PPAGP fibers were thinner than poly(D,L-lactic acid) fibers, and possessed a more irregular and porous surface. All polymers increased EC adhesion, compared with polystyrene, but only polyphosphazenes were able to improve EC growth. The highest increase in EC proliferation was induced by PPAGP, which, as revealed by environmental scanning electron microscopy, was also able to induce ECs to arrange into tubular structures. The conclusion is drawn that PPAGP may provide the best scaffold for engineered blood vessels, because it promotes adhesion, growth, and organization of ECs into capillary-like structures.

Building In Vitro Models of Organs

Tissue-engineering techniques are being used to build in vitro models of organs as substitutes for human donor organs for transplantation as well as in vitro toxicology testing (as alternatives to use of animals). Tissue engineering involves the fabrication of scaffolds from materials that are biologically compatible to serve as cellular supports and microhabitats in order to reconstitute a desired tissue or organ. Three organ systems that are currently the foci of tissue engineering efforts for both transplantation and in vitro toxicology testing purposes are discussed. These are models of the cornea, nerves (peripheral nerves specifically), and cardiovascular components. In each of these organ systems, a variety of techniques and materials are being used to achieve the same end results. In general, models that are designed with consideration of the developmental and cellular biology of the target tissues or organs have tended to result in morphologically and physiologically accurate models. Many of the models, with further development and refinement, have the potential to be useful as functional substitute tissues and organs for transplantation or for in vitro toxicology testing.

Diffusion Limits of an in Vitro Thick Prevascularized Tissue

Although tissue engineering promises to replace or restore lost function to nearly every tissue in the body, successful applications are currently limited to tissue less than 2 mm in thickness. in vivo capillary networks deliver oxygen and nutrients to thicker (> 2 mm) tissues, suggesting that introduction of a preformed in vitro vascular network may be a useful strategy for engineered tissues. This article describes a system for generating capillary-like networks within a thick fibrin matrix. Human umbilical vein endothelial cells, growing on the surface of microcarrier beads, were embedded in fibrin gels a known distance (Delta = 1.8-4.5 mm) from a monolayer of human dermal fibroblasts. The distance of the growth medium, which contained vascular endothelial growth factor and basic fibroblast growth factor, from the beads, C, was varied from 2.7 to 7.2 mm. Capillaries with visible lumens sprouted in 2-3 days, reaching lengths that exceeded 500 microm within 6-8 days. On day 7, capillary network formation was largely independent of C; however, a strong inverse correlation with Delta was observed, with the maximum network formation at Delta = 1.8 mm. Surprisingly, the thickness of the gel was not a limiting factor for oxygen diffusion as these tissue constructs retained a relatively high oxygen tension of > 125 mmHg. We conclude that diffusion of oxygen in vitro is not limiting, allowing the development of tissue constructs on the order of centimeters in thickness. In addition, diffusion of fibroblast-derived soluble mediators is necessary for stable capillary formation, but is significantly impeded relative to that of nutrients present in the medium.

Hepatic tissue engineering for adjunct and temporary liver support: critical technologies

The severe donor liver shortage, high cost, and complexity of orthotopic liver transplantation have prompted the search for alternative treatment strategies for end-stage liver disease, which would require less donor material, be cheaper, and less invasive. Hepatic tissue engineering encompasses several approaches to develop adjunct internal liver support methods, such as hepatocyte transplantation and implantable hepatocyte-based devices, as well as temporary extracorporeal liver support techniques, such as bioartificial liver assist devices. Many tissue engineered liver support systems have passed the "proof of principle" test in preclinical and clinical studies; however, they have not yet been found sufficiently reliably effective for routine clinical use. In this review we describe, from an engineering perspective, the progress and remaining challenges that must be resolved in order to develop the next generation of implantable and extracorporeal devices for adjunct or temporary liver assist.

Principals of neovascularization for tissue engineering

The goals in tissue engineering include the replacement of damaged, injured or missing body tissues with biological compatible substitutes such as bioengineered tissues. However, due to an initial mass loss after implantation, improved vascularization of the regenerated tissue is essential. Recent advances in understanding the process of blood vessel growth has offered significant tools for therapeutic neovascularization. Several angiogenic growth factors including vascular endothelial cell growth factor (VEGF) and basic fibroblast growth factor (bFGF) were used for vascularization of ischemic tissues. Three approaches have been used for vascularization of bioengineered tissue: incorporation of angiogenic factors in the bioengineered tissue, seeding endothelial cells with other cell types and prevascularization of matrices prior to cell seeding. This paper reviews the process of blood vessel growth and tissue vascularization, and discuss strategies for efficient vascularization of engineered tissues.

A hydrogel material for plastic and reconstructive applications injected into the subcutaneous space of a sheep

Soft tissue reconstruction using tissue-engineered constructs requires the development of materials that are biocompatible and support cell adhesion and growth. The objective of this study was to evaluate the use of macroporous hydrogel fragments that were formed using either unmodified alginate or alginate covalently linked with the fibronectin cell adhesion peptide RGD (alginate-RGD). These materials were injected into the subcutaneous space of adult, domesticated female sheep and harvested for histological comparisons at 1 and 3 months. In addition, the alginate-RGD porous fragments were seeded with autologous sheep preadipocytes isolated from the omentum, and these cell-based constructs were also implanted. The results from this study indicate that both the alginate and alginate-RGD subcutaneous implants supported tissue and vascular ingrowth. Furthermore, at all time points of the experiment, a minimal inflammatory response and capsule formation surrounding the implant were observed. The implanted materials also maintained their sizes over the 3-month study period. In addition, the alginate-RGD fragments supported the adhesion and proliferation of sheep preadipocytes, and adipose tissue was present within the transplant site of these cellular constructs, which was not present within the biomaterial control sites.

Host response to tissue engineered devices

The two main components of a tissue engineered device are the transplanted cells and the biomaterial, creating a device for the restoration or modification of tissue or organ function. The implantation of polymer/cell constructs combines concepts of biomaterials and cell transplantation. The interconnections between the host responses to the biomaterial and transplanted cells determines the biocompatibility of the device. This review describes the inflammatory response to the biomaterial component and immune response towards transplanted cells. Emphasis is on how the presence of the transplanted cell construct affects the host response. The inflammatory response towards a biomaterial can impact the immune response towards transplanted cells and vice versa. Immune rejection is the most important host response towards the cellular component of tissue engineered devices containing allogeneic, xenogeneic or immunogenic ex vivo manipulated autologous cells. The immune mechanisms towards allografts and xenografts are outlined to provide a basis for the mechanistic hypotheses of the immune response towards encapsulated cells, with antigen shedding and the indirect pathway of antigen presentation predominating. A review of experimental evidence illustrates examples of the inflammatory response towards biodegradable polymer scaffold materials, examples of devices appropriately integrated as assessed morphologically with the host for various applications including bone, nerve, and skin regeneration, and of the immune response towards encapsulated allogeneic and xenogeneic cells.