Delivery of whole liver-equivalent hepatocyte mass using polymer devices and hepatotrophic stimulation

Hepatocyte and Islet Cell Cotransplantation on Poly-L-Lactide Matrix for the Treatment of Liver Cirrhosis

The human autologous hepatocyte matrix implant is a promising alternative procedure to counter liver damage. We assessed the outcome of human hepatocytes isolation from cirrhotic liver compared to the clinical and histological scores of disease severity. A total of 11 patients with various clinical scores (CTP and MELD) and histological score (Metavir, fibrosis) of liver cirrhosis were included in the hepatocyte matrix implant clinical phase I study. The liver segment and pancreatic tissue were harvested from each patient, and hepatocytes and cells of islets of Langerhans were isolated. The freshly isolated human hepatocytes were coseeded with the islet cells onto poly(l-lactic acid) (PLLA) scaffolds, cultured, and transplanted back into the patient. Human hepatocytes were isolated from 11 cirrhotic liver specimens with a resulting yield of 1.4 ± 0.5 × 106 cells per gram of the liver specimen and a viability rate of 52 ± 13%. It was found that the yield and viability of the liver cells were not correlated with the clinical and histological scores of the liver cirrhosis. A correlation was found between the hepatocyte yield obtained and the average number of hepatocytes counted in 10 microscopic fields of view. More viable cells were obtained from cirrhotic livers caused by chronic hepatitis B as compared to chronic hepatitis C in the same MELD score range. There was no correlation between the clinical and histological disease severity scores of liver cirrhosis and the outcome of hepatocytes isolation. It seems that the yield could depend on the type of hepatitis underlying the cirrhotic tissue. The study was registered at www.clinicaltrial.gov with the study identifier: NCT01335568.

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.

Dynamics of Fibrovascular Tissue Ingrowth in Hydrogel Foams

We have investigated and quantified the degree of fibrovascular tissue ingrowth in cylindrical poly(vinyl alcohol) (PVA) foams of 12.5 mm diameter, 5 mm thickness, and 71% porosity implanted in the mesentery of rats over a period of 25 days. Fibrovascular tissue penetrated the center of PVA foams 5 days postimplantation yet the void fraction available for cell seeding was 55% and the volume average pore diameter was 190 (±39) μm. By 10 days postimplantation the void fraction had decreased to 32% and the volume average pore diameter was 121 (±20) μm. As time elapsed fibrovascular tissue continued to expand and fill the remaining pore space. At 15 days postimplantation the void space was impractical for cell seeding and continued to decrease through the remainder of the study. Our data suggest that hydrogel foams with a polydispersed pore morphology can be prevascularized with adequate space for cell seeding as the volume of tissue penetrating the foam is limited by the smaller pores in the foam structure; however, available void space for cell seeding decreases with time.

Stem cells and biopharmaceuticals: vital roles in the growth of tissue-engineered small intestine

Tissue engineering currently constitutes a complex, multidisciplinary field exploring ideal sources of cells in combination with scaffolds or delivery systems in order to form a new, functional organ to replace native organ lack or loss. Short bowel syndrome (SBS) is a life-threatening condition with high morbidity and mortality rates in children. Current therapeutic strategies consist of costly and risky allotransplants that demand lifelong immunosuppression. A promising alternative is the implantation of autologous organoid units (OU) to create a tissue-engineered small intestine (TESI). This strategy is proven to be stem cell and mesenchyme dependent. Intestinal stem cells (ISCs) are located at the base of the crypt and are responsible for repopulating the cycling mucosa up to the villus tip. The stem cell niche governs the biology of ISCs and, together with the rest of the epithelium, communicates with the underlying mesenchyme to sustain intestinal homeostasis. Biopharmaceuticals are broadly used in the clinic to activate or enhance known signaling pathways and may greatly contribute to the development of a full-thickness intestine by increasing mucosal surface area, improving blood supply, and determining stem cell fate. This review will focus on tissue engineering as a means of building the new small intestine, highlighting the importance of stem cells and recombinant peptide growth factors as biopharmaceuticals.

Engineered liver for transplantation

Orthotopic liver transplantation is the only definitive treatment for end stage liver failure and the shortage of donor organs severely limits the number of patients receiving transplants. Liver tissue engineering aims to address the donor liver shortage by creating functional tissue constructs to replace a damaged or failing liver. Despite decades of work, various bottoms-up, synthetic biomaterials approaches have failed to produce a functional construct suitable for transplantation. Recently, a new strategy has emerged using whole organ scaffolds as a vehicle for tissue engineering. This technique involves preparation of these organ scaffolds via perfusion decellularization with the resulting scaffold retaining the circulatory network of the native organ. This important phenomenon allows for the construct to be repopulated with cells and to be connected to the blood torrent upon transplantation. This opinion paper presents the current advances and discusses the challenges of creating fully functional transplantable liver grafts with this whole liver engineering approach.

Control of Liver Tissue Reconstitution in Mesenteric Leaves: The Effect of Preculture on Mouse Hepatic Progenitor Cells Prior to Transplantation

Our objective is to control the reconstitution of liverlike tissues at extrahepatic sites using hepatic progenitor cells (HPCs) andin vitropreculture prior to transplantation. We prepared cell-based hybrid grafts by culturing HPCs isolated from fetal E14.5 mouse livers on biodegradable, highly porous 3-dimensional poly-L-lactic acid (PLLA) scaffolds for 1 week in basal medium (the basal condition) or 10 mM nicotinamide (NA) and 1% dimethyl sulfoxide (DMSO) supplemented conditions (the ND-positive condition) prior to implantation. Sections of hybrid grafts cultured for 1 week showed that HPCs grew and spread on the surface of scaffolds under both basal and ND (+) conditions. Most of these cells were albumin (+) and CK18 (+). CK19 (+) cells were also present under the basal condition but not the ND (+) condition. Cultured hybrid grafts were implanted into the mesenteric leaves of mice and removed after 1 month. Transplanted tissues cultured under the basal condition consisted of albumin (+) hepatocyte-like and CK19 (+) biliary epithelial cell (BEC)-like cells organized in duct-like structures. In contrast, integrated tissues cultured under the ND (+) condition alone had differentiated albumin (+) hepatocyte-like cells and were relatively larger than those under the basal condition. Hepatocyte-like cells of transplanted hybrid grafts cultured under both conditions were periodic acid-Schiff (PAS) staining-positive and expressed transcription factors, hepatocyte nuclear factor (HNF) 4 and CCAAT/enhancer-binding protein (C/EBP) α. These findings suggest that combining progenitor cells andin vitropreculture may potentially regulate liverlike tissues at extrahepatic sites.

Application of whole-organ tissue engineering in hepatology

Initially hailed as the ultimate solution to organ failure, engineering of vascularized tissues such as the liver has stalled because of the need for a well-structured circulatory system that can maintain the cells seeded inside the construct. A new approach has evolved to overcome this obstacle. Whole-organ decellularization is a method that retains most of the native vascular structures of the organ, providing microcirculatory support and structure, which can be anastomosed with the recipient circulation. The technique was first applied to the heart and then adapted for the liver. Several studies have shown that cells can be eliminated, the extracellular matrix and vasculature are reasonably preserved and, after repopulation with hepatocytes, these grafts can perform hepatic functions in vitro and in vivo. Progress is rapidly being made as researchers are addressing several key challenges to whole-organ tissue engineering, such as ensuring correct cell distribution, nonparenchymal cell seeding, blood compatibility, immunological concerns, and the source of cells and matrices.

[The cultivation of bone marrow cells and cell lines on polymeric films]

The cultivation of multipotent mesenchymal stromal bone marrow cells and cells of A-431, MDCK, Vero, 3T3 and Hep-G2 was performed on polymeric films (PVA) with different hydrophobic fatty acid residues. The cells of different types grew on these films with different intensity, but in the most cases comparable with the cultivation control on usual plastic. The examined films were nontoxic to cells and sufficiently adhesive. They did not changed pH of cultural media, were optically transparent under microscope and comfortable in the experimental work. These films can be used as a model for the artificial organ construction. The covalent binding of different fatty acids to PVA shows possibility of the adaptable changes of films properties (hydrophobity and adhesiveness), and therefore possibility of the creation of optimal conditions for different cell types attachement and growth.

Engineering new tissue: formation of neo-cartilage

One approach to tissue-engineering combines isolated cells with polymer scaffolds for the purpose of generating new tissue or tissue equivalents. It has met with much success when applied to the formation of new cartilage (neo-cartilage). In this review, we will examine the development of polyglycolic acid fiber meshes and calcium alginate gels that when combined with chondrocytes generate new cartilage. Animal models designed to mimic clinical problems will also be discussed. Using the chondrocyte-polymer systems as paradigms, we will attempt to extract some principles of tissue engineering.

Hepatocyte transplantation in the dalmatian dog model of hyperuricosuria

Hepatocyte transplantation shows promise as a therapy to support liver function. We have previously shown that hepatocytes can be transplanted into prevascularized synthetic polymers in rat and pig models. This study was designed to assess the possibility of correction of a liver metabolic defect in a dog model. The Dalmatian dog is known to have an inborn error of metabolism in the hepatocyte that causes a decrease in the degradation of uric acid into allantoin. This leads to a rise in uric acid levels in blood and urine. Four male Dalmatian dogs 18-28 kg in weight were studied as recipients of normal hepatocytes. Poly vinyl alcohol sponges measuring 250 cm(2) x 5 mm were first implanted between the leaves of the mesentery and the omentum to form prevascularized implantation beds, and an end-to-side portacaval shunt to induce hepatotrophic stimulation was made. After 7 days 1.5 x 10(10) normal hepatocytes obtained from donor beagles receiving cyclosporine immunosuppression were implanted into the prevascularized sponges. Excretion of uric acid in urine decreased from 136.3 +/- 18.1 to 44.1 +/- 20.4 micromol/kg/day (p < 0.05) in these animals at 2 weeks, and continued for 6 weeks. Uric acid levels in serum did not change significantly from 26.77 +/- 10.30 to 39.65 +/- 9.09 micromol/liter (mean +/- SD) 4 weeks after transplantation. Control Dalmatians, which were transplanted with Dalmatian hepatocytes, remained at baseline. Creatinine clearance was unchanged. We conclude that hepatocyte transplantation is possible in this Dalmatian dog model of hyperuricosuria and that urinary excretion of uric acid can be decreased by beagle hepatocyte transplantation delivered in prevascularized synthetic polymer sponges using portacaval shunts as hepatotrophic stimulation.

Integrating cell Transplantation and Controlled Drug Delivery Technologies to Engineer Liver Tissue

Engineering liver tissue using hepatocyte transplantation may provide a new approach for treating a variety of liver diseases. However, techniques to transplant hepatocytes and promote their survival must be developed. We have developed systems to transplant hepatocytes on highly porous (95%), biodegradable sponges, and to regulate the survival of cultured hepatocytes by releasing specific growth factors in the cellular environment. Sponges were fabricated from poly (L, lactic acid) (PLLA) and polyvinyl alcohol using a particulate leaching technique. Epidermal growth factor and insulin, critical factors for hepatocyte growth and survival in culture, were incorporated into microspheres fabricated from poly (lactic-co-glycolic acid) (PLGA) utilizing a double emulsion technique. The incorporated factors were released in a controlled manner over one month in vitro, and the released factors maintained their biological activity, as measured by their ability to promote hepatocyte growth and survival in culture. The growth factor-containing microspheres could be transplanted with hepatocytes using the porous sponges, and the localized, sustained release of these factors improved hepatocyte engraftment 2-fold. These studies suggest that hepatocyte containing tissues can be engineered using cell transplantation, and that regulating the microenvironment of transplanted cells can control their engraftment.

Poly(α-Hydroxy Ester)/Short Fiber Hydroxyapatite Composite Foams for Orthopedic Application

A process has been developed to manufacture biodegradable composite foams of poly(DL-lactic- co-glycolic acid) (PLGA) and hydroxyapatite short fibers for use in bone regeneration. The processing technique allows the manufacture of three-dimensional foam scaffolds and involves the formation of a composite material consisting of a porogen material (either gelatin microspheres or salt particles) and hydroxyapatite short fibers embedded in a PLGA matrix. After the porogen is leached out, an open-cell composite foam remains which has a pore size and morphology defined by the porogen. The foam porosity can be controlled by altering the volume fraction of porogen used to make the composite material. Foams made using NaCl particles as a porogen were manufactured with porosities as high as 0.84±0.01 (n=3). The short hydroxyapatite fibers served to reinforce the PLGA. The compressive yield strength of foams manufactured using gelatin microspheres as a porogen was found to increase with fiber content. Foams with compressive yield strengths up to 2.82±0.63 MPa (n=3) with porosities of 0.47±0.01 (n=3) were manufactured using 30% by weight hydroxyapatite fibers in the initial composite prior to leaching. These composite foams with improved mechanical properties may also be expected to have enhanced osteoconductivity and hence provide a novel material which may prove useful in the field of bone regeneration.

Transplantation of a fetal liver cell-loaded hyaluronic acid sponge onto the mesentery recovers a Wilson's disease model rat

An auxiliary liver represents a promising alternative for liver transplantation. The use of a large amount of mature hepatocytes, however, despite their high function, is limited in a clinical setting. Here, we propose a novel transplantation system that dramatically improved a diseased animal by incorporating fetal liver cells (FLCs) as a cell source, the mesentery as a transplantation site and a hyaluronic acid (HA) sponge as a cell scaffold. We transplanted wild-type Long Evans Agouti rat FLCs embedded in HA sponges onto the mesentery of Long Evans Cinnamon (LEC) rats, an animal model for Wilson's disease. The FLC-loaded HA sponges successfully grafted and consequently prevented jaundice. Accordingly, the treated animals showed a significant reduction in blood copper concentration, which consequently led to significant decreases in serum total bilirubin and direct bilirubin, and to a significant increase in albumin productivity. Furthermore, haematoxylin and eosin staining of the host livers demonstrated that fibrosis at the periportal area was moderated in the treated animals. In conclusion, we transplanted FLC-loaded HA sponges onto the mesenteric blood vessels, leading to thick, liver-like tissue possessing blood vessels, and the liver tissue engineered thus exhibited a remarkable therapeutic effect on the copper metabolism deficiency of LEC rats.

Foetal hepatocyte transplantation in a vascularized AV-Loop transplantation model in the rat

The use of foetal liver cells (FLC) in the context of hepatic tissue engineering might permit efficient in vitro expansion and cryopreservation in a cell bank. A prerequisite for successful application of bioartificial liver tissue is sufficient initial vascularization. In this study, we evaluated the transplantation of fibrin gel-immobilized FLC in a vascularized arterio-veno-venous (AV)-loop model. FLC were isolated from embryonic/foetal (ED 16) rat livers and were enriched by using magnetic cell sorting (MACS). After cryopreservation, FLC were labelled by pkh-26. Cells were transplanted in a fibrin matrix into a subcutaneous chamber containing a microsurgically created AV-loop in the femoral region of the recipient rat. The chambers were explanted after 14 days. Subcutaneous implants without an AV-loop and cell-free implants served as controls. Fluorescence microscopy of the constructs was used to identify pkh-26(+)- donor cells. Characterization was performed by RT-PCR and immunhistology (IH) for CK-18 and CD31. Transplantation of FLC using the AV-loop permitted a neo-tissue formation in the fibrin matrix. A high-density vascularization was observed in the AV-loop constructs as shown by CD31 IH. Viable foetal donor cells were detected which expressed CK-18. FLC can be successfully used for heterotopic transplantation. Fibrin matrix permits rapid blood vessel ingrowth from the AV-loop and supports engraftment of FLC. It is therefore an appropriate environment for hepatocyte transplantation in combination with microsurgical vascularization strategies. Transplantation of fibrin gel-immobilized FLC may be a promising approach for the development of highly vascularized in vivo tissue-engineering-based liver support systems.

Development of hepatic tissue engineering

Liver transplantation is still the only treatment for end-staged liver diseases in children. However, donor organ shortage and immunosuppression are major limitations. Thus, approaches of hepatocyte transplantation are under investigation. Using cells might permit mass expansion, cryopreservation, and the ex vivo genetic modification of cells. For the development of cell-transplantation techniques, the use of three-dimensional scaffolds as carrier was shown to be advantageous. Polymeric matrices permit the formation of a neo-tissue and stimulation by the modification of the matrix surface. Another important issue is to define the right cell type for transplantation. Adult hepatocytes have a limited growth and differentiation potential. In contrast, fetal liver cells (FLC) possess an enormous growth and a bipotential differentiation potential. Thus, these cells may be very attractive as a cell resource for developing cell-based liver replacement. A third major issue in this approach is the neo-vascularization. Therefore, the transplantation in a recently developed model using a microsurgically created arterioveno-venous (AV) loop as a central vessel for the neo-tissue was used for transplantation of FLC in a fibrin-matrix. Initial results indicated that the transplantation of FLC using the AV-loop transplantation model may be promising for the development of highly vascularized in vivo tissue-engineered liver support systems.

Liver tissue engineering: The future of liver therapeutics

The applications derived from the concept of tissue engineering have spurred significant interest in the field of regenerative medicine as novel, next generation therapies. Due to a lack of treatment modalities for patients suffering from many forms of liver diseases, recent studies have touted that engineering hepatic tissues de novo in culture may be a viable method to address this therapeutic void. Liver tissue engineering is a new and emerging field in which a functional liver system is created in vivo using isolated hepatocytes and/or other cells types to treat acute and chronic liver diseases. Under circumstances in which a small, but functional liver tissue system could be engineered to provide the equivalent biological function proportional to a few percent of a normal, well-functioning liver, it would be possible to correct many disease phenotypes as a result of various forms of inherited metabolic deficiencies. Alternatively, hepatic tissues can be engineered rapidly to produce therapeutic effects allowing this approach to become an effective modality in the treatment of acute liver failure. Strategies to achieve high levels of hepatocyte survival and the development of methods to engineer a functional liver system in vivo will be discussed in this review.

Nanofabrication and Microfabrication of Functional Materials for Tissue Engineering

The burgeoning field of regenerative medicine promises significant progress in the treatment of cardiac ischemia, liver disease, and spinal cord injury. Key to its success will be the ability to engineer tissue safely and reliably. Tissue functionality must be recapitulated in the laboratory and then integrated into surrounding tissue upon transfer to the patient. Scaffolding materials must be chosen such that the microenvironment surrounding the cells is a close analog of the native environment. In the early days of tissue engineering, these materials were largely borrowed from other fields, with much of the focus on biocompatibility and biodegradation. However, attention has shifted recently to cell-cell and cell-surface interactions, largely because of enabling technologies at the nanoscale and microscale. Studies on cellular behavior in response to various stimuli are now easily realized by using microfabrication techniques and devices (e.g., biomedical microelectromechanical systems). These experiments are reproducible and moderate in cost, and often can be accomplished at high throughput, providing the fundamental knowledge required to design biomaterials that closely mimic the biological system. It is our opinion that these novel materials and technologies will bring engineered tissues one step closer to practical application in the clinic. This review discusses their application to cardiac, liver, and nerve tissue engineering.

Fetal and adult liver stem cells for liver regeneration and tissue engineering

For the development of innovative cell-based liver directed therapies, e.g. liver tissue engineering, the use of stem cells might be very attractive to overcome the limitation of donor liver tissue. Liver specific differentiation of embryonic, fetal or adult stem cells is currently under investigation. Different types of fetal liver (stem) cells during development were identified, and their advantageous growth potential and bipotential differentiation capacity were shown. However, ethical and legal issues have to be addressed before using fetal cells. Use of adult stem cells is clinically established, e.g. transplantation of hematopoietic stem cells. Other bone marrow derived liver stem cells might be mesenchymal stem cells (MSC). However, the transdifferentiation potential is still in question due to the observation of cellular fusion in several in vivo experiments. In vitro experiments revealed a crucial role of the environment (e.g. growth factors and extracellular matrix) for specific differentiation of stem cells. Co-cultured liver cells also seemed to be important for hepatic gene expression of MSC. For successful liver cell transplantation, a novel approach of tissue engineering by orthotopic transplantation of gel-immobilized cells could be promising, providing optimal environment for the injected cells. Moreover, an orthotopic tissue engineering approach using bipotential stem cells could lead to a repopulation of the recipients liver with healthy liver and biliary cells, thus providing both hepatic functions and biliary excretion. Future studies have to investigate, which stem cell and environmental conditions would be most suitable for the use of stem cells for liver regeneration or tissue engineering approaches.

Morphological and functional analysis of rat hepatocyte spheroids generated on poly(L-lactic acid) polymer in a pulsatile flow bioreactor

Liver neo-tissue suitable for transplantation has not been established. Primary rat hepatocytes were cultured on three-dimensional biodegradable polymer matrices in a pulsatile flow bioreactor with the intention of inducing tissue formation and improving cell survival. Functional and structural analysis of the hepatocytes forming liver neo-tissue was performed. Biodegradable poly(L-lactic acid) (PLLA) polymer discs were seeded with 4 x 10(6) primary rat hepatocytes each, were exposed to a pulsatile medium flow of 24 mL/min for 1, 2, 4, or 6 days and were investigated for monoethylglycinexylidine (MEGX) formation, ammonia detoxification, Cytokeratin 18 (CK18) expression, and preserved glycogen storage. Fine structural details were obtained using scanning and transmission electron microscopy. Spheroids of viable hepatocytes were formed. MEGX-specific production was maintained and ammonia removal capacity remained high during the entire flow-culture period of 6 days. CK18 distribution was normal. Periodic-acid- Schiff reaction demonstrated homogenous glycogen storage. The hepatocytes reassembled to form intercellular junctions and bile canaliculi. Functional and morphological analysis of rat hepatocytes forming spheroids in a pulsatile flow bioreactor indicated preserved and intact hepatocyte morphology and specific function. Pulsatile flow culture on PLLA scaffolds is a promising new method of hepatic tissue engineering leading to liver neo-tissue formation.

Injectable Liver: A Novel Approach Using Fibrin Gel as a Matrix for Culture and Intrahepatic Transplantation of Hepatocytes

Cell transplantation and tissue engineering with liver cells are currently under investigation as experimental therapies for certain liver diseases. In this study we evaluated a fibrin-based gel matrix as carrier for hepatocytes in culture. Furthermore, a novel technique for direct intrahepatic injection of fibrin gel-immobilized hepatocytes was developed and evaluated in a rat model. Hepatocytes were harvested from rats. Fibrin matrix was generated with modified fibrin sealant. Cells, in medium containing epidermal growth factor and insulin, were seeded in a drop of fibrin matrix onto plastic culture dishes. Cell numbers were assessed by DNA content. Hepatocyte differentiation was evaluated by reverse transcription-polymerase chain reaction (RT-PCR) and immunohistology (IH) for cytokeratin (CK)-18 and albumin. PKH26-labeled fibrin gel-immobilized hepatocytes were transplanted into liver by direct injection underneath the capsule. Fluorescence microscopy of explanted liver was performed to identify PKH26+ donor cells. Neotissue was characterized by IH for the markers CK-18, ED1, and desmin. Culture in a fibrin matrix allowed stable cell numbers and three-dimensional neotissue formation. RT-PCR and IH showed preservation of liver-specific markers CK-18 and albumin in vitro. Transplanted cells were identified by fluorescence microscopy after 2 and 7 days. CK-18 and desmin staining showed integration of hepatocytes and hepatic stellate cells into the host liver. Fibrin matrix is an appropriate environment for hepatocytes in culture. Direct intrahepatic injection of fibrin gel-immobilized hepatocytes is technically feasible. We conclude that fibrin gel immobilization is an attractive tool for the development of tissue engineering-based liver support systems.

The generation of functionally differentiated, three-dimensional hepatic tissue from two-dimensional sheets of progenitor small hepatocytes and nonparenchymal cells

BACKGROUND

The authors' laboratory has investigated tissue engineering of the liver as a novel approach for treating end-stage liver disease. Fabrication of thick, viable, three-dimensional liver tissue is limited by the lack of vascularity in the tissue-engineered constructs. To overcome this limitation, the authors fabricated three-dimensional, vascularized liver tissue in vivo from two-dimensional cell sheets created from small hepatocytes (SHC) and nonparenchymal cells (NPC) implanted into rat omentum.

METHODS

SHC and NPC were cultured on a silicon wafer and lifted as monolayer cell sheets. After maximal hepatotrophic stimulation was induced in the host by injecting retrorsine, creating a portacaval shunt, and performing a partial hepatectomy, these sheets were placed onto the omentum and then rolled into a three-dimensional cylinder.

RESULTS

New tissue consisting of both hepatocytes and bile duct-like structures formed by 2 weeks, and the mass of hepatocytes increased in size up to 2 months. The hepatocytes in these constructs were immunohistochemically positive for albumin and transferrin, and bile duct-like structures were positive for gamma-glutamyl transpeptidase, which suggests that they possess liver-specific function. Electron microscopy also revealed structures resembling bile canaliculi.

CONCLUSIONS

Functional, morphologically complex new tissue was generated from morphologically simple monolayer cell sheets of SHC and NPC. These results represent an essential step toward the design of tissue-engineered complex vascularized thick tissue.

Tissue engineering in surgery

The demands for repair and renewal of worn out or injured human tissue continue to increase and it is now apparent that this demand cannot be met from human donors. A partial solution may be found in living related and trans-species transplantation but these approaches invoke the problems of disease transfer and ethical dilemmas. Tissue engineering is a new technology that seeks to meet these increasing demands by utilising novel cell culture methods in vitro to provide tissue replacements in vivo. This article reviews the current state of tissue engineering and its potential for use in surgery.

Tissue engineering of the gastrointestinal tract for surgical replacement: a nutrition tool of the future?

Optimal nutrition depends on the multiple complex functions performed by the gastrointestinal tract, which range from basic functions such as storage, conduit and mechanical processing to more finely regulated capabilities such as vectorial transport, immune defence and cell signalling. Surgical strategies to supply lacking gastrointestinal tract tissues have relied on either replacement by proxy (surgical substitution) or the introduction of prostheses. Tissue engineering seeks to replace missing tissues with engineered tissues that more accurately reproduce the native physiological and anatomical milieu. It is now possible to engineer several areas of the gastrointestinal tract with high fidelity, and to employ tissue-engineered bowel in replacement in animal models. These replacement models have reflected excellent anatomical and physiological recapitulation of native bowel by the tissue-engineered constructs in vivo.

Nanoscale clustering of RGD peptides at surfaces using comb polymers. 2. Surface segregation of comb polymers in polylactide

Part 1 of these studies described poly(methyl methacrylate-r-polyoxyethylene methacrylate) P(MMA-r-POEM) comb polymers that present Arg-Gly-Asp (RGD) peptides at a surface in nanoscale clusters on a protein-resistant background for control of cell adhesion. Here in part 2, we examine surface segregation of these peptide-modified and unmodified comb polymers blended with polylactide (PLA) as a self-assembly approach suitable for surface modification of porous tissue engineering scaffolds. Multiple thermodynamic driving forces for surface enrichment of the comb polymer are exploited by annealing PLA/P(MMA-r-POEM) blends above the glass transition of the blend components but below the melting point of PLA, while in contact with water. Predictions of the interfacial composition profiles of annealed blends were made using a self-consistent field (SCF) lattice model. The calculations predict strong enrichment of the comb in the top approximately 50 A of blends, and organization of comb molecules in quasi-2D conformations at the interface, similar to the apparent structure of pure comb surfaces in contact with water described in part 1. Experimentally, PLA/comb blend surfaces were characterized by contact angle measurements, XPS, quantification of ligand-cluster surface density and stability by AFM and fluorescent nanosphere labeling, and cell attachment assays. These data were consistent with SCF predictions, showing significant enrichment of the comb at water-annealed surfaces and RGD cluster densities consistent with 2D conformations for comb molecules in the surface layer. Bulk miscibility of the blends was verified by dynamic rheometry, small-angle neutron scattering, DSC and X-ray diffraction studies. Surface segregation of combs provided tunable cell adhesion on PLA through surface-localized nanoclusters of RGD atop a cell-resistant background.

Local delivery of basic fibroblast growth factor increases both angiogenesis and engraftment of hepatocytes in tissue-engineered polymer devices

BACKGROUND

We investigated heterotopic hepatocyte transplantation on biodegradable polymers as a potential treatment for end-stage liver disease. The primary problem has been insufficient engraftment of transplanted cells partly because of insufficient vascularization. Increasing vascularization through locally delivered angiogenic factors may increase angiogenesis and hepatocyte engraftment.

METHODS

We studied the effect of local delivery of basic fibroblast growth factor (bFGF) on angiogenesis and hepatocyte engraftment within tissue-engineered liver constructs. Poly-l-lactic acid discs were fabricated and coated with either a mixture of saline, sucralfate, and Hydron (control group) or bFGF, sucralfate, and Hydron (bFGF group). bFGF release from polymers in vitro was tested using an ELISA. Hepatocytes were isolated from Lewis rats, seeded on control (n=9) or bFGF (n=11) polymers, and implanted into the small bowel mesentery of syngeneic animals. Specimens were harvested after 2 weeks and analyzed for hepatocyte engraftment. Microvascular density was compared between control (n=6) and bFGF groups (n=5).

RESULTS

Three hundred twenty-three thousandths of a microgram of bFGF were incorporated per polymer. Greater than 99% of the bFGF was released into solution by 72 hr in vitro. Two weeks after implantation, microvascular density, as measured by capillaries per high-powered field (c/hpf), was significantly greater in the bFGF group (43.8 c/hpf), compared with the control group (30.5 c/hpf; P<0.005). Specimens from the bFGF group (mean engraftment, 61,355 microm2) showed a 2.5-fold increase in hepatocyte engraftment as compared with control (24,197 microm2; P<0.002).

CONCLUSIONS

The angiogenic growth factor bFGF can be incorporated into degradable polymers used as delivery devices for hepatocyte transplantation. Implantation of these devices increases angiogenesis into the device and increases hepatocyte engraftment.

Building drug delivery into tissue engineering

The creation of efficient methods for manufacturing biotechnology drugs--many of which influence fundamental but complex cell behaviours, such as proliferation, migration and differentiation--is creating new opportunities for tissue repair. Many agents are potent and multifunctional; that is, they produce different effects within different tissues. Therefore, control of tissue concentration and spatial localization of delivery is essential for safety and effectiveness. Synthetic systems that can control agent delivery are particularly promising as materials for enhancing tissue regeneration. This review discusses the state of the art in controlled-release and microfluidic drug delivery technologies, and outlines their potential applications for tissue engineering.

Hepatocyte transplantation using biodegradable matrices in ascorbic acid-deficient rats: comparison with heterotopically transplanted liver grafts

BACKGROUND

Hepatocyte transplantation using polymeric matrices is under investigation as an alternative therapy for metabolic liver diseases. Long-term engraftment of hepatocytes in polymers has been demonstrated. However, the metabolic activity of hepatocytes in such devices has never been assessed in direct comparison with liver grafts.

METHODS

Hepatocyte and partial liver transplantation were evaluated in the scurvy-prone osteogenic disorder Shionogi rat model. Biodegradable poly glycolic acid matrices seeded with hepatocytes equivalent to 20% of the recipient's liver mass, or 20% liver grafts were heterotopically transplanted into ascorbic acid- (AsA) deficient recipients. Recipients of cell-free matrices or AsA-deficient liver grafts served as controls. Recipients were set on AsA-free diet after transplantation. Plasma AsA levels, AsA concentrations in liver and adrenal gland tissue, and body weight ratios were assessed and H&E histology was performed.

RESULTS

Recipients from the control groups showed symptoms of scurvy at 1 month after cessation of AsA supply. Hepatocyte transplantation and auxiliary liver transplantation prevented symptoms of scurvy and increased plasma and tissue AsA levels and body weight ratios. AsA levels in recipients of 20% liver grafts were comparable to normal control animals.

CONCLUSIONS

Hepatocytes transplanted in polymeric matrices are able to compensate for liver-based metabolic deficiencies. Hepatocyte transplantation improves plasma AsA levels in AsA-deficient recipients. However, auxiliary liver grafts are superior to hepatocyte grafts in improving metabolic parameters. Further research work is needed to increase the efficiency of liver cell transplantation with regard to a clinical application.

Priming of hepatocytes for cell culture by partial hepatectomy prior to cell isolation

The combination of ex vivo gene transfer and a sufficient transplant model for hepatocytes may permit treatment of single enzyme-based metabolic liver diseases. Induction of replicative potential (priming) in hepatocyte cultures may enhance the efficiency of gene transfer under stable in vitro conditions. It is known that hepatocyte replication is increased in vivo after partial hepatectomy. We investigated the effect of partial hepatectomy prior to cell isolation on hepatocytes in vitro. Male Lewis rats served as donors. Hepatocytes were isolated by collagenase digestion from either intact livers or from livers 48 h after 70% hepatectomy (PH). Cells were seeded on collagen-coated culture dishes with hormone-supplemented culture media. Hepatocyte morphology, number, albumin secretion rate, and mono-ethyl-glycin-xylidid (MEGX)-biotransformation capacity were assessed on days 1, 3, and 5 in culture. PH significantly increased hepatocyte number and albumin secretion of cultured hepatocytes over the whole observation period. In contrast, MEGX-biotransformation capacity was significantly decreased. Morphology of cultured hepatocytes was not affected by PH prior to hepatocyte isolation. These results suggest a prolonged and complex response of hepatocytes to PH in vitro. Hepatocyte priming by PH is a promising approach toward stable cultures of proliferating hepatocytes and may provide a model for in vitro studies of hepatic regeneration mechanisms. Further research on hepatocyte priming toward an application in ex vivo gene transfer and hepatic tissue engineering seems justified.

Survival and function of rat hepatocytes cocultured with nonparenchymal cells or sinusoidal endothelial cells on biodegradable polymers under flow conditions

BACKGROUND/PURPOSE

The authors have investigated hepatocyte transplantation using biodegradable polymer scaffolds as a possible treatment of end-stage liver disease. The purpose of this study was to investigate the survival rate and function of hepatocytes alone or cocultured with other cell types on 3-dimensional biodegradable polymers for 7 days under continuous flow conditions in vitro.

METHODS

Hepatocytes (group 1, n = 8), hepatocytes with nonparenchymal cells (group 2, n = 7), or hepatocytes with sinusoidal endothelial cells (group 3, n = 6) were isolated from Lewis rats and seeded onto the polymer scaffolds. The polymer devices subsequently were placed under continuous flow conditions for 7 days. Albumin production from the constructs was measured each day, and urea nitrogen synthesis was examined on day 7. The devices also were examined by histology at day 7.

RESULTS

Histology results showed the presence of numerous viable hepatocytes on polymer devices, with no differences in hepatocyte viability between the 3 groups. Albumin secretion in the culture medium gradually decreased by day 7. There also were no significant differences in albumin production or urea nitrogen synthesis between the 3 groups at day 7.

CONCLUSIONS

Hepatocytes could survive on the 3-dimensional polymer scaffolds under flow conditions for 7 days, and albumin secretion and urea synthesis of hepatocytes were seen at day 7. Nonparenchymal cells and sinusoidal endothelial cells had no measurable effect on hepatocyte function in our continuous flow culture system.

Long-term differentiated function of heterotopically transplanted hepatocytes on three-dimensional polymer matrices

Hepatocyte transplantation using porous matrices is under investigation as an alternative therapy for certain liver diseases. For this purpose, long-term function of transplanted hepatocytes is mandatory. This problem has not been sufficiently investigated yet. In this study Lewis rats were used as donors and recipients. Stimulated (group A, portocaval shunt) or unstimulated (group B) hepatocytes were transplanted into prevascularized polyvinyl-alcohol matrices. Cell-free matrices served as controls (group C). Matrices were harvested between 1 h and 1 year after implantation and analyzed by morphometry; albumin RNA in situ hybridization; and cytokeratin-, actin-, desmin-, and macrophage-specific antigen immunohistology. The hepatocyte number significantly decreased within the first week following implantation. Between 1 month and 1 year after transplantation a significant increase in hepatocyte number was noted in groups A and B. Albumin transcripts of transplanted hepatocytes were at normal levels at all times except for group B after 1 year. The immunohistology suggested engraftment of nonparenchymal liver cells. We conclude that 3-dimensional matrices provide a sufficient environment for long-term engraftment of transplanted liver cells. The hepatocytes are able, despite suboptimal initial engraftment, to repopulate the scaffold for at least half of the recipient's life span and maintain cell-specific function after sufficient stimulation.

Influence of pancreatic islets on growth and differentiation of hepatocytes in co-culture

Improvement of cell culture conditions in hepatic tissue engineering may permit cell/tissue banking and the generation of liver tissue equivalents for transplantation. In these systems, continuous hepatotrophic stimulation is still necessary. We investigated the stimulatory effects of pancreatic islets on hepatocytes in co-culture and characterized the stimulatory mechanisms. Hepatocytes and pancreatic islets were harvested from Lewis rats. Cells were cultured on collagen dishes either with nonstimulated media (controls and co-cultures with low or high islet rate) or stimulated media (controls and co-cultures). To characterize stimulatory mechanisms, additional co-cultures with membrane separation, with antiinsulin, antiglucagon, and with both antibodies were examined. Hepatocyte numbers, albumin secretion rate by enzyme-linked immunoadsorbent assay, and monoethylglycinxylidid biotransformation values by fluorescence polarization immunoassay were assessed. A radioimmunoassay measured insulin and glucagon concentrations. In groups with nonstimulated media, cell number was higher in co-cultures with low islet rate, and albumin secretion rate was increased in co-cultures with high islet rate compared to controls. MEGX biotransformation was decreased in co-cultures. In groups with stimulated media, co-culture had no impact on cell number or albumin secretion rate. Hepatocyte numbers and albumin secretion rates were not changed in co-cultures after membrane separation. Islet effects on hepatocytes were reduced in co-cultures with antiinsulin, antiglucagon, or both antibodies. Pancreatic islets provide stimulation for hepatocytes in vitro. Islet effects were mediated by soluble factors, and are dependent on insulin and glucagon. These results permit further investigations towards three-dimensional transplantable hepatocyte-islet devices for continuous in vitro and in vivo stimulation.

Is there an optimal concentration of cotransplanted islets of Langerhans for stimulation of hepatocytes in three dimensional matrices?

BACKGROUND

Hepatocyte transplantation using three-dimensional matrices is under investigation as an alternative therapy for several liver diseases. For sufficient transplantation results hepatotrophic stimulation is necessary. We investigated the stimulatory effect of cotransplanted pancreatic islets in different ratios.

METHODS

Lewis rats were used as donors and recipients. A portocaval shunt (group A) or sham operation (groups B-G) was performed 1 week before hepatocyte transplantation. Four polyvinyl-alcohol matrices each containing 1.25 x 10(7) hepatocytes (groups A and B) or 1.25 x 10(7) hepatocytes and 125 (C), 250 (D), 500 (E), or 750 (F) islets were implanted between small bowel mesenteric leaves. In group G, medium soaked matrices were implanted. One month after implantation, specimens were harvested and investigated using albumin-RNA in situ hybridization, and insulin, glucagon, and bromodesoxy uridine immunohistochemistry. The hepatocyte area was assessed using image analysis.

RESULTS

Hepatocyte area and proliferation ratio increased depending on the number of cotransplanted islets with a peak at 40 islets per 1 million hepatocytes (group E). Cotransplantation of islets in higher concentrations did not further increase hepatocyte area or proliferation ratio. Hepatocytes in all groups expressed albumin RNA at normal transcription levels as compared to standard liver sections. Islets displayed insulin and glucagon in physiological distribution.

DISCUSSION

Three-dimensional matrices provide a sufficient environment for transplanted hepatocytes and islets. The hepatotrophic effect of cotransplanted islets is comparable to portocaval shunting and has a saturation limit at 40 islets per 1 million hepatocytes. For further application of islet cotransplantation, this ratio seems to be preferable.

Evaluation of methods of hepatotrophic stimulation in rat heterotopic hepatocyte transplantation using polymers

BACKGROUND

Hepatocyte transplantation has been studied as an alternative to organ transplantation. Hepatocyte transplant models should provide sufficient cell mass for replacement function and hepatotrophic stimulation of the transplanted cells in heterotopic locations.

METHOD

The authors used three-dimensional porous polyvinyl-alcohol matrices as cell carriers, which were implanted between mesenteric leaves of the intestine. In this study, different methods were evaluated for hepatotrophic stimulation. Fifty million transplanted hepatocytes (approximately 10% liver mass) were implanted in Lewis rats. We compared 70% partial hepatectomy, portacaval shunt, cotransplantation of enterocytes, cotransplantation of islets of Langerhans, and methylprednisolone injection to a control group with only hepatocyte transplantation. Portacaval shunt and islet cotransplantation also were used in combination. Specimens were harvested 2 weeks after transplantation, and area per histological cross section compromised by hepatocytes was measured.

RESULTS

Seventy percent partial hepatectomy, enterocyte cotransplantation, and methylprednisolone injection resulted in hepatocyte maintenance similar to control group (3,100 +/- 7,592 microm2). Portacaval shunt (96,866 +/- 55,039 microm2) and islet cotransplantation (173,020 +/- 75,977 microm2) yielded a highly significant increase in hepatocyte area. The combination of portacaval shunt and islet cotransplantation resulted in a significant increase compared with using these methods individually (288,930 +/- 86,726 microm2). Additional immunohistochemical stains for active DNA synthesis, insulin, and glucagon demonstrated the proliferative abilities of the hepatocytes and the synthesis of insulin and glucagon in the cotransplanted islets.

CONCLUSION

Hepatocyte transplantation can be performed using polymer carriers and that hepatocyte survival and maintenance can be improved with portacaval shunt and islet cotransplantation.

Cellular ingrowth and thickness changes in poly-L-lactide and polyglycolide matrices implanted subcutaneously in the rat

Highly porous matrices of poly-L-lactide (PL) and polyglycolide (PG), 24, 50, or 95 mg/cc in the form of 10 x 10 x 3 mm wafers, were implanted subcutaneously (two per rat) in the flanks of 8-12-week-old female Lewis rats (n = 120). Matrices were harvested, two rats per week, for 15 weeks and examined histologically. At weeks 1 and 2, a thin fibrous capsule was present and matrices showed capillary beds and host-cell infiltration along the implant margins. By week 4, the PL specimens had some arterioles while the PG specimens still had only capillary beds. At week 7, PL had well developed arterioles, venules, and capillaries while PG began to show modest vascular beds of capillaries only. In terms of cellular ingrowth, PL remained unchanged from 7 to 15 weeks. Giant cell formation was observed wherever polymer was present. There was a loss of thickness and cell mass for both matrices over time (PG > PL) despite initial host-cell ingrowth. As both polymers degraded and were absorbed, the ingrown cells mass regressed. There was little remaining PG at 15 weeks, leaving no trace of cells that previously had ingrown and no evidence of scar tissue.

Transplantation of cells in matrices for tissue regeneration

Tissue engineering is a field that has truly emerged in the last decade. It has brought together diverse technologies, e.g. cell culture, polymer chemistry and transplantation. The creation of matrices to guide tissue regeneration allows manipulation at several levels, i.e. the cells employed, the choice of polymer and the design of construct assembly methods. We present experience using such constructs to guide regeneration of diverse tissues, e.g. liver, intestine, urologic tissue, skin, cartilage, bone and cardiovascular structures. Emerging concepts in using cell/polymer constructs include the need for appropriate modeling of the micromechanical environments of different tissues, as well as the necessity of finding new strategies to achieve vascularization of tissues for transplant. Finally, the concept of applying tissue-engineered structures to non-native sites is discussed.

Morphological changes during hepatocellular maturity in neonatal rats

BACKGROUND

Hepatocellular maturation is characterised by the progressive transition from an architecture in which hepatocyte plates are at least two cells thick to the familiar adult pattern in which liver cell plates are predominantly single-cell in thickness. A similar process also has been noted during compensatory hyperplasia following damage, or destruction to the hepatic parenchyma. The pathological events that underlie these processes remain inadequately explained. A new morphological approach has been developed to study the maturity of rat neonatal livers in order to identify the factors which govern the structured morphogenesis of the liver in greater detail.

METHODS

Sections of hepatic tissue obtained from the left lateral and right posterior lobes from neonatal rats were studied at 8, 10, 11, 13, 14, 18, 23, and 28 days postpartum. These were mounted, fixed, and stained with haematoxylin and eosin and analysed using a modified point-counting technique. Following validation of the technique, the proportion of single-cell plates to double-cell plates was then calculated at each time point.

RESULTS

Liver sections from 8-day neonatal rats had the lowest percentage of single-cell thick plates of 16.9 +/- 4.6% and the lowest standard deviation (mean +/- SD). The hepatic architecture had fully matured by 28 days and was characterised by predominantly single-cell plates (84.6 +/- 4.6%) lining the hepatic sinusoids. During the intervening time, the standard deviations increased significantly, peaking between 18-23 days, and reflected the rapidly changing morphology of the liver during this maturation process of the conversion of double-cell plates to single-cell plates.

CONCLUSIONS

It is concluded that the process of hepatocellular maturity in the neonatal Sprague-Dawley rat is reproducibly complete by 28 days and that further studies may now be conducted to determine the anatomical and pathophysiological changes that govern this important transition.

The effect of donor and recipient age on engraftment of tissue-engineered liver

A novel treatment for end-stage liver disease using heterotopic hepatocyte transplantation on biodegradable polymers has been investigated. Survival and repopulation of adequate cell mass to replace hepatic function has been the principal difficulty of this method. Hence the authors have begun to investigate the role of donor and recipient age on the efficiency of hepatocyte transplantation. Lewis rats were used as donors and recipients. Hepatocytes were isolated with a collagenase digestion, both for the adult and fetal livers (17 days estimated gestational age). After digestion, the hepatocytes were seeded onto 95% porous poly-(L)-lactic acid matrices. The polymer-cell constructs with adult or fetal cells were then implanted between mesenteric leaves of three different recipient groups: adults (approximately 200 g), 2-week, and 4-week neonates (two to five animals per group, depending on litter size). The specimens were harvested at 4 weeks, stained with Hematoxylin and Eosin (H&E), and the cell area of each specimen (24 sections per group) was quantitated using morphometric analysis. Results were statistically analyzed using an unpaired, two-tailed Student's t test. At 4 weeks, all specimens showed survival of groups of hepatocytes, especially along the periphery of the polymers and near blood vessels. The hepatocyte cell area for the six groups was calculated in square micrometers: the adult cells transplanted into adult recipients, 0.16 x 10(5) microns2; fetal cells into adults, 0.47 x 10(5) microns2; adult into 4-week neonates, 1.17 x 10(5) microns2; fetal into 4-week neonates, 4.54 x 10(5) microns2; adult into 2-week neonates, 2.98 x 10(5) microns2, and fetal into 2-week neonates, 5.81 x 10(5) microns2. In all three recipient groups, the area of fetal hepatocytes was approximately two to three times the area of the adult hepatocytes (P < .05 for 2-week and 4-week neonatal recipients, P = .06 for adult recipients). Also, as the recipient age decreased, there was an increase in the hepatocyte cell area (P < .05 for fetal or adult groups). The authors conclude that fetal hepatocytes heterotopically transplanted have a significant survival advantage over adult hepatocytes, independent of recipient age. The authors further conclude that the neonatal environment is more favorable than the adult environment for implantation of hepatocytes.

Bromodeoxyuridine: a diagnostic tool in biology and medicine, Part III. Proliferation in normal, injured and diseased tissue, growth factors, differentiation, DNA replication sites and in situ hybridization

This paper is a continuation of parts I (history, methods and cell kinetics) and II (clinical applications and carcinogenesis) published previously (Dolbeare, 1995 Histochem. J. 27, 339, 923). Incorporation of bromodeoxyuridine (BrdUrd) into DNA is used to measure proliferation in normal, diseased and injured tissue and to follow the effect of growth factors. Immunochemical detection of BrdUrd can be used to determine proliferative characteristics of differentiating tissues and to obtain birth dates for actual differentiation events. Studies are also described in which BrdUrd is used to follow the order of DNA replication in specific chromosomes, DNA replication sites in the nucleus and to monitor DNA repair. BrdUrd incorporation has been used as a tool for in situ hybridization experiments.

Outcome of subcutaneous islet transplantation improved by polymer device

Syngeneic transplantation of rat islets into subcutaneous tissue failed to cure streptozocin diabetes. The reason is unknown, but poor vascularization may play a role. We hypothesize that if a well-vascularized subcutaneous site could be created, islet grafts would do well. Four hundred freshly isolated mouse islets were transplanted syngeneically under the renal capsule or into the intraperitoneal cavity and compared with 800 islets in subcutaneous tissue of streptozocin-diabetic mice. Four weeks after transplantation, 14 of 14 under the renal capsule, 4 of 8 in the intraperitoneal site, and 0 of 7 in the subcutaneous tissue site achieved normoglycemia. To create vascularized organoids, we transplanted 800 mouse islets into polyvinyl alcohol (PVA) or polyglycolic acid (PGA) polymers in subcutaneous tissue of streptozocin-diabetic mice either immediately (four in PVA and six in PGA) or 7 days (four in PVA and four in PGA) after implantation. Four weeks after transplantation, the mean blood glucose level and body weight had no change with PVA. However, the mean body weight increased significantly with PGA and 3/10 became normoglycemic. When transplanting 400 islets with PGA polymers intraperitoneally, all animals (n=5) remained hyperglycemic 3 months later. In contrast, four of five recipients transplanted with 800 islets with PGA polymers subcutaneously became normoglycemic. The grafts from successful animals contained numerous revascularized islets containing a substantial amount of insulin. These preliminary results indicate that subcutaneous islet transplantation using PGA polymers can improve the metabolic status and, in some cases, even cure diabetes in streptozocin-diabetic mice.