Liver tissue engineering using decellularized scaffolds: Current progress, challenges, and opportunities

Liver transplantation represents the only definitive treatment for patients with end-stage liver disease. However, the shortage of liver donors provokes a dramatic gap between available grafts and patients on the waiting list. Whole liver bioengineering, an emerging field of tissue engineering, holds great potential to overcome this gap. This approach involves two main steps; the first is liver decellularization and the second is recellularization. Liver decellularization aims to remove cellular and nuclear materials from the organ, leaving behind extracellular matrices containing different structural proteins and growth factors while retaining both the vascular and biliary networks. Recellularization involves repopulating the decellularized liver with appropriate cells, theoretically from the recipient patient, to reconstruct the parenchyma, vascular tree, and biliary network. The aim of this review is to identify the major advances in decellularization and recellularization strategies and investigate obstacles for the clinical application of bioengineered liver, including immunogenicity of the designed liver extracellular matrices, the need for standardization of scaffold fabrication techniques, selection of suitable cell sources for parenchymal repopulation, vascular, and biliary tree reconstruction. In vivo transplantation models are also summarized for evaluating the functionality of bioengineered livers. Finally, the regulatory measures and future directions for confirming the safety and efficacy of bioengineered liver are also discussed. Addressing these challenges in whole liver bioengineering may offer new solutions to meet the demand for liver transplantation and improve patient outcomes.

Biological characteristics of tissue engineered-nerve grafts enhancing peripheral nerve regeneration

BACKGROUND

A favorable regenerative microenvironment is essential for peripheral nerve regeneration. Neural tissue-specific extracellular matrix (ECM) is a natural material that helps direct cell behavior and promote axon regeneration. Both bone marrow-derived mesenchymal stem cells (BMSCs) and adipose-derived mesenchymal stem cells (ADSCs) transplantation are effective in repairing peripheral nerve injury (PNI). However, there is no study that characterizes the in vivo microenvironmental characteristics of these two MSCs for the early repair of PNI when combined with neural tissue-derived ECM materials, i.e., acellular nerve allograft (ANA).

METHODS

In order to investigate biological characteristics, molecular mechanisms of early stage, and effectiveness of ADSCs- or BMSCs-injected into ANA for repairing PNI in vivo, a rat 10 mm long sciatic nerve defect model was used. We isolated primary BMSCs and ADSCs from bone marrow and adipose tissue, respectively. First, to investigate the in vivo response characteristics and underlying molecular mechanisms of ANA combined with BMSCs or ADSCs, eighty-four rats were randomly divided into three groups: ANA group, ANA+BMSC group, and ANA+ADSC group. We performed flow cytometry, RT-PCR, and immunofluorescence staining up to 4 weeks postoperatively. To further elucidate the underlying molecular mechanisms, changes in long noncoding RNAs (lncRNAs), circular RNAs (circRNAs), microRNAs (miRNAs), and messenger RNAs (mRNAs) were systematically investigated using whole transcriptome sequencing. We then constructed protein-protein interaction networks to find 10 top ranked hub genes among differentially expressed mRNAs. Second, in order to explore the effectiveness of BMSCs and ADSCs on neural tissue-derived ECM materials for repairing PNI, sixty-eight rats were randomized into four groups: ANA group, ANA+BMSC group, ANA+ADSC group, and AUTO group. In the ANA+BMSC and ANA+ADSC groups, ADSCs/BMSCs were equally injected along the long axis of the 10-mm ANA. Then, we performed histological and functional assessments up to 12 weeks postoperatively.

RESULTS

The results of flow cytometry and RT-PCR showed that ANA combined with BMSCs exhibited more significant immunomodulatory effects, as evidenced by the up-regulation of interleukin (IL)-10, down-regulation of IL-1β and tumor necrosis factor-alpha (TNF-α) expression, promotion of M1-type macrophage polarization to M2-type, and a significant increase in the number of regulatory T cells (Tregs). ANA combined with ADSCs exhibited more pronounced features of pro-myelination and angiogenesis, as evidenced by the up-regulation of myelin-associated protein gene (MBP and MPZ) and angiogenesis-related factors (TGF-β, VEGF). Moreover, differentially expressed genes from whole transcriptome sequencing results further indicated that ANA loaded with BMSCs exhibited notable immunomodulatory effects and ANA loaded with ADSCs was more associated with angiogenesis, axonal growth, and myelin formation. Notably, ANA infused with BMSCs or ADSCs enhanced peripheral nerve regeneration and motor function recovery with no statistically significant differences.

CONCLUSIONS

This study revealed that both ANA combined with BMSCs and ADSCs enhance peripheral nerve regeneration and motor function recovery, but their biological characteristics (mainly including immunomodulatory effects, pro-vascular regenerative effects, and pro-myelin regenerative effects) and underlying molecular mechanisms in the process of repairing PNI in vivo are different, providing new insights into MSC therapy for peripheral nerve injury and its clinical translation.

The journey of decellularized vessel: from laboratory to operating room

Over the past few decades, there has been a remarkable advancement in the field of transplantation. But the shortage of donors is still an urgent problem that requires immediate attention. As with xenotransplantation, bioengineered organs are promising solutions to the current shortage situation. And decellularization is a unique technology in organ-bioengineering. However, at present, there is no unified decellularization method for different tissues, and there is no gold-standard for evaluating decellularization efficiency. Meanwhile, recellularization, re-endothelialization and modification are needed to form transplantable organs. With this mind, we can start with decellularization and re-endothelialization or modification of small blood vessels, which would serve to address the shortage of small-diameter vessels while simultaneously gathering the requisite data and inspiration for further recellularization of the whole organ-scale vascular network. In this review, we collect the related experiments of decellularization and post-decellularization approaches of small vessels in recent years. Subsequently, we summarize the experience in relation to the decellularization and post-decellularization combinations, and put forward obstacle we face and possible solutions.

Exosome-loaded decellularized tissue: Opening a new window for regenerative medicine

Mesenchymal stem cell-derived exosomes (MSCs-EXO) have received a lot of interest recently as a potential therapeutic tool in regenerative medicine. Extracellular vesicles (EVs) known as exosomes (EXOs) are crucial for cell-cell communication throughout a variety of activities including stress response, aging, angiogenesis, and cell differentiation. Exploration of the potential use of EXOs as essential therapeutic effectors of MSCs to encourage tissue regeneration was motivated by success in the field of regenerative medicine. EXOs have been administered to target tissues using a variety of methods, including direct, intravenous, intraperitoneal injection, oral delivery, and hydrogel-based encapsulation, in various disease models. Despite the significant advances in EXO therapy, various methods are still being researched to optimize the therapeutic applications of these nanoparticles, and it is not completely clear which approach to EXO administration will have the greatest effects. Here, we will review emerging developments in the applications of EXOs loaded into decellularized tissues as therapeutic agents for use in regenerative medicine in various tissues.

Strategies for decellularization, re-cellularIzation and crosslinking in liver bioengineering

Liver transplantation is the only definitive treatment for end-stage liver disease and its availability is restricted by organ donor shortages. The development of liver bioengineering provides the probability to create a functional alternative to reduce the gap in organ demand and supply. Decellularized liver scaffolds have been widely applied in bioengineering because they can mimic the native liver microenvironment and retain extracellular matrix (ECM) components. Multiple approaches including chemical, physical and biological methods have been developed for liver decellularization in current studies, but a full set of unified criteria has not yet been established. Each method has its advantages and drawbacks that influence the microstructure and ligand landscape of decellularized liver scaffolds. Optimizing a decellularization method to eliminate cell material while retaining as much of the ECM intact as possible is therefore important for biological scaffold applications. Furthermore, crosslinking strategies can improve the biological performance of scaffolds, including reinforcing biomechanics, delaying degradation in vivo and reducing immune rejection, which can better promote the integration of re-cellularized scaffolds with host tissue and influence the reconstruction process. In this review, we aim to present the different liver decellularization techniques, the crosslinking methods to improve scaffold characteristics with crosslinking and the preparation of soluble ECM.

Proteomic analysis of decellularized mice liver and kidney extracellular matrices

BACKGROUND

The extracellular matrix (ECM) is a three-dimensional network of proteins that encases and supports cells within a tissue and promotes physiological and pathological cellular differentiation and functionality. Understanding the complex composition of the ECM is essential to decrypt physiological processes as well as pathogenesis. In this context, the method of decellularization is a useful technique to eliminate cellular components from tissues while preserving the majority of the structural and functional integrity of the ECM.

RESULTS

In this study, we employed a bottom-up proteomic approach to elucidate the intricate network of proteins in the decellularized extracellular matrices of murine liver and kidney tissues. This approach involved the use of a novel, perfusion-based decellularization protocol to generate acellular whole organ scaffolds. Proteomic analysis of decellularized mice liver and kidney ECM scaffolds revealed tissue-specific differences in matrisome composition, while we found a predominantly stable composition of the core matrisome, consisting of collagens, glycoproteins, and proteoglycans. Liver matrisome analysis revealed unique proteins such as collagen type VI alpha-6, fibrillin-2 or biglycan. In the kidney, specific ECM-regulators such as cathepsin z were detected.

CONCLUSION

The identification of distinct proteomic signatures provides insights into how different matrisome compositions might influence the biological properties of distinct tissues. This experimental workflow will help to further elucidate the proteomic landscape of decellularized extracellular matrix scaffolds of mice in order to decipher complex cell-matrix interactions and their contribution to a tissue-specific microenvironment.

MMP inhibition as a novel strategy for extracellular matrix preservation during whole liver decellularization

As the only reliable treatment option for end-stage liver diseases, conventional liver transplantation confronts major supply limitations. Accordingly, the decellularization of discarded livers to produce bioscaffolds that support recellularization with progenitor/stem cells has emerged as a promising translational medicine approach. The success of this approach will substantially be determined by the extent of extracellular matrix (ECM) preservation during the decellularization process. Here, we assumed that the matrix metalloproteinase (MMP) inhibition could reduce the ECM damage during the whole liver decellularization of an animal model using a perfusion-based system. We demonstrated that the application of doxycycline as an MMP inhibitor led to significantly higher preservation of collagen, glycosaminoglycans, and hepatic growth factor (HGF) contents, as well as mechanical and structural features, including tensile strength, fiber integrity, and porosity. Notably, produced bioscaffolds were biocompatible and efficiently supported cell viability and proliferation in vitro. We also indicated that produced bioscaffolds efficiently supported HepG2 cell function upon seeding onto liver ECM discs using albumin and urea assay. Additionally, MMP inhibitor pretreated decellularized livers were more durable in contact with collagenase digestion compared to control bioscaffolds in vitro. Using zymography, we confirmed the underlying mechanism that results in these promising effects is through the inhibition of MMP2 and MMP9. Overall, we demonstrated a novel method based on MMP inhibition to ameliorate the ECM structure and composition preservation during liver decellularization as a critical step in fabricating transplantable bioengineered livers.

Polyethylene glycol crosslinked decellularized single liver lobe scaffolds with vascular endothelial growth factor promotes angiogenesis in vivo

BACKGROUND

Improving the mechanical properties and angiogenesis of acellular scaffolds before transplantation is an important challenge facing the development of acellular liver grafts. The present study aimed to evaluate the cytotoxicity and angiogenesis of polyethylene glycol (PEG) crosslinked decellularized single liver lobe scaffolds (DLSs), and establish its suitability as a graft for long-term liver tissue engineering.

METHODS

Using mercaptoacrylate produced by the Michael addition reaction, DLSs were first modified using N-succinimidyl S-acetylthioacetate (SATA), followed by cross-linking with PEG as well as vascular endothelial growth factor (VEGF). The optimal concentration of agents and time of the individual steps were identified in this procedure through biomechanical testing and morphological analysis. Subsequently, human umbilical vein endothelial cells (HUVECs) were seeded on the PEG crosslinked scaffolds to detect the proliferation and viability of cells. The scaffolds were then transplanted into the subcutaneous tissue of Sprague-Dawley rats to evaluate angiogenesis. In addition, the average number of blood vessels was evaluated in the grafts with or without PEG at days 7, 14, and 21 after implantation.

RESULTS

The PEG crosslinked DLS maintained their three-dimensional structure and were more translucent after decellularization than native DLS, which presented a denser and more porous network structure. The results for Young's modulus proved that the mechanical properties of 0.5 PEG crosslinked DLS were the best and close to that of native livers. The PEG-VEGF-DLS could better promote cell proliferation and differentiation of HUVECs compared with the groups without PEG cross-linking. Importantly, the average density of blood vessels was higher in the PEG-VEGF-DLS than that in other groups at days 7, 14, and 21 after implantation in vivo.

CONCLUSIONS

The PEG crosslinked DLS with VEGF could improve the biomechanical properties of native DLS, and most importantly, their lack of cytotoxicity provides a new route to promote the proliferation of cells in vitro and angiogenesis in vivo in liver tissue engineering.

Whole Liver Derived Acellular Extracellular Matrix for Bioengineering of Liver Constructs: An Updated Review

Biomaterial templates play a critical role in establishing and bioinstructing three-dimensional cellular growth, proliferation and spatial morphogenetic processes that culminate in the development of physiologically relevant in vitro liver models. Various natural and synthetic polymeric biomaterials are currently available to construct biomimetic cell culture environments to investigate hepatic cell-matrix interactions, drug response assessment, toxicity, and disease mechanisms. One specific class of natural biomaterials consists of the decellularized liver extracellular matrix (dECM) derived from xenogeneic or allogeneic sources, which is rich in bioconstituents essential for the ultrastructural stability, function, repair, and regeneration of tissues/organs. Considering the significance of the key design blueprints of organ-specific acellular substrates for physiologically active graft reconstruction, herein we showcased the latest updates in the field of liver decellularization-recellularization technologies. Overall, this review highlights the potential of acellular matrix as a promising biomaterial in light of recent advances in the preparation of liver-specific whole organ scaffolds. The review concludes with a discussion of the challenges and future prospects of liver-specific decellularized materials in the direction of translational research.

Bioengineering liver tissue by repopulation of decellularised scaffolds

Liver transplantation is the only curative therapy for end stage liver disease, but is limited by the organ shortage, and is associated with the adverse consequences of immunosuppression. Repopulation of decellularised whole organ scaffolds with appropriate cells of recipient origin offers a theoretically attractive solution, allowing reliable and timely organ sourcing without the need for immunosuppression. Decellularisation methodologies vary widely but seek to address the conflicting objectives of removing the cellular component of tissues whilst keeping the 3D structure of the extra-cellular matrix intact, as well as retaining the instructive cell fate determining biochemicals contained therein. Liver scaffold recellularisation has progressed from small rodent in vitro studies to large animal in vivo perfusion models, using a wide range of cell types including primary cells, cell lines, foetal stem cells, and induced pluripotent stem cells. Within these models, a limited but measurable degree of physiologically significant hepatocyte function has been reported with demonstrable ammonia metabolism in vivo. Biliary repopulation and function have been restricted by challenges relating to the culture and propagations of cholangiocytes, though advances in organoid culture may help address this. Hepatic vasculature repopulation has enabled sustainable blood perfusion in vivo, but with cell types that would limit clinical applications, and which have not been shown to have the specific functions of liver sinusoidal endothelial cells. Minority cell groups such as Kupffer cells and stellate cells have not been repopulated. Bioengineering by repopulation of decellularised scaffolds has significantly progressed, but there remain significant experimental challenges to be addressed before therapeutic applications may be envisaged.

Functional acellular matrix for tissue repair

In view of their low immunogenicity, biomimetic internal environment, tissue- and organ-like physicochemical properties, and functionalization potential, decellularized extracellular matrix (dECM) materials attract considerable attention and are widely used in tissue engineering. This review describes the composition of extracellular matrices and their role in stem-cell differentiation, discusses the advantages and disadvantages of existing decellularization techniques, and presents methods for the functionalization and characterization of decellularized scaffolds. In addition, we discuss progress in the use of dECMs for cartilage, skin, nerve, and muscle repair and the transplantation or regeneration of different whole organs (e.g., kidneys, liver, uterus, lungs, and heart), summarize the shortcomings of using dECMs for tissue and organ repair after refunctionalization, and examine the corresponding future prospects. Thus, the present review helps to further systematize the application of functionalized dECMs in tissue/organ transplantation and keep researchers up to date on recent progress in dECM usage.

Rat liver ECM incorporated into electrospun polycaprolactone scaffolds as a platform for hepatocyte culture

Liver disease is expanding across the globe; however, health-care systems still lack approved pharmaceutical treatment strategies to mitigate potential liver failures. Organ transplantation is the only treatment for liver failure and with increasing cases of liver disease, transplant programs increasingly cannot provide timely transplant availability for all patients. The development of pharmaceutical mitigation strategies is clearly necessary and methods to improve drug development processes are considered vital for this purpose. Herein, we present a methodology for incorporating whole organ decellularised rat liver ECM (rLECM) into polycaprolactone (PCL) electrospun scaffolds with the aim of producing biologically relevant liver tissue models. rLECM PCL scaffolds have been produced with 5 w/w% and 10 w/w% rLECM:PCL and were analyzed by SEM imaging, tensile mechanical analyses and FTIR spectroscopy. The hepatocellular carcinoma cell line, HepG2, was cultured upon the scaffolds for 14 days and were analyzed through cell viability assay, DNA quantification, albumin quantification, immunohistochemistry, and RT-qPCR gene expression analysis. Results showed significant increases in proliferative activity of HepG2 on rLECM containing scaffolds alongside maintained key gene expression. This study confirms that rLECM can be utilized to modulate the bioactivity of electrospun PCL scaffolds and has the potential to produce electrospun scaffolds suitable for enhanced hepatocyte cultures and in-vitro liver tissue models.

Preparation and Use of Decellularized Extracellular Matrix for Tissue Engineering

The multidisciplinary fields of tissue engineering and regenerative medicine have the potential to revolutionize the practise of medicine through the abilities to repair, regenerate, or replace tissues and organs with functional engineered constructs. To this end, tissue engineering combines scaffolding materials with cells and biologically active molecules into constructs with the appropriate structures and properties for tissue/organ regeneration, where scaffolding materials and biomolecules are the keys to mimic the native extracellular matrix (ECM). For this, one emerging way is to decellularize the native ECM into the materials suitable for, directly or in combination with other materials, creating functional constructs. Over the past decade, decellularized ECM (or dECM) has greatly facilitated the advance of tissue engineering and regenerative medicine, while being challenged in many ways. This article reviews the recent development of dECM for tissue engineering and regenerative medicine, with a focus on the preparation of dECM along with its influence on cell culture, the modification of dECM for use as a scaffolding material, and the novel techniques and emerging trends in processing dECM into functional constructs. We highlight the success of dECM and constructs in the in vitro, in vivo, and clinical applications and further identify the key issues and challenges involved, along with a discussion of future research directions.

Solid fraction determines stiffness and viscosity in decellularized pancreatic tissues

The role of extracellular matrix (ECM) composition and turnover in mechano-signaling and the metamorphic fate of cells seeded into decellularized tissue can be elucidated by recent developments in non-invasive imaging and biotechnological analysis methods. Because these methods allow accurate quantification of the composition and structural integrity of the ECM, they can be critical in establishing standardized decellularization protocols. This study proposes quantification of the solid fraction, the single-component fraction and the viscoelasticity of decellularized pancreatic tissues using compact multifrequency magnetic resonance elastography (MRE) to assess the efficiency and quality of decellularization protocols. MRE of native and decellularized pancreatic tissues showed that viscoelasticity parameters depend according to a power law on the solid fraction of the decellularized matrix. The parameters can thus be used as highly sensitive markers of the mechanical integrity of soft tissues. Compact MRE allows consistent and noninvasive quantification of the viscoelastic properties of decellularized tissue. Such a method is urgently needed for the standardized monitoring of decellularization processes, evaluation of mechanical ECM properties, and quantification of the integrity of solid structural elements remaining in the decellularized tissue matrix.

Decellularization for the retention of tissue niches

Decellularization of natural tissues to produce extracellular matrix is a promising method for three-dimensional scaffolding and for understanding microenvironment of the tissue of interest. Due to the lack of a universal standard protocol for tissue decellularization, recent investigations seek to develop novel methods for whole or partial organ decellularization capable of supporting cell differentiation and implantation towards appropriate tissue regeneration. This review provides a comprehensive and updated perspective on the most recent advances in decellularization strategies for a variety of organs and tissues, highlighting techniques of chemical, physical, biological, enzymatic, or combinative-based methods to remove cellular contents from tissues. In addition, the review presents modernized approaches for improving standard decellularization protocols for numerous organ types.

Decellularized extracellular matrix scaffolds: Recent trends and emerging strategies in tissue engineering

The application of scaffolding materials is believed to hold enormous potential for tissue regeneration. Despite the widespread application and rapid advance of several tissue-engineered scaffolds such as natural and synthetic polymer-based scaffolds, they have limited repair capacity due to the difficulties in overcoming the immunogenicity, simulating in-vivo microenvironment, and performing mechanical or biochemical properties similar to native organs/tissues. Fortunately, the emergence of decellularized extracellular matrix (dECM) scaffolds provides an attractive way to overcome these hurdles, which mimic an optimal non-immune environment with native three-dimensional structures and various bioactive components. The consequent cell-seeded construct based on dECM scaffolds, especially stem cell-recellularized construct, is considered an ideal choice for regenerating functional organs/tissues. Herein, we review recent developments in dECM scaffolds and put forward perspectives accordingly, with particular focus on the concept and fabrication of decellularized scaffolds, as well as the application of decellularized scaffolds and their combinations with stem cells (recellularized scaffolds) in tissue engineering, including skin, bone, nerve, heart, along with lung, liver and kidney.

Perfusion-Based Recellularization of Rat Livers with Islets of Langerhans

Purpose

Artificial organs might serve as alternative solutions for whole organ transplantation. Decellularization of a liver provides a non-immunogenic matrix with the advantage of three afferent systems, the portal vein, the hepatic artery and the bile duct. This study aims to evaluate the recellularization of rat livers with islets of Langerhans via the bile duct and the portal vein for the comparison of different perfusion routes.

Methods

Rat livers were decellularized in a pressure-controlled perfusion manner and repopulated with intact isolated islets of Langerhans via either the portal vein or the bile duct.

Results

Repopulation via the portal vein showed islet clusters stuck within the vascular system demonstrated by ellipsoid borders of thick reticular tissue around the islet cluster in Azan staining. After recellularization via the bile duct, islets were distributed close to the vessels within the parenchymal space and without a surrounding reticular layer. Large clusters of islets had a diameter of up to 1000 µm without clear shapes.

Conclusion

We demonstrated the bile duct to be superior to the portal vein for repopulation of a decellularized rat liver with islets of Langerhans. This technique may serve as a bioengineering platform to generate an implantable and functional endocrine neo-pancreas and provide scaffolds with the anatomic benefit of three afferent systems to facilitate co-population of cells.

Design by Nature: Emerging Applications of Native Liver Extracellular Matrix for Cholangiocyte Organoid-Based Regenerative Medicine

Organoid technology holds great promise for regenerative medicine. Recent studies show feasibility for bile duct tissue repair in humans by successfully transplanting cholangiocyte organoids in liver grafts during perfusion. Large-scale expansion of cholangiocytes is essential for extending these regenerative medicine applications. Human cholangiocyte organoids have a high and stable proliferation capacity, making them an attractive source of cholangiocytes. Commercially available basement membrane extract (BME) is used to expand the organoids. BME allows the cells to self-organize into 3D structures and stimulates cell proliferation. However, the use of BME is limiting the clinical applications of the organoids. There is a need for alternative tissue-specific and clinically relevant culture substrates capable of supporting organoid proliferation. Hydrogels prepared from decellularized and solubilized native livers are an attractive alternative for BME. These hydrogels can be used for the culture and expansion of cholangiocyte organoids in a clinically relevant manner. Moreover, the liver-derived hydrogels retain tissue-specific aspects of the extracellular microenvironment. They are composed of a complex mixture of bioactive and biodegradable extracellular matrix (ECM) components and can support the growth of various hepatobiliary cells. In this review, we provide an overview of the clinical potential of native liver ECM-based hydrogels for applications with human cholangiocyte organoids. We discuss the current limitations of BME for the clinical applications of organoids and how native ECM hydrogels can potentially overcome these problems in an effort to unlock the full regenerative clinical potential of the organoids.

A new strategy for the decellularization of whole organs by hydrostatic pressure

Tissue engineering has been able to develop novel decellularization-recellularization techniques, which facilitates the research for the generation of functional organs. This is based in the initial obtention of the organ's extracellular matrix (ECM). Therefore, any improvement in the decellularization process would have a positive impact in the results of the recellularization process. Nevertheless, commonly the methods and equipment employed for this process are expensive and thus limit the access of this technique to various research groups globally.

AIM

To develop a decellularization technique with the exclusive use of hydrostatic pressure of detergent solutions, to have an easily accessible and low-cost technique that meets the basic requirements of acellularity and functionality of the ECM.

METHODS

This experimental study was performed in 10 male Wistar rats, obtaining the liver to carry out serial washes, with 1, 2 and 3% Triton X-100 solutions and 0.1% SDS. The washes were performed by using a Gravity Perfusion System (GPS), which assured us a continuous hydrostatic pressure of 7.5 mmHg. The obtained ECM was processed using stains and immunostaining to determine the residual cell content and preservation of its components.

RESULTS

The staining showed a removal of cellular and nuclear components of approximately 97% of the acellular ECM, with an adequate three-dimensional pattern of collagen and proteoglycans. Furthermore, the acellular ECM allowed the viability of a primary hepatocyte culture.

CONCLUSIONS

The use of the GPS decellularization technique allowed us to obtain an acellular and functional ECM, drastically reducing experimentation costs. This article is protected by copyright. All rights reserved.

Recent Advances in Liver Engineering With Decellularized Scaffold

Liver transplantation is currently the only effective treatment for patients with end-stage liver disease; however, donor liver scarcity is a notable concern. As a result, extensive endeavors have been made to diversify the source of donor livers. For example, the use of a decellularized scaffold in liver engineering has gained considerable attention in recent years. The decellularized scaffold preserves the original orchestral structure and bioactive chemicals of the liver, and has the potential to create a de novo liver that is fit for transplantation after recellularization. The structure of the liver and hepatic extracellular matrix, decellularization, recellularization, and recent developments are discussed in this review. Additionally, the criteria for assessment and major obstacles in using a decellularized scaffold are covered in detail.

Upgrading Hepatic Differentiation and Functions on 3D Printed Silk-Decellularized Liver Hybrid Scaffolds

We developed hybrid liver-specific three-dimensional (3D) printed scaffolds using a solubilized native decellularized liver (DCL) matrix and silk fibroin (SF) and investigated their ability to support functional cultures of hepatic cells. Rat livers were decellularized by perfusing detergents via the portal vein, solubilized using pepsin to form DCL, and characterized. SF blended with gelatin (8% w/v) was optimized with varying percentages of DCL to obtain silk gelatin-DCL bioink (SG-DCL). Different compositions of SG-DCL were studied by rheology for optimum versatility and print fidelity. 3D printed six-layered scaffolds were fabricated using a sophisticated direct-write 3D bioprinter. Huh7 cells were cultured on the 3D printed scaffolds for 3 weeks. 3D printed SG scaffolds without DCL along with 2D films (SG and SG-DCL) and 2D culture on tissue culture Petri dish control were used for comparative studies. The DCL matrix showed the absence of cells in histology and SEM. The combined SG-DCL ink at all of the studied DCL percentages (1-10%) revealed shear-thinning behavior in the printable range. The storage modulus value for the SG-DCL ink at all DCL percentages was higher than the loss modulus. In comparison to 2D controls, hepatic cells cultured on 3D SG-DCL revealed increased proliferation until 2 weeks and an upregulated expression of hepatocyte markers, including asialoglycoprotein receptor 1 (ASGR1). The Wnt pathway gene β-catenin was upregulated by more than 4-fold in 3D SG-DCL on day 3, while it showed a decline on day 7 as compared to 3D SG and also 2D controls. The expression of the epithelial cell adhesion molecule (EpCAM) was however lower in both 2D SG-DCL (2-fold) and 3D SG-DCL (2.5-fold) as compared to that in 2D controls. Immunofluorescence studies validated the protein expression of ASGR1 in 3D SG-DCL. Albumin (ALB) was not identified on SG scaffolds but prominently expressed in 3D SG-DCL constructs. In comparison to 2D SG, both ALB (1.8-fold) and urea (5-fold) were enhanced in cells cultured on 3D SG-DCL on day 7 of culture. Hence, the SG-DCL 3D printed scaffolds provide a conducive microenvironment for elevating differentiation and functions of hepatic cells possibly through an involvement of the Wnt/β-catenin signaling pathway.

Recent advances in optimization of liver decellularization procedures used for liver regeneration

Severe liver diseases have been considered the most common causes of adult deaths worldwide. Until now, liver transplantation is known as the only effective treatment for end stage liver disease. However, it is associated with several problems, most importantly, the side effects of immunosuppressive drugs that should be used after transplantation, and the shortage of tissue donors compared to the increasing number of patients requiring liver transplantation. Currently, tissue/organ decellularization as a new approach in tissue engineering is becoming a valid substitute for managing these kinds of problems. Decellularization of a whole liver is an attractive procedure to create three-dimensional (3D) scaffolds that micro-architecturally and structurally are similar to the native one and could support the repair or replacement of damaged or injured tissue. In this review, the different methods used for decellularization of liver tissue have been reviewed. In addition, the current approaches to overcome the challenges in these techniques are discussed.

Decellularized xenogeneic scaffolds in transplantation and tissue engineering: Immunogenicity versus positive cell stimulation

Seriously compromised function of some organs can only be restored by transplantation. Due to the shortage of human donors, the need to find another source of organs is of primary importance. Decellularized scaffolds of non-human origin are being studied as highly potential biomaterials for tissue engineering. Their biological nature and thus the ability to provide a naturally-derived environment for human cells to adhere and grow highlights their great advantage in comparison to synthetic scaffolds. Nevertheless, since every biomaterial implanted in the body generates immune reaction, studying the interaction of the scaffold with the surrounding tissues is necessary. This review aims to summarize current knowledge on the immunogenicity of semi-xenografts involved in transplantation. Moreover, positive aspects of the interaction between xenogeneic scaffold and human cells are discussed, focusing on specific roles of proteins associated with extracellular matrix in cell adhesion and signalling.

Identification of tissue sections from decellularized liver scaffolds for repopulation experiments

BACKGROUND

Biological organ engineering is a novel experimental approach to generate functional liver grafts by decellularization and repopulation. Currently, healthy organs of small or large animals and human organs with preexisting liver diseases are used to optimize decellularization and repopulation.However, the effects of morphological changes on allo- and xenogeneic cell-scaffold interactions during repopulation procedure, e.g., using scaffold-sections, are unknown. We present a sequential morphological workflow to identify murine liver scaffold-sections with well-preserved microarchitecture.

METHODS

Native livers (CONT, n = 9) and livers with experimentally induced pathologies (hepatics steatosis: STEA, n = 7; hepatic fibrosis induced by bile duct ligation: BDL, n = 9; nodular regenerative hyperplasia induced by 90% partial hepatectomy: PH, n = 8) were decellularized using SDS and Triton X-100 to generate cell-free scaffolds. Scaffold-sections were assessed using a sequential morphological workflow consisting of macroscopic, microscopic and morphological evaluation: (1) The scaffold was evaluated by a macroscopic decellularization score. (2) Regions without visible tissue remnants were localized for sampling and histological processing. Subsequent microscopical examination served to identify tissue samples without cell remnants. (3) Only cell-free tissue sections were subjected to detailed liver-specific morphological assessment using a histological and immunohistochemical decellularization score.

RESULTS

Decellularization was feasible in 33 livers, which were subjected to the sequential morphological workflow. In 11 of 33 scaffolds we achieved a good macroscopic decellularization result (CONT: 3 scaffolds; STEA: 3 scaffolds; BDL: 3 scaffolds; PH: 2 scaffolds). The microscopic assessment resulted in the selection of 88 cell-free tissue sections (CONT: 15 sections; STEA: 38 sections; BDL: 30 sections; PH: 5 sections). In 27 of those sections we obtained a good histological decellularization result (CONT: 3 sections; STEA: 6 sections; BDL: 17 sections; PH: 1 section). All experimental groups contained sections with a good immunohistochemical decellularization result (CONT: 6 sections; STEA: 5 sections; BDL: 4 sections; PH: 1 section).

DISCUSSION

Decellularization was possible in all experimental groups, irrespectively of the underlying morphological alteration. Furthermore, our proposed sequential morphological workflow was suitable to detect tissue sections with well-preserved hepatic microarchitecture, as needed for further repopulation experiments.

Decellularization systems and devices: State-of-the-art

Extracellular matrix (ECM) is a natural biomaterial scaffold that provides biochemical and structural support to its surrounding cells, forming tissue and respective organs. These ECM proteins can be extracted from organs and tissues through decellularization, which is the process of removing cellular content and nuclear material from the organs to obtain decellularized ECM (dECM). dECM is a versatile and functional biomaterial that can be used as the base component of bioinks for rebuilding tissue and organs. Intact dECM of whole organs can be used as a scaffold for recellularization with human stem cells to produce a functioning organ. As decellularization is a relatively new lab process, the associated technologies and devices are largely non-standardized and only available in small, lab-specific scales. Additionally, there is a lack of standardized protocols to analyze the quality and consistency of harvested dECM for medical applications. This review discusses the relevant decellularization systems and devices currently available to facilitate further development of this process for larger scales with the intention to commercialize dECM materials. STATEMENT OF SIGNIFICANCE: Extracellular matrix (ECM) is a natural cocktail of biomaterials that provides biochemical and structural support to its surrounding cells. ECM proteins are extracted from organs and tissues through decellularization. Being a versatile and functional biomaterial, decellularized extracellular matrix (dECM) is being used as base component of bioinks/hydrogels for rebuilding of tissue and organ constructs. Decellularization is a relatively new lab process with associated technologies/devices being largely non-standardized and only available in lab-specific scales. We discuss categories of decellularization systems and devices for the first time being used in academic and commercial settings. We highlight inherent challenges with the current systems and suggest possible solutions. We comment on further development of these processes for large-scale and commercial applications of dECM.

Development and Characterization of a Porcine Liver Scaffold

The growing number of patients requiring liver transplantation for chronic liver disease cannot be currently met due to a shortage in donor tissue. As such, alternative tissue engineering approaches combining the use of acellular biological scaffolds and different cell populations (hepatic or progenitor) are being explored to augment the demand for functional organs. Our goal was to produce a clinically relevant sized scaffold from a sustainable source within 24 h, while preserving the extracellular matrix (ECM) to facilitate cell repopulation at a later stage. Whole porcine livers underwent perfusion decellularization via the hepatic artery and hepatic portal vein using a combination of saponin, sodium deoxycholate, and deionized water washes resulting in an acellular scaffold with an intact vasculature and preserved ECM. Molecular and immunohistochemical analysis (collagen I and IV and laminin) showed complete removal of any DNA material, together with excellent retention of glycosaminoglycans and collagen. Fourier-transform infrared spectroscopy (FTIR) analysis showed both absence of nuclear material and removal of any detergent residue, which was successfully achieved after additional ethanol gradient washes. Samples of the decellularized scaffold were assessed for cytotoxicity by seeding with porcine adipose-derived mesenchymal stem cells in vitro, these cells over a 10-day period showed attachment and proliferation. Perfusion of the vascular tree with contrast media followed by computed tomography (CT) imaging showed an intact vascular network. In vivo implantation of whole intact nonseeded livers, into a porcine model (as auxiliary graft) showed uniform perfusion macroscopically and histologically. Using this method, it is possible to create an acellular, clinically sized, liver scaffold with intact vasculature in less than 24 h.

A Survival Model of In Vivo Partial Liver Lobe Decellularization Towards In Vivo Liver Engineering

In vivo liver decellularization has become a promising strategy to study in vivo liver engineering. However, long-term survival after in vivo liver decellularization has not yet been achieved due to anatomical and technical challenges. This study aimed at establishing a survival model of in vivo partial liver lobe perfusion-decellularization in rats. We compared three decellularization protocols (1% Triton X100 followed by 1% sodium dodecyl sulfate [SDS], 1% SDS vs. 1% Triton X100, n = 6/group). Using the optimal one as judged by macroscopy, histology and DNA content, we characterized the structural integrity and matrix proteins by using histology, scanning electron microscopy, computed tomography scanning, and immunohistochemistry (IHC). We prevented contamination of the abdominal cavity with the corrosive detergents by using polyvinylidene chloride (PVDC) film + dry gauze in comparison to PVDC film + dry gauze + aspiration tube (n = 6/group). Physiological reperfusion was assessed by histology. Survival rate was determined after a 7-day observation period. Only perfusion with 1% SDS resulted in an acellular scaffold (fully translucent without histologically detectable tissue remnants, DNA concentration is <2% of that in native lobe) with remarkable structural and ultrastructural integrity as well as preservation of main matrix proteins (IHC positive for collagen IV, laminin, and elastin). Contamination of abdominal organs with the potentially toxic SDS solution was achieved by placing a suction tube in addition to the PVDC film + dry gauze and allowed a 7-day survival of all animals without severe postoperative complications. On reperfusion, the liver turned red within seconds without any leakage from the surface of the liver. About 12 h after reperfusion, not only blood cells but also some clots were visible in the portal vein, sinusoidal matrix network, and central vein, suggesting physiological perfusion. In conclusion, our results of this study show the first available data on generation of a survival model of in vivo parenchymal organ decellularization, creating a critical step toward in vivo organ engineering. Impact Statement Recently, in vivo liver decellularization has been considered a promising approach to study in vivo liver repopulation of a scaffold compared with ex vivo liver repopulation. However, long-term survival of in vivo liver decellularization has not yet been achieved. Here, despite anatomical and technical challenges, we successfully created a survival model of in vivo selected liver lobe decellularization in rats, providing a major step toward in vivo organ engineering.

A Hepatic Scaffold from Decellularized Liver Tissue: Food for Thought

Allogeneic liver transplantation is still deemed the gold standard solution for end-stage organ failure; however, donor organ shortages have led to extended waiting lists for organ transplants. In order to overcome the lack of donors, the development of new therapeutic options is mandatory. In the last several years, organ bioengineering has been extensively explored to provide transplantable tissues or whole organs with the final goal of creating a three-dimensional growth microenvironment mimicking the native structure. It has been frequently reported that an extracellular matrix-based scaffold offers a structural support and important biological molecules that could help cellular proliferation during the recellularization process. The aim of the present review is to underline the recent developments in cell-on-scaffold technology for liver bioengineering, taking into account: (1) biological and synthetic scaffolds; (2) animal and human tissue decellularization; (3) scaffold recellularization; (4) 3D bioprinting; and (5) organoid technology. Future possible clinical applications in regenerative medicine for liver tissue engineering and for drug testing were underlined and dissected.

Current progress in hepatic tissue regeneration by tissue engineering

Liver, as a vital organ, is responsible for a wide range of biological functions to maintain homeostasis and any type of damages to hepatic tissue contributes to disease progression and death. Viral infection, trauma, carcinoma, alcohol misuse and inborn errors of metabolism are common causes of liver diseases are a severe known reason for leading to end-stage liver disease or liver failure. In either way, liver transplantation is the only treatment option which is, however, hampered by the increasing scarcity of organ donor. Over the past years, considerable efforts have been directed toward liver regeneration aiming at developing new approaches and methodologies to enhance the transplantation process. These approaches include producing decellularized scaffolds from the liver organ, 3D bio-printing system, and nano-based 3D scaffolds to simulate the native liver microenvironment. The application of small molecules and micro-RNAs and genetic manipulation in favor of hepatic differentiation of distinct stem cells could also be exploited. All of these strategies will help to facilitate the application of stem cells in human medicine. This article reviews the most recent strategies to generate a high amount of mature hepatocyte-like cells and updates current knowledge on liver regenerative medicine.

Tissue and organ decellularization in regenerative medicine

The advancement and improvement in decellularization methods can be attributed to the increasing demand for tissues and organs for transplantation. Decellularized tissues and organs, which are free of cells and genetic materials while retaining the complex ultrastructure of the extracellular matrix (ECM), can serve as scaffolds to subsequently embed cells for transplantation. They have the potential to mimic the native physiology of the targeted anatomic site. ECM from different tissues and organs harvested from various sources have been applied. Many techniques are currently involved in the decellularization process, which come along with their own advantages and disadvantages. This review focuses on recent developments in decellularization methods, the importance and nature of detergents used for decellularization, as well as on the role of the ECM either as merely a physical support or as a scaffold in retaining and providing cues for cell survival, differentiation and homeostasis. In addition, application, status, and perspectives on commercialization of bioproducts derived from decellularized tissues and organs are addressed. © 2018 American Institute of Chemical Engineers Biotechnol. Prog., 34:1494-1505, 2018.

Three-dimensional liver-derived extracellular matrix hydrogel promotes liver organoids function

An important advantage of employing extracellular matrix (ECM)-derived biomaterials in tissue engineering is the ability to tailor the biochemical and biophysical microenvironment of the cells. This study aims to assess whether three-dimensional (3D) liver-derived ECM hydrogel (LEMgel) promotes physiological function of liver organoids generated by self-organization of human hepatocarcinoma cells together with human mesenchymal and endothelial cells. We have optimized the decellularization method to fabricate liver ECM derived from sheep to preserve the greatest content of glycosaminoglycans, collagen, laminin, and fibronectin in produced LEMgel. During gelation, complex viscoelasticity modulus of the LEMgel (3 mg/mL) increased from 186.7 to 1570.5 Pa and Tan Delta decreased from 0.27 to 0.18. Scanning electron microscopy (SEM) determined that the LEMgel had a pore size of 382 ± 71 µm. Hepatocarcinoma cells in the self-organized liver organoids in 3D LEMgel (LEMgel organoids) showed an epithelial phenotype and expressed ALB, CYP3A4, E-cadherin, and ASGPR. The LEMgel organoid had significant upregulation of transcripts of ALB, CYP3A4, CYP3A7, and TAT as well as downregulation of AFP compared to collagen type I- and hydrogel-free-organoids or organoids in solubilized LEM and 2D culture of hepatocarcinoma cells. Generated 3D LEMgel organoids had significantly more ALB and AAT secretion, urea production, CYP3A4 enzyme activity, and inducibility. In conclusion, 3D LEMgel enhanced the functional activity of self-organized liver organoids compared to traditional 2D, 3D, and collagen gel cultures. Our novel 3D LEMgel organoid could potentially be used in liver tissue engineering, drug discovery, toxicology studies, or bio-artificial liver fabrication.

[Liver engineering as a new source of donor organs : A systematic review]

BACKGROUND

Organ engineering is a new strategy to cope with the shortage of donor organs. A functional scaffold from explanted organs is prepared by removing all cellular components (decellularization) and the reseeding (repopulation) of the organ scaffold to generate a functional organ in vitro for transplantation. This technique was also applied to the liver (liver engineering).

OBJECTIVES

Outline of the current state of the art and resulting approaches for future research strategies.

MATERIAL AND METHODS

Systematic review according to the PRISMA guidelines: a PubMed-based literature search (search terms liver, decellularization), selection of relevant articles based on predetermined criteria for relevance (e.g. decellularization, repopulation and transplantation), extraction and critical appraisal of data and results concerning the conditions for decellularization, repopulation and transplantation.

RESULTS

Decellularization was successfully performed in small and large animal models. Hepatocytes as well as stem cells and hepatic cell lines were applied for repopulation and 7 publications could show the successful transplantation of acellular and repopulated organ scaffolds. The current scientific need for further studies concerning the source of donor organs, optimization of the decellularization process, the cell type for the reseeding process and the establishment of the optimal conditions for the repopulation of the scaffold is still tremendous. For successful recellularization of the liver three goals need to be achieved: (1) reseeding of the organ scaffold with a sufficient amount of parenchymal cells, (2) endothelialization of the vascular tree to ensure the supply of oxygen and nutrients to parenchymal cells and (3) an appropriate epithelialization of the biliary tree. In order to progress to clinical trials a suitable transplantation model to verify the function of the organ constructs must be established.

CONCLUSION

Liver engineering using biological cell-free organ scaffolds represents a scientific and ethical challenge. The existing results emphasize the potential of this new and promising strategy to create organs for transplantation in the future.

Effect of pulsatile flow perfusion on decellularization

Background

Decellularized animal organs have been used as scaffolds for tissue engineering. To make a properly functioning scaffolds, the extracellular matrix (ECM) components must be preserved after decellularization. Because pulsatile flow is known to be beneficial for tissue perfusion, pulsatile perfusion of a detergent might decrease the exposure time of the tissues to the detergent used for decellularization. Using Energy Equivalent Pressure (EEP) as a pulsatility parameter, the effect of pulsatile flow in decellularization process is studied.

Results

Twelve rat hearts were decellularization with 1% sodium dodecyl sulfate (SDS) solution for 2 h. They are divided into two groups, one with pulsatile perfusion (n = 6), the other with non-pulsatile perfusion (n = 6) of SDS. The initial mean perfusion pressures were same in both group. The result indicated that the EEP and the perfusion flow were increased significantly in the pulsatile group compared to the non-pulsatile group. Photographs taken during the decellularization showed more profound decellularization in the pulsatile group. The residual DNA content in the scaffolds was significantly lower in the pulsatile group. However, the level of ECM components, collagen and GAG showed no significant differences between the groups.

Conclusions

Decellularization is more efficient in pulsatile flow than in non-pulsatile flow but still preserves the ECM molecules.

Recent Advances in Decellularization and Recellularization for Tissue-Engineered Liver Grafts

Liver transplantation from deceased or living human donors remains the only proven option for patients with end-stage liver disease. However, the shortage of donor organs is a significant clinical concern that has led to the pursuit of tissue-engineered liver grafts generated from decellularized liver extracellular matrix and functional cells. Investigative efforts on optimizing both liver decellularization and recellularization protocols have been made in recent decades. In the current review, we briefly summarize these advances, including the generation of high-quality liver extracellular matrix scaffolds, evaluation criteria for quality control, modification of matrix for enhanced properties, and reseeding strategies. These efforts to optimize the methods of decellularization and recellularization lay the groundwork towards generating a transplantable, human-sized liver graft for the treatment of patients with severe liver disease.

Decellularization of Whole Human Liver Grafts Using Controlled Perfusion for Transplantable Organ Bioscaffolds

Liver transplantation is the only effective treatment for end-stage liver disease, but absolute donor shortage remains a limiting factor. Recent advances in tissue engineering focus on generation of native extracellular matrix (ECM) by decellularized complete livers in animal models. Although proof of concept has been reported for human livers, this study aims to perform whole liver decellularization in a clinically relevant series using controlled machine perfusion. In this study, we describe a mild nondestructive decellularization protocol, effective in 11 discarded human whole liver grafts to generate constructs that reliably maintain hepatic architecture and ECM components using machine perfusion, while completely removing cellular DNA and RNA. The decellularization process preserved the ultrastructural ECM components confirmed by histology, electron microscopy, and proteomic analysis. Anatomical characteristics of the native microvascular network and biliary drainage of the liver were confirmed by contrast computed tomography scanning. Decellularized vascular matrix remained suitable for normal suturing and no major histocompatibility complex molecules were detected, suggesting absence of allo-reactivity when used for transplantation. After extensive washing, decellularized scaffolds were nontoxic for cells after reseeding human mesenchymal stromal or umbilical vein endothelial endothelium cells. Indeed, evidence of effective recellularization of the vascular lining was obtained. In conclusion, we established an effective method to generate clinically applicable liver scaffolds from human discarded whole liver grafts and show proof of concept that reseeding of normal human cells in the scaffold is feasible. This supports new opportunities for bioengineering of transplantable grafts in the future.

Engineering an endocrine Neo-Pancreas by repopulation of a decellularized rat pancreas with islets of Langerhans

Decellularization of pancreata and repopulation of these non-immunogenic matrices with islets and endothelial cells could provide transplantable, endocrine Neo- Pancreata. In this study, rat pancreata were perfusion decellularized and repopulated with intact islets, comparing three perfusion routes (Artery, Portal Vein, Pancreatic Duct). Decellularization effectively removed all cellular components but conserved the pancreas specific extracellular matrix. Digital subtraction angiography of the matrices showed a conserved integrity of the decellularized vascular system but a contrast emersion into the parenchyma via the decellularized pancreatic duct. Islets infused via the pancreatic duct leaked from the ductular system into the peri-ductular decellularized space despite their magnitude. TUNEL staining and Glucose stimulated insulin secretion revealed that islets were viable and functional after the process. We present the first available protocol for perfusion decellularization of rat pancreata via three different perfusion routes. Furthermore, we provide first proof-of-concept for the repopulation of the decellularized rat pancreata with functional islets of Langerhans. The presented technique can serve as a bioengineering platform to generate implantable and functional endocrine Neo-Pancreata.

Bioengineered Livers: A New Tool for Drug Testing and a Promising Solution to Meet the Growing Demand for Donor Organs

BACKGROUND

Organ engineering is a new innovative strategy to cope with two problems: the need for physiological models for pharmacological research and donor organs for transplantation. A functional scaffold is generated from explanted organs by removing all cells (decellularization) by perfusing the organ with ionic or nonionic detergents via the vascular system. Subsequently the acellular scaffold is reseeded with organ-specific cells (repopulation) to generate a functional organ.

SUMMARY

This review gives an overview of the state of the art describing the decellularization process, the subsequent quality assessment, the repopulation techniques and the functional assessment. It emphasizes the use of scaffolds as matrix for culturing human liver cells for drug testing. Further, it highlights the techniques for transplanting these engineered scaffolds in allogeneic or xenogeneic animals in order to test their biocompatibility and use as organ grafts. Key Messages: The first issue is the so-called decellularization, which is best explored and resulted in a multitude of different protocols. The most promising approach seems to be the combination of pulsatile perfusion of the liver with Triton X-100 and SDS via hepatic artery and portal vein. Widely accepted parameters of quality control include the quantitative assessment of the DNA content and the visualization of eventually remaining nuclei confirmed by HE staining. Investigations regarding the composition of the extracellular matrix focused on histological determination of laminin, collagen, fibronectin and elastin and remained qualitatively. Repopulation is the second issue which is addressed. Selection of the most suitable cell type is a highly controversial topic. Currently, the highest potential is seen for progenitor and stem cells. Cells are infused into the scaffold and cultured under static conditions or in a bioreactor allowing dynamic perfusion of the scaffold. The quality of repopulation is mainly assessed by routine histology and basic functional assays. These promising results prompted to consider the use of a liver scaffold repopulated with human cells for pharmacological research. Transplantation of the (repopulated) scaffold is the third topic which is not yet widely addressed. Few studies report the heterotopic transplantation of repopulated liver tissue without vascular anastomosis. Even fewer studies deal with the heterotopic transplantation of a scaffold or a repopulated liver lobe. However, observation time was still limited to hours, and long-term graft survival has not been reported yet. These exciting results emphasize the potential of this new and promising strategy to create physiological models for pharmacological research and to generate liver grafts for the transplant community to treat organ failure. However, the scientific need for further development in the field of liver engineering is still tremendous.

Arterial Decellularized Scaffolds Produced Using an Innovative Automatic System

There is still an unmet clinical need for small-caliber artery substitution. Decellularized scaffolds in tissue engineering represent a promising solution. We have developed an innovative system for the automatic decellularization of blood vessels, used to process pig arteries. The system is able to automatically drive a decellularization process in a safe and reliable environment, with complex time patterns, using up to three different decellularization solutions, and providing at the same time a physical stress to improve the decellularization. The decellularization of pig arteries was evaluated by means of histology, DNA quantification and mechanical testing. Outcomes showed scaffolds with no cellular or nuclear remnants and a well-preserved tissue structure, corroborated by mechanical properties similar to native tissue. Decellularized scaffolds were seeded on the inner layer with human endothelial cells and implanted as iliac artery replacement in 4 pharmacologically immune-compromised pigs. This chimeric model was performed as a very preliminary evaluation to investigate the performances of these scaffolds in vivo, and to investigate the fate of seeded cells. Recipients were sacrificed on day 14 and day 70 after surgery, and vessels were found to be patent and with no evidence of thrombi formation. The inner layer was covered by endothelial cells, and the migration of cells positive for α-smooth-muscle actin was observed from the outer layer towards the tunica media. Intriguingly, the endothelial cells on explanted vessels were entirely derived from the host while the seeded cells were lost. In conclusion, this work presents a novel tool for a safe and controlled production of arterial scaffolds, with good decellularization outcomes and a good performance in a short-term, large-animal implantation.

Procedure for Decellularization of Rat Livers in an Oscillating-pressure Perfusion Device

Decellularization and recellularization of parenchymal organs may enable the generation of functional organs in vitro, and several protocols for rodent liver decellularization have already been published. We aimed to improve the decellularization process by construction of a proprietary perfusion device enabling selective perfusion via the portal vein and/or the hepatic artery. Furthermore, we sought to perform perfusion under oscillating surrounding pressure conditions to improve the homogeneity of decellularization. The homogeneity of perfusion decellularization has been an underestimated factor to date. During decellularization, areas within the organ that are poorly perfused may still contain cells, whereas the extracellular matrix (ECM) in well-perfused areas may already be affected by alkaline detergents. Oscillating pressure changes can mimic the intraabdominal pressure changes that occur during respiration to optimize microperfusion inside the liver. In the study presented here, decellularized rat liver matrices were analyzed by histological staining, DNA content analysis and corrosion casting. Perfusion via the hepatic artery showed more homogenous results than portal venous perfusion did. The application of oscillating pressure conditions improved the effectiveness of perfusion decellularization. Livers perfused via the hepatic artery and under oscillating pressure conditions showed the best results. The presented techniques for liver harvesting, cannulation and perfusion using our proprietary device enable sophisticated perfusion set-ups to improve decellularization and recellularization experiments in rat livers.

Pressure- and flow-controlled media perfusion differently modify vascular mechanics in lung decellularization

Organ biofabrication is a potential future alternative for obtaining viable organs for transplantation. Achieving intact scaffolds to be recellularized is a key step in lung bioengineering. Perfusion of decellularizing media through the pulmonary artery has shown to be effective. How vascular perfusion pressure and flow vary throughout lung decellularization, which is not well known, is important for optimizing the process (minimizing time) while ensuring scaffold integrity (no barotrauma). This work was aimed at characterizing the pressure/flow relationship at the pulmonary vasculature and at how effective vascular resistance depends on pressure- and flow-controlled variables when applying different methods of media perfusion for lung decellularization. Lungs from 43 healthy mice (C57BL/6; 7-8 weeks old) were investigated. After excision and tracheal cannulation, lungs were inflated at 10 cmH2O airway pressure and subjected to conventional decellularization with a solution of 1% sodium dodecyl sulfate (SDS). Pressure (PPA) and flow (V'PA) at the pulmonary artery were continuously measured. Decellularization media was perfused through the pulmonary artery: (a) at constant PPA=20 cmH2O or (b) at constant V'PA=0.5 and 0.2 ml/min. Effective vascular resistance was computed as Rv=PPA/V'PA. Rv (in cmH2O/(ml/min)); mean±SE) considerably varied throughout lung decellularization, particularly for pressure-controlled perfusion (from 29.1±3.0 in baseline to a maximum of 664.1±164.3 (p<0.05), as compared with flow-controlled perfusion (from 49.9±3.3 and 79.5±5.1 in baseline to a maximum of 114.4±13.9 and 211.7±70.5 (p<0.05, both), for V'PA of 0.5 and 0.2 ml/min respectively. Most of the media infused to the pulmonary artery throughout decellularization circulated to the airways compartment across the alveolar-capillary membrane. This study shows that monitoring perfusion mechanics throughout decellularization provides information relevant for optimizing the process time while ensuring that vascular pressure is kept within a safety range to preserve the organ scaffold integrity.