Decellularization and Recellularization of Rat Livers With Hepatocytes and Endothelial Progenitor Cells

Decellularized Liver Matrices for Expanding the Donor Pool-An Evaluation of Existing Protocols and Future Trends

Liver transplantation is the only curative option for end-stage liver disease and is necessary for an increasing number of patients with advanced primary or secondary liver cancer. Many patient groups can benefit from this treatment, however the shortage of liver grafts remains an unsolved problem. Liver bioengineering offers a promising method for expanding the donor pool through the production of acellular scaffolds that can be seeded with recipient cells. Decellularization protocols involve the removal of cells using various chemical, physical, and enzymatic steps to create a collagenous network that provides support for introduced cells and future vascular and biliary beds. However, the removal of the cells causes varying degrees of matrix damage, that can affect cell seeding and future organ performance. The main objective of this review is to present the existing techniques of producing decellularized livers, with an emphasis on the assessment and definition of acellularity. Decellularization agents are discussed, and the standard process of acellular matrix production is evaluated. We also introduce the concept of the stepwise assessment of the matrix during decellularization through decellularization cycles. This method may lead to shorter detergent exposure times and less scaffold damage. The introduction of apoptosis induction in the field of organ engineering may provide a valuable alternative to existing long perfusion protocols, which lead to significant matrix damage. A thorough understanding of the decellularization process and the action of the various factors influencing the final composition of the scaffold is essential to produce a biocompatible matrix, which can be the basis for further studies regarding recellularization and retransplantation.

Exploiting the Potential of Decellularized Extracellular Matrix (ECM) in Tissue Engineering: A Review Study

While significant progress has been made in creating polymeric structures for tissue engineering, the therapeutic application of these scaffolds remains challenging owing to the intricate nature of replicating the conditions of native organs and tissues. The use of human-derived biomaterials for therapeutic purposes closely imitates the properties of natural tissue, thereby assisting in tissue regeneration. Decellularized extracellular matrix (dECM) scaffolds derived from natural tissues have become popular because of their unique biomimetic properties. These dECM scaffolds can enhance the body's ability to heal itself or be used to generate new tissues for restoration, expanding beyond traditional tissue transfers and transplants. Enhanced knowledge of how ECM scaffold materials affect the microenvironment at the injury site is expected to improve clinical outcomes. In this review, recent advancements in dECM scaffolds are explored and relevant perspectives are offered, highlighting the development and application of these scaffolds in tissue engineering for various organs, such as the skin, nerve, bone, heart, liver, lung, and kidney.

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.

Two Decades of Advances and Limitations in Organ Recellularization

The recellularization of tissues after decellularization is a relatively new technology in the field of tissue engineering (TE). Decellularization involves removing cells from a tissue or organ, leaving only the extracellular matrix (ECM). This can then be recellularized with new cells to create functional tissues or organs. The first significant mention of recellularization in decellularized tissues can be traced to research conducted in the early 2000s. One of the landmark studies in this field was published in 2008 by Ott, where researchers demonstrated the recellularization of a decellularized rat heart with cardiac cells, resulting in a functional organ capable of contraction. Since then, other important studies have been published. These studies paved the way for the widespread application of recellularization in TE, demonstrating the potential of decellularized ECM to serve as a scaffold for regenerating functional tissues. Thus, although the concept of recellularization was initially explored in previous decades, these studies from the 2000s marked a major turning point in the development and practical application of the technology for the recellularization of decellularized tissues. The article reviews the historical advances and limitations in organ recellularization in TE over the last two decades.

Decellularized skin pretreatment by monophosphoryl lipid A and lactobacillus casei supernatant accelerate skin recellularization

BACKGROUND

Bioscaffolds and cells are two main components in the regeneration of damaged tissues via cell therapy. Umbilical cord stem cells are among the most well-known cell types for this purpose. The main objective of the present study was to evaluate the effect of the pretreatment of the foreskin acellular matrix (FAM) by monophosphoryl lipid A (MPLA) and Lactobacillus casei supernatant (LCS) on the attraction of human umbilical cord mesenchymal stem cells (hucMSC).

METHODS AND RESULTS

The expression of certain cell migration genes was studied using qRT-PCR. In addition to cell migration, transdifferentiation of these cells to the epidermal-like cells was evaluated via immunohistochemistry (IHC) and immunocytochemistry (ICC) of cytokeratin 19 (CK19). The hucMSC showed more tissue tropism in the presence of MPLA and LCS pretreated FAM compared to the untreated control group. We confirmed this result by scanning electron microscopy (SEM) analysis, glycosaminoglycan (GAG), collagen, and DNA content. Furthermore, IHC and ICC data demonstrated that both treatments increase the protein expression level of CK19.

CONCLUSION

Pretreatment of acellular bioscaffolds by MPLA or LCS can increase the migration rate of cells and also transdifferentiation of hucMSC to epidermal-like cells without growth factors. This strategy suggests a new approach in regenerative medicine.

A decellularized matrix enriched collagen microscaffold for a 3D in vitro liver model

The development of liver scaffolds retaining their three-dimensional (3D) structure and extra-cellular matrix (ECM) composition is essential for the advancement of liver tissue engineering. We report the design and validation of an alginate-based platform using a combination of decellularized matrices and collagen to preserve the functionality of liver cells. The scaffolds were characterized using SEM and fluorescence microscopy techniques. The proliferation and functional behaviours of hepatocellular carcinoma HuH7 cells were observed. It was found that the decellularized skin scaffold with collagen was better for maintaining the growth of cells in comparison to other decellularized matrices. In addition, we observed a significant increase in the functional profile once exogenous collagen was added to the liver matrix. Our study also suggests that a cirrhotic liver model should have a different matrix composition as compared to a healthy liver model. When primary rat hepatocytes were used for developing a healthy liver model, the proliferation studies with hepatocytes showed a decellularized skin matrix as the better option, but the functionality was only maintained in a decellularized liver matrix with addition of exogenous collagen. We further checked if these platforms can be used for studying drug induced toxicity observed in the liver by studying the activation of cytochrome P450 upon drug exposure of the cells growing in our model. We observed a significant induction of the CYP1A1 gene on administering the drugs for 6 days. Thus, this platform could be used for drug-toxicity screening studies using primary hepatocytes in a short span of time. Being a microscaffold based system, this platform offers some advantages, such as smaller volumes of samples, analysing multiple samples simultaneously and a minimal amount of decellularized matrix in the matrix composition, making it an economical option compared to a completely dECM based platform.

Vascular reconstruction of the decellularized biomatrix for whole-organ engineering-a critical perspective and future strategies

Whole-organ re-engineering is the most challenging goal yet to be achieved in tissue engineering and regenerative medicine. One essential factor in any transplantable and functional tissue engineering is fabricating a perfusable vascular network with macro- and micro-sized blood vessels. Whole-organ development has become more practical with the use of the decellularized organ biomatrix (DOB) as it provides a native biochemical and structural framework for a particular organ. However, reconstructing vasculature and re-endothelialization in the DOB is a highly challenging task and has not been achieved for constructing a clinically transplantable vascularized organ with an efficient perfusable capability. Here, we critically and articulately emphasized factors that have been studied for the vascular reconstruction in the DOB. Furthermore, we highlighted the factors used for vasculature development studies in general and their application in whole-organ vascular reconstruction. We also analyzed in detail the strategies explored so far for vascular reconstruction and angiogenesis in the DOB for functional and perfusable vasculature development. Finally, we discussed some of the crucial factors that have been largely ignored in the vascular reconstruction of the DOB and the future directions that should be addressed systematically.

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.

Angiogenesis and Re-endothelialization in decellularized scaffolds: Recent advances and current challenges in tissue engineering

Decellularization of tissues and organs has recently become a promising approach in tissue engineering and regenerative medicine to circumvent the challenges of organ donation and complications of transplantations. However, one main obstacle to reaching this goal is acellular vasculature angiogenesis and endothelialization. Achieving an intact and functional vascular structure as a vital pathway for supplying oxygen and nutrients remains the decisive challenge in the decellularization/re-endothelialization procedure. In order to better understand and overcome this issue, complete and appropriate knowledge of endothelialization and its determining variables is required. Decellularization methods and their effectiveness, biological and mechanical characteristics of acellular scaffolds, artificial and biological bioreactors, and their possible applications, extracellular matrix surface modification, and different types of utilized cells are factors affecting endothelialization consequences. This review focuses on the characteristics of endothelialization and how to optimize them, as well as discussing recent developments in the process of re-endothelialization.

Advances in Recellularization of Decellularized Liver Grafts with Different Liver (Stem) Cells: Towards Clinical Applications

Liver transplantation is currently the only curative therapy for patients with acute or chronic liver failure. However, a dramatic gap between the number of available liver grafts and the number of patients on the transplantation waiting list emphasizes the need for valid liver substitutes. Whole-organ engineering is an emerging field of tissue engineering and regenerative medicine. It aims to generate transplantable and functional organs to support patients on transplantation waiting lists until a graft becomes available. It comprises two base technologies developed in the last decade; (1) organ decellularization to generate a three-dimensional (3D) extracellular matrix scaffold of an organ, and (2) scaffold recellularization to repopulate both the parenchymal and vascular compartments of a decellularized organ. In this review article, recent advancements in both technologies, in relation to liver whole-organ engineering, are presented. We address the potential sources of hepatocytes and non-parenchymal liver cells for repopulation studies, and the role of stem-cell-derived liver progeny is discussed. In addition, different cell seeding strategies, possible graft modifications, and methods used to evaluate the functionality of recellularized liver grafts are outlined. Based on the knowledge gathered from recent transplantation studies, future directions are summarized.

Analysis of decellularized mouse liver fragment and its recellularization with human endometrial mesenchymal cells as a candidate for clinical usage

Decellularized tissue has been used as a natural extracellular matrix (ECM) or bioactive biomaterial for tissue engineering. The present study aims to compare and analyze different decellularization protocols for mouse liver fragments and cell seeding and attachment in the created scaffold using human endometrial mesenchymal cells (hEMCs).After collecting and dissecting the mouse liver into small fragments, they were decellularized by Triton X-100 and six concentrations of sodium dodecyl sulfate (SDS; 0.025, 0.05, 0.1, 0.25, 0.5, and 1%) at different exposure times. The morphology and DNA content of decellularized tissues were studied, and the group with better morphology and lower DNA content was selected for additional assessments. Masson's tri-chrome and periodic acid Schiff staining were performed to evaluate ECM materials. Raman confocal spectroscopy analysis was used to quantify the amount of collagen type I, III and IV, glycosaminoglycans and elastin. Scanning electron microscopy and MTT assay were applied to assess the ultrastructure and porosity and cytotoxicity of decellularized scaffolds, respectively. In the final step, hEMCs were seeded on the decellularized scaffold and cultured for one week, and finally the cell attachment and homing were studied morphologically.The treated group with 0.1% SDS for 24 h showed a well preserved ECM morphology similar to native control and showing the minimum level of DNA. Raman spectroscopy results demonstrated that the amount of collagen type I and IV was not significantly changed in this group compared to the control, but a significant reduction in collagen III and elastin protein levels was seen (P < 0.001). The micrographs showed a porous ECM in decellularized sample similar to the native control with the range of 2.25 µm to 7.86 µm. After cell seeding, the infiltration and migration of cells in different areas of the scaffold were seen. In conclusion, this combined protocol for mouse liver decellularization is effective and its recellularization with hEMCs could be suitable for clinical applications in the future.

Normothermic liver machine perfusion as a dynamic platform for regenerative purposes: What does the future have in store for us?

Liver transplantation has become an immense success; nevertheless, far more recipients are registered on waiting lists than there are available donor livers for transplantation. High-risk, extended criteria donor livers are increasingly used to reduce the discrepancy between organ demand and supply. Especially for high-risk livers, dynamic preservation using machine perfusion can decrease post-transplantation complications and may increase donor liver utilisation by improving graft quality and enabling viability testing before transplantation. To further increase the availability of donor livers suitable for transplantation, new strategies are required that make it possible to use organs that are initially too damaged to be transplanted. With the current progress in experimental liver transplantation research, (long-term) normothermic machine perfusion may be used in the future as a dynamic platform for regenerative medicine approaches, enabling repair and regeneration of injured donor livers. Currently explored therapeutics such as defatting cocktails, RNA interference, senolytics, and stem cell therapy may assist in the repair and/or regeneration of injured livers before transplantation. This review will provide a forecast of the future utility of normothermic machine perfusion in decreasing the imbalance between donor liver demand and supply by enabling the repair and regeneration of damaged donor livers.

Bioartificial livers: a review of their design and manufacture

Acute liver failure (ALF) is a rapidly progressive disease with high morbidity and mortality rates. Liver transplantation and artificial liver support systems, such as artificial livers (ALs) and bioartificial livers (BALs), are the two major therapies for ALF. Compared to ALs, BALs are composed of functional hepatocytes that provide essential liver functions, including detoxification, metabolite synthesis, and biotransformation. Furthermore, BALs can potentially provide effective support as a form of bridging therapy to liver transplantation or spontaneous recovery for patients with ALF. In this review, we systematically discussed the currently available state-of-the-art designs and manufacturing processes for BAL support systems. Specifically, we classified the cell sources and bioreactors that are applied in BALs, highlighted the advanced technologies of hepatocyte culturing and bioreactor fabrication, and discussed the current challenges and future trends in developing next generation BALs for large scale clinical applications.

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.

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.

Biomimicked hierarchical 2D and 3D structures from natural templates: applications in cell biology

Intricate structures of natural surfaces and materials have amazed people over the ages. The unique properties of various surfaces also created interest and curiosity in researchers. In the recent past, with the advent of superior microscopy techniques, we have started to understand how these complex structures provide superior properties. With that knowledge, scientists have developed various biomimicked and bio-inspired surfaces for different non-biological applications. In the last two decades, we have also started to learn how structures of the tissue microenvironment influence cell function and behaviour, both in physiological and pathological conditions. Hence, it became essential to decipher the role and importance of structural hierarchy in the cellular context. With advances in microfabricated techniques, such complex structures were made by superimposing features of different dimensions. However, the fabricated topographies are far from matching the complexities presentin vivo. Hence, the need of biomimicking the natural surfaces for cellular applications was felt. In this review, we discuss a few examples of hierarchical surfaces found in plants, insects, and vertebrates. Such structures have been widely biomimicked for various applications but rarely studied for cell-substrate interaction and cellular response. Here, we discuss the research work wherein 2D hierarchical substrates were prepared using biomimicking to understand cellular functions such as adhesion, orientation, differentiation, and formation of spheroids. Further, we also present the status of ongoing research in mimicking 3D tissue architecture using de-cellularized plant-based and tissue/organ-based scaffolds. We will also discuss 3D printing for fabricating 2D and 3D hierarchical structures. The review will end by highlighting the various advantages and research challenges in this approach. The biomimickedin-vivolike substrate can be used to better understand cellular physiology, and for tissue engineering.

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.

Cell Therapy and Bioengineering in Experimental Liver Regenerative Medicine: In Vivo Injury Models and Grafting Strategies

Purpose of Review

To describe experimental liver injury models used in regenerative medicine, cell therapy strategies to repopulate damaged livers and the efficacy of liver bioengineering.

Recent Findings

Several animal models have been developed to study different liver conditions. Multiple strategies and modified protocols of cell delivery have been also reported. Furthermore, using bioengineered liver scaffolds has shown promising results that could help in generating a highly functional cell delivery system and/or a whole transplantable liver.

Summary

To optimize the most effective strategies for liver cell therapy, further studies are required to compare among the performed strategies in the literature and/or innovate a novel modifying technique to overcome the potential limitations. Coating of cells with polymers, decellularized scaffolds, or microbeads could be the most appropriate solution to improve cellular efficacy. Besides, overcoming the problems of liver bioengineering may offer a radical treatment for end-stage liver diseases.

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.

Assembly and Function of a Bioengineered Human Liver for Transplantation Generated Solely from Induced Pluripotent Stem Cells

The availability of an autologous transplantable auxiliary liver would dramatically affect the treatment of liver disease. Assembly and function in vivo of a bioengineered human liver derived from induced pluripotent stem cells (iPSCs) has not been previously described. By improving methods for liver decellularization, recellularization, and differentiation of different liver cellular lineages of human iPSCs in an organ-like environment, we generated functional engineered human mini livers and performed transplantation in a rat model. Whereas previous studies recellularized liver scaffolds largely with rodent hepatocytes, we repopulated not only the parenchyma with human iPSC-hepatocytes but also the vascular system with human iPS-endothelial cells, and the bile duct network with human iPSC-biliary epithelial cells. The regenerated human iPSC-derived mini liver containing multiple cell types was tested in vivo and remained functional for 4 days after auxiliary liver transplantation in immunocompromised, engineered (IL2rg^-/-) rats.

Liver bioengineering: Recent trends/advances in decellularization and cell sheet technologies towards translation into the clinic

Development of novel technologies provides the best tissue constructs engineering and maximizes their therapeutic effects in regenerative therapy, especially for liver dysfunctions. Among the currently investigated approaches of tissue engineering, scaffold-based and scaffold-free tissues are widely suggested for liver regeneration. Analogs of liver acellular extracellular matrix (ECM) are utilized in native scaffolds to increase the self-repair and healing ability of organs. Native ECM analog could improve liver repairing through providing the supportive framework for cells and signaling molecules, exerting normal biomechanical, biochemical, and physiological signal complexes. Recently, innovative cell sheet technology is introduced as an alternative for conventional tissue engineering with the advantage of fewer scaffold restrictions and cell culture on a Thermo-Responsive Polymer Surface. These sheets release the layered cells through a temperature-controlled procedure without enzymatic digestion, while preserving the cell-ECM contacts and adhesive molecules on cell-cell junctions. In addition, several novelties have been introduced into the cell sheet and decellularization technologies to aid cell growth, instruct differentiation/angiogenesis, and promote cell migration. In this review, recent trends, advancements, and issues linked to translation into clinical practice are dissected and compared regarding the decellularization and cell sheet technologies for liver tissue engineering.

Biomaterial-based cell delivery strategies to promote liver regeneration

Chronic liver disease and cirrhosis is a widespread and untreatable condition that leads to lifelong impairment and eventual death. The scarcity of liver transplantation options requires the development of new strategies to attenuate disease progression and reestablish liver function by promoting regeneration. Biomaterials are becoming an increasingly promising option to both culture and deliver cells to support in vivo viability and long-term function. There is a wide variety of both natural and synthetic biomaterials that are becoming established as delivery vehicles with their own unique advantages and disadvantages for liver regeneration. We review the latest developments in cell transplantation strategies to promote liver regeneration, with a focus on the use of both natural and synthetic biomaterials for cell culture and delivery. We conclude that future work will need to refine the use of these biomaterials and combine them with novel strategies that recapitulate liver organization and function in order to translate this strategy to clinical use.

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.

Stem cell therapy and tissue engineering strategies using cell aggregates and decellularized scaffolds for the rescue of liver failure

Liver failure is a lethal condition with hepatocellular dysfunction, and liver transplantation is presently the only effective treatment. However, due to the limited availability of donors and the potential immune rejection, novel therapeutic strategies are actively sought to restore the normal hepatic architectures and functions, especially for livers with inherited metabolic dysfunctions or chronic diseases. Although the conventional cell therapy has shown promising results, the direct infusion of hepatocytes is hampered by limited hepatocyte sources, poor cell viability, and engraftment. Hence, this review mainly highlights the role of stem cells and progenitors as the alternative cell source and summarizes the potential approaches based on tissue engineering to improve the delivery efficiency of cells. Particularly, the underlying mechanisms for cell therapy using stem cells and progenitors are discussed in two main aspects: paracrine effect and cell differentiation. Moreover, tissue-engineering approaches using cell aggregates and decellularized liver scaffolds for bioengineering of functional hepatic constructs are discussed and compared in terms of the potential to replicate liver physiological structures. In the end, a potentially effective strategy combining the premium advantages of stem cell aggregates and decellularized liver scaffolds is proposed as the future direction of liver tissue engineering and regeneration.

Pre-coating decellularized liver with HepG2-conditioned medium improves hepatic recellularization

Liver transplantation from compatible donors has been the main therapy available for patients with irreversible hepatic injuries. Due to the increasing shortage of organs suitable for transplantation, tissue engineering technologies are important alternatives or surrogate approaches for the future of human organ transplantations. New bioengineering tools have been designed to produce decellularized organs (i.e. scaffolds) which could be recellularized with human cells. Specifically, there is an unmet need for developing reproducible protocols for inducing better cellular spreading in decellularized liver scaffolds. The aim of the present work was to investigate the possibility to improve liver scaffold recellularization by pre-coating decellularized tissue scaffolds with HepG2-conditioned medium (CM). Furthermore, we evaluated the capability of commercial human liver cells (HepG2) to adhere to several types of extracellular matrices (ECM) as well as CM components. Wistar rat livers were decellularized and analyzed by histology, scanning electron microscopy (SEM), immunohistochemistry and residual DNA-content analysis. Human induced pluripotent stem cells (hiPSCs)-derived mesenchymal cells (hiMSCs), and human commercial hepatic (HepG2) and endothelial (HAEC) cells were used for liver scaffold recellularization with or without CM pre-coating. Recellularization occurred for up to 5 weeks. Hepatic tissues and CM were analyzed by proteomic assays. We show that integrity and anatomical organization of the hepatic ECM were maintained after decellularization, and proteomic analysis suggested that pre-coating with CM enriched the decellularized liver ECM. Pre-coating with HepG2-CM highly improved liver recellularization and revealed the positive effects of liver ECM and CM components association.

Advanced Biomaterials and Processing Methods for Liver Regeneration: State-of-the-Art and Future Trends

Liver diseases contribute markedly to the global burden of mortality and disease. The limited organ disposal for orthotopic liver transplantation results in a continuing need for alternative strategies. Over the past years, important progress has been made in the field of tissue engineering (TE). Many of the early trials to improve the development of an engineered tissue construct are based on seeding cells onto biomaterial scaffolds. Nowadays, several TE approaches have been developed and are applied to one vital organ: the liver. Essential elements must be considered in liver TE-cells and culturing systems, bioactive agents or growth factors (GF), and biomaterials and processing methods. The potential of hepatocytes, mesenchymal stem cells, and others as cell sources is demonstrated. They need engineered biomaterial-based scaffolds with perfect biocompatibility and bioactivity to support cell proliferation and hepatic differentiation as well as allowing extracellular matrix deposition and vascularization. Moreover, they require a microenvironment provided using conventional or advanced processing technologies in order to supply oxygen, nutrients, and GF. Herein the biomaterials and the conventional and advanced processing technologies, including cell-sheets process, 3D bioprinting, and microfluidic systems, as well as the future trends in these major fields are discussed.

Lung Microvascular Niche, Repair, and Engineering

Biomaterials have been used for a long time in the field of medicine. Since the success of "tissue engineering" pioneered by Langer and Vacanti in 1993, tissue engineering studies have advanced from simple tissue generation to whole organ generation with three-dimensional reconstruction. Decellularized scaffolds have been widely used in the field of reconstructive surgery because the tissues used to generate decellularized scaffolds can be easily harvested from animals or humans. When a patient's own cells can be seeded onto decellularized biomaterials, theoretically this will create immunocompatible organs generated from allo- or xeno-organs. The most important aspect of lung tissue engineering is that the delicate three-dimensional structure of the organ is maintained during the tissue engineering process. Therefore, organ decellularization has special advantages for lung tissue engineering where it is essential to maintain the extremely thin basement membrane in the alveoli. Since 2010, there have been many methodological developments in the decellularization and recellularization of lung scaffolds, which includes improvements in the decellularization protocols and the selection and preparation of seeding cells. However, early transplanted engineered lungs terminated in organ failure in a short period. Immature vasculature reconstruction is considered to be the main cause of engineered organ failure. Immature vasculature causes thrombus formation in the engineered lung. Successful reconstruction of a mature vasculature network would be a major breakthrough in achieving success in lung engineering. In order to regenerate the mature vasculature network, we need to remodel the vascular niche, especially the microvasculature, in the organ scaffold. This review highlights the reconstruction of the vascular niche in a decellularized lung scaffold. Because the vascular niche consists of endothelial cells (ECs), pericytes, extracellular matrix (ECM), and the epithelial-endothelial interface, all of which might affect the vascular tight junction (TJ), we discuss ECM composition and reconstruction, the contribution of ECs and perivascular cells, the air-blood barrier (ABB) function, and the effects of physiological factors during the lung microvasculature repair and engineering process. The goal of the present review is to confirm the possibility of success in lung microvascular engineering in whole organ engineering and explore the future direction of the current methodology.

Vascular reconstruction: A major challenge in developing a functional whole solid organ graft from decellularized organs

Bioengineering a functional organ holds great potential to overcome the current gap between the organ need and shortage of available organs. Whole organ decellularization allows the removal of cells from large-scale organs, leaving behind extracellular matrices containing different growth factors, structural proteins, and a vascular network with a bare surface. Successful application of decellularized tissues as transplantable organs is hampered by the inability to completely reline the vasculature by endothelial cells (ECs), leading to blood coagulation, loss of vascular patency, and subsequent death of reseeded cells. Therefore, an intact, continuous layer of endothelium is essential to maintain proper functioning of the vascular system, which includes the transfer of nutrients to surrounding tissues and protecting other types of cells from shear stress. Here, we aimed to summarize the available cell sources that can be used for reendothelialization in addition to different trials performed by researchers to reconstruct vascularization of decellularized solid organs. Additionally, different techniques for enhancing reendothelialization and the methods used for evaluating reendothelialization efficiency along with the future prospective applications of this field are discussed. STATEMENT OF SIGNIFICANCE: Despite the great progress in whole organ decellularization, reconstruction of vasculature within the engineered constructs is still a major roadblock. Reconstructed endothelium acts as a multifunctional barrier of vessels, which can reduce thrombosis and help delivering of oxygen and nutrients throughout the whole organ. Successful reendothelialization can be achieved through reseeding of appropriate cell types on the naked vasculature with or without modification of its surface. Here, we present the current research milestones that so far established to reconstruct the vascular network in addition to the methods used for evaluating the efficiency of reendotheilization. Thus, this review is quite significant and will aid the researchers to know where we stand toward biofabricating a transplantable organ from decellularizd extracellular matrix.

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.

Recent Trends in Decellularized Extracellular Matrix Bioinks for 3D Printing: An Updated Review

The promise of regenerative medicine and tissue engineering is founded on the ability to regenerate diseased or damaged tissues and organs into functional tissues and organs or the creation of new tissues and organs altogether. In theory, damaged and diseased tissues and organs can be regenerated or created using different configurations and combinations of extracellular matrix (ECM), cells, and inductive biomolecules. Regenerative medicine and tissue engineering can allow the improvement of patients' quality of life through availing novel treatment options. The coupling of regenerative medicine and tissue engineering with 3D printing, big data, and computational algorithms is revolutionizing the treatment of patients in a huge way. 3D bioprinting allows the proper placement of cells and ECMs, allowing the recapitulation of native microenvironments of tissues and organs. 3D bioprinting utilizes different bioinks made up of different formulations of ECM/biomaterials, biomolecules, and even cells. The choice of the bioink used during 3D bioprinting is very important as properties such as printability, compatibility, and physical strength influence the final construct printed. The extracellular matrix (ECM) provides both physical and mechanical microenvironment needed by cells to survive and proliferate. Decellularized ECM bioink contains biochemical cues from the original native ECM and also the right proportions of ECM proteins. Different techniques and characterization methods are used to derive bioinks from several tissues and organs and to evaluate their quality. This review discusses the uses of decellularized ECM bioinks and argues that they represent the most biomimetic bioinks available. In addition, we briefly discuss some polymer-based bioinks utilized in 3D bioprinting.

lncRNA Gm10451 regulates PTIP to facilitate iPSCs-derived β-like cell differentiation by targeting miR-338-3p as a ceRNA

iPSCs-derived insulin-producing cell transplantation is a promising strategy for diabetes therapy. Although there have been many protocols of mature, glucose-responsive β cells induced in vitro over the past few years, many underlying problems remain to be resolved. As a crucial regulator, long noncoding RNAs (lncRNAs) participate in numerous biological processes, including the maintenance of pluripotency, and stem cell differentiation. In this study, we identified a novel lncRNA Gm10451 as a functional regulator for β-like cell differentiation. Localized to the cytoplasm, Gm10451 regulates histone H3K4 methyltransferase complex PTIP to facilitate Insulin⁺/Nkx6.1⁺ β-like cell differentiation by targeting miR-338-3p as a competing endogenous RNA (ceRNA). miR-338-3p has also been shown to suppress Nkx6.1⁺ early-stage β-like cell differentiation by targeting PTIP. Following transplantation into streptozotocin (STZ)-mice, Gm10451 loss in β-like cells prevented the expression of mature β-cell makers, such as Insulin, Nkx6.1, and Mafa. Accordingly, hyperglycemia in the mice was not resolved. Taken together, this study provides an efficient epigenetic target for generating more mature and functional iPSCs-derived β-like cells. We anticipate that pancreatic organoids, which are generated from human stem cells, biological materials, and epigenetic modifications, can be used in the future as a novel diabetes treatment option.

Advances in Hepatic Tissue Bioengineering with Decellularized Liver Bioscaffold

The burden of liver diseases continues to grow worldwide, and liver transplantation is the only option for patients with end-stage liver disease. This procedure is limited by critical issues, including the low availability of donor organs; thus, novel therapeutic strategies are greatly needed. Recently, bioengineering approaches using decellularized liver scaffolds have been proposed as a novel strategy to overcome these challenges. The aim of this systematic literature review was to identify the major advances in the field of bioengineering using decellularized liver scaffolds and to identify obstacles and challenges for clinical application. The main findings of the articles and each contribution for technique optimization were highlighted, including the protocols of perfusion and decellularization, duration, demonstration of quality control—scaffold acellularity, matrix composition, and preservation of growth factors—and tissue functionality after recellularization. In previous years, many advances have been made as this technique has evolved from studies in animal models to human livers. As the field develops and this promising technique has become much more feasible, many challenges remain, including the selection of appropriate cell types for recellularization, route of cell administration, cell-seeding protocol, and scalability that must be standardized prior to clinical application.

Vasculature reconstruction of decellularized liver scaffolds via gelatin-based re-endothelialization

Decellularized liver scaffolds based liver engineering is a promising approach toward developing functional liver surrogates. However, a major obstacle to long-term transplantation is the hemocompatibility of the bioengineered liver surrogates. One approach to improve the hemocompatibility of engineered liver surrogates is re-endothelialization. In the current study, immortalized endothelial cells were perfused for re-endothelialization of decellularized rat liver scaffolds. When compared to the media-based perfusion approach, gelatin hydrogels-based perfusion significantly increased the number of cells that were retained in the decellularized liver scaffolds and the vascular lumen coverage ratio. Endothelial cells were lining along the vasculatures of the decellularized liver scaffolds and actively proliferating. Re-endothelialization improved the blood retention ability of the liver scaffold vasculatures. Doppler ultrasound detected active blood flows within the re-endothelialized liver scaffold transplants 8 days post-transplantation. Our results strengthened the feasibility of developing bioengineered liver surrogates utilizing decellularized liver scaffolds. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 107A: 392-402, 2019.

A Novel Surgical Technique As a Foundation for In Vivo Partial Liver Engineering in Rat

Organ engineering is a novel strategy to generate liver organ substitutes that can potentially be used in transplantation. Recently, in vivo liver engineering, including in vivo organ decellularization followed by repopulation, has emerged as a promising approach over ex vivo liver engineering. However, postoperative survival was not achieved. The aim of this study is to develop a novel surgical technique of in vivo selective liver lobe perfusion in rats as a prerequisite for in vivo liver engineering. We generate a circuit bypass only through the left lateral lobe. Then, the left lateral lobe is perfused with heparinized saline. The experiment is performed with 4 groups (n = 3 rats per group) based on different perfusion times of 20 min, 2 h, 3 h, and 4 h. Survival, as well as the macroscopically visible change of color and the histologically determined absence of blood cells in the portal triad and the sinusoids, is taken as an indicator for a successful model establishment. After selective perfusion of the left lateral lobe, we observe that the left lateral lobe, indeed, turned from red to faint yellow. In a histological assessment, no blood cells are visible in the branch of the portal vein, the central vein, and the sinusoids. The left lateral lobe turns red after reopening the blocked vessels. 12/12 rats survived the procedure for more than one week. We are the first to report a surgical model for in vivo single liver lobe perfusion with a long survival period of more than one week. In contrast to the previously published report, the most important advantage of the technique presented here is that perfusion of 70% of the liver is maintained throughout the whole procedure. The establishment of this technique provides a foundation for in vivo partial liver engineering in rats, including decellularization and recellularization.

Innovation for hepatotoxicity in vitro research models: A review

Many categories of drugs can induce hepatotoxicity, so improving the prediction of toxic drugs is important. In vitro models using human hepatocytes are more accurate than in vivo animal models. Good in vitro models require an abundance of metabolic enzyme activities and normal cellular polarity. However, none of the in vitro models can completely simulate hepatocytes in the human body. There are two ways to overcome this limitation: enhancing the metabolic function of hepatocytes and changing the cultural environment. In this review, we summarize the current state of research, including the main characteristics of in vitro models and their limitations, as well as improved technology and developmental prospects. We hope that this review provides some new ideas for hepatotoxicity research.

Canine Placenta Recellularized Using Yolk Sac Cells with Vascular Endothelial Growth Factor

Regenerative medicine has been growing because of the emergent need for tissues/organs for transplants and restorative surgeries. Biological scaffolds are important tools to try to solve this problem. The one used in this reserach was developed by an acellular biological scaffold from canine placenta with a rich source of cellular matrix. After decellularization, the cellular matrix demonstrated structural preservation with the presence of important functional proteins such as collagen, fibronectin, and laminin. We used cells transduced with vascular endothelial growth factor (VEGF) to recellularize this scaffold. It was succeeded by seeding the cells in nonadherent plaques in the presence of the sterelized placenta scaffold. Cells were adhered to the scaffold when analyzed by immunocytochemistry and scanning electron microscopy, both showing sprouting of yolk sac VEGF (YSVEGF) cells. This recellularized scaffold is a promissory biomaterial for repairing injured areas where neovascularization is required.

Establishment of practical recellularized liver graft for blood perfusion using primary rat hepatocytes and liver sinusoidal endothelial cells

Tissue decellularization produces a three-dimensional scaffold that can be used to fabricate functional liver grafts following recellularization. Inappropriate cell distribution and clotting during blood perfusion hinder the practical use of recellularized livers. Here we aimed to establish a seeding method for the optimal distribution of parenchymal and endothelial cells, and to evaluate the effect of liver sinusoidal endothelial cells (LSECs) in the decellularized liver. Primary rat hepatocytes and LSECs were seeded into decellularized whole-liver scaffolds via the biliary duct and portal vein, respectively. Biliary duct seeding provided appropriate hepatocyte distribution into the parenchymal space, and portal vein-seeded LSECs simultaneously lined the portal lumen, thereby maintaining function and morphology. Hepatocytes co-seeded with LSECs retained their function compared with those seeded alone. Platelet deposition was significantly decreased and hepatocyte viability was maintained in the co-seeded group after extracorporeal blood perfusion. In conclusion, our seeding method provided optimal cell distribution into the parenchyma and vasculature according to the three-dimensional structure of the decellularized liver. LSECs maintained hepatic function, and supported hepatocyte viability under blood perfusion in the engineered liver graft owing to their antithrombogenicity. This recellularization procedure could help produce practical liver grafts with blood perfusion.

Shear stress: An essential driver of endothelial progenitor cells

The blood flow through vessels produces a tangential, or shear, stress sensed by their innermost layer (i.e., endothelium) and representing a major hemodynamic force. In humans, endothelial repair and blood vessel formation are mainly performed by circulating endothelial progenitor cells (EPCs) characterized by a considerable expression of vascular endothelial growth factor receptor 2 (VEGFR2), CD34, and CD133, pronounced tube formation activity in vitro, and strong reendothelialization or neovascularization capacity in vivo. EPCs have been proposed as a promising agent to induce reendothelialization of injured arteries, neovascularization of ischemic tissues, and endothelialization or vascularization of bioartificial constructs. A number of preconditioning approaches have been suggested to improve the regenerative potential of EPCs, including the use of biophysical stimuli such as shear stress. However, in spite of well-defined influence of shear stress on mature endothelial cells (ECs), articles summarizing how it affects EPCs are lacking. Here we discuss the impact of shear stress on homing, paracrine effects, and differentiation of EPCs. Unidirectional laminar shear stress significantly promotes homing of circulating EPCs to endothelial injury sites, induces anti-thrombotic and anti-atherosclerotic phenotype of EPCs, increases their capability to form capillary-like tubes in vitro, and enhances differentiation of EPCs into mature ECs in a dose-dependent manner. These effects are mediated by VEGFR2, Tie2, Notch, and β1/3 integrin signaling and can be abrogated by means of complementary siRNA/shRNA or selective pharmacological inhibitors of the respective proteins. Although the testing of sheared EPCs for vascular tissue engineering or regenerative medicine applications is still an unaccomplished task, favorable effects of unidirectional laminar shear stress on EPCs suggest its usefulness for their preconditioning.

Liver tissue engineering: From implantable tissue to whole organ engineering

The term “liver tissue engineering” summarizes one of the ultimate goals of modern biotechnology: the possibility of reproducing in total or in part the functions of the liver in order to treat acute or chronic liver disorders and, ultimately, create a fully functional organ to be transplanted or used as an extracorporeal device. All the technical approaches in the area of liver tissue engineering are based on allocating adult hepatocytes or stem cell‐derived hepatocyte‐like cells within a three‐dimensional structure able to ensure their survival and to maintain their functional phenotype. The hosting structure can be a construct in which hepatocytes are embedded in alginate and/or gelatin or are seeded in a pre‐arranged scaffold made with different types of biomaterials. According to a more advanced methodology termed three‐dimensional bioprinting, hepatocytes are mixed with a bio‐ink and the mixture is printed in different forms, such as tissue‐like layers or spheroids. In the last decade, efforts to engineer a cell microenvironment recapitulating the dynamic native extracellular matrix have become increasingly successful, leading to the hope of satisfying the clinical demand for tissue (or organ) repair and replacement within a reasonable timeframe. Indeed, the preclinical work performed in recent years has shown promising results, and the advancement in the biotechnology of bioreactors, ex vivo perfusion machines, and cell expansion systems associated with a better understanding of liver development and the extracellular matrix microenvironment will facilitate and expedite the translation to technical applications. (Hepatology Communications 2018;2:131–141)

Whole liver engineering: A promising approach to develop functional liver surrogates

Liver donor shortage remains the biggest challenge for patients with end-stage liver failures. While bioartificial liver devices have been developed as temporary supports for patients waiting for transplantation, their applications have been limited clinically. Whole liver engineering is a biological scaffold based regenerative medicine approach that holds promise for developing functional liver surrogates. Significant advancements have been made since the first report in 2010. This review focuses on the recent achievements of whole liver engineering studies.

Decellularization of placentas: establishing a protocol

Biological biomaterials for tissue engineering purposes can be produced through tissue and/or organ decellularization. The remaining extracellular matrix (ECM) must be acellular and preserve its proteins and physical features. Placentas are organs of great interest because they are discarded after birth and present large amounts of ECM. Protocols for decellularization are tissue-specific and have not been established for canine placentas yet. This study aimed at analyzing a favorable method for decellularization of maternal and fetal portions of canine placentas. Canine placentas were subjected to ten preliminary tests to analyze the efficacy of parameters such as the type of detergents, freezing temperatures and perfusion. Two protocols were chosen for further analyses using histology, scanning electron microscopy, immunofluorescence and DNA quantification. Sodium dodecyl sulfate (SDS) was the most effective detergent for cell removal. Freezing placentas before decellularization required longer periods of incubation in different detergents. Both perfusion and immersion methods were capable of removing cells. Placentas decellularized using Protocol I (1% SDS, 5 mM EDTA, 50 mM TRIS, and 0.5% antibiotic) preserved the ECM structure better, but Protocol I was less efficient to remove cells and DNA content from the ECM than Protocol II (1% SDS, 5 mM EDTA, 0.05% trypsin, and 0.5% antibiotic).

"Tissue Papers" from Organ-Specific Decellularized Extracellular Matrices

Using an innovative, tissue-independent approach to decellularized tissue processing and biomaterial fabrication, the development of a series of "tissue papers" derived from native porcine tissues/organs (heart, kidney, liver, muscle), native bovine tissue/organ (ovary and uterus), and purified bovine Achilles tendon collagen as a control from decellularized extracellular matrix particle ink suspensions cast into molds is described. Each tissue paper type has distinct microstructural characteristics as well as physical and mechanical properties, is capable of absorbing up to 300% of its own weight in liquid, and remains mechanically robust (E = 1-18 MPa) when hydrated; permitting it to be cut, rolled, folded, and sutured, as needed. In vitro characterization with human mesenchymal stem cells reveals that all tissue paper types support cell adhesion, viability, and proliferation over four weeks. Ovarian tissue papers support mouse ovarian follicle adhesion, viability, and health in vitro, as well as support, and maintain the viability and hormonal function of nonhuman primate and human follicle-containing, live ovarian cortical tissues ex vivo for eight weeks postmortem. "Tissue papers" can be further augmented with additional synthetic and natural biomaterials, as well as integrated with recently developed, advanced 3D-printable biomaterials, providing a versatile platform for future multi-biomaterial construct manufacturing.

Towards a Bioengineered Kidney: Recellularization Strategies for Decellularized Native Kidney Scaffolds

Patients with end-stage renal disease often undergo dialysis as a partial substitute for kidney function while waiting for their only treatment option: a kidney transplant. Several research directions emerged for alternatives in support of the ever-growing numbers of patients. Recent years brought big steps forward in the field, with researchers questioning and improving the current dialysis devices as well as moving towards the design of a bioengineered kidney. Whole-organ engineering is also being explored as a possibility, making use of animal or human kidney scaffolds for engineering a transplantable organ. While this is not a new strategy, having been applied so far for thin tissues, it is a novel approach for complex organs such as the kidneys. Kidneys can be decellularized and the remaining scaffold consisting of an extracellular matrix can be repopulated with (autologous) cells, aiming at growing ex vivo a fully transplantable organ. In a broader view, such organs might also be used for a better understanding of fundamental biological concepts and disease mechanisms, drug screening and toxicological investigations, opening new pathways in the treatment of kidney disease.

Decellularization of whole organs has been widely explored and described; therefore, this manuscript only briefly reviews some important considerations with an emphasis on scaffold decontamination, but focuses further on recellularization strategies. Critical aspects, including cell types and sources that can be used for recellularization, seeding strategies and possible applications beyond renal replacement are discussed.