Induced pluripotent stem cell-derived hepatocytes and endothelial cells in multi-component hydrogel fibers for liver tissue engineering

Assembly of Interfacial Polyelectrolyte Complexation Fibers with Mineralization Gradient for Physiologically-Inspired Ligament Regeneration

Current synthetic grafts for ligament rupture repair often fail to integrate well with the surrounding biological tissue, leading to complications such as graft wear, fatigue, and subsequent re-rupture. To address this medical challenge, this study aims at advancing the development of a biological ligament through the integration of physiologically-inspired principles and tissue engineering strategies. In this study, interfacial polyelectrolyte complexation (IPC) spinning technique, along with a custom-designed collection system, to fabricate a hierarchical scaffold mimicking native ligament structure, is utilized. To emulate the bone-ligament interface and alleviate stress concentration, a hydroxyapatite (HAp) mineral gradient is strategically introduced near both ends of the scaffold to enhance interface integration and diminish the risk of avulsion rupture. Biomimetic viscoelasticity is successfully displayed to provide similar mechanical support to native ligamentous tissue under physiological conditions. By introducing the connective tissue growth factor (CTGF) and conducting mesenchymal stem cells transplantation, the regenerative potential of the synthetic ligament is significantly amplified. This pioneering study offers a multifaceted solution combining biomimetic materials, regenerative therapies, and advanced techniques to potentially transform ligament rupture treatment.

3D Printing of a Vascularized Mini-Liver Based on the Size-Dependent Functional Enhancements of Cell Spheroids for Rescue of Liver Failure

The emerging stem cell-derived hepatocyte-like cells (HLCs) are the alternative cell sources of hepatocytes for treatment of highly lethal acute liver failure (ALF). However, the hostile local environment and the immature cell differentiation may compromise their therapeutic efficacy. To this end, human adipose-derived mesenchymal stromal/stem cells (hASCs) are engineered into different-sized multicellular spheroids and co-cultured with 3D coaxially and hexagonally patterned human umbilical vein endothelial cells (HUVECs) in a liver lobule-like manner to enhance their hepatic differentiation efficiency. It is found that small-sized hASC spheroids, with a diameter of ≈50 µm, show superior pro-angiogenic effects and hepatic differentiation compared to the other counterparts. The size-dependent functional enhancements are mediated by the Wnt signaling pathway. Meanwhile, co-culture of hASCs with HUVECs, at a HUVECs/hASCs seeding density ratio of 2:1, distinctly promotes hepatic differentiation and vascularization both in vitro and in vivo, especially when endothelial cells are patterned into hollow hexagons. After subcutaneous implantation, the mini-liver, consisting of HLC spheroids and 3D-printed interconnected vasculatures, can effectively improve liver regeneration in two ALF animal models through amelioration of local oxidative stress and inflammation, reduction of liver necrosis, as well as increase of cell proliferation, thereby showing great promise for clinical translation.

Fabrication and Cell Culture Applications of Core-Shell Hydrogel Fibers Composed of Chitosan/DNA Interfacial Polyelectrolyte Complexation and Calcium Alginate: Straight and Beaded Core Variations

Core-shell hydrogel fibers are widely used in cell culture applications. A simple and rapid method is presented for fabricating core-shell hydrogel fibers, consisting of straight or beaded core fibers, for cell culture applications. The core fibers are prepared using interfacial polyelectrolyte complexation (IPC) with chitosan and DNA. Briefly, two droplets of chitosan and DNA are brought in contact to form an IPC film, which is dragged to prepare an IPC fiber. The incubation time and DNA concentration are adjusted to prepare straight and beaded IPC fibers. The fibers with Ca2+ are immersed in an alginate solution to form calcium alginate shell hydrogels around the core IPC fibers. To the best of the knowledge, this is the first report of core-shell hydrogel fibers with IPC fiber cores. To demonstrate cell culture, straight hydrogel fibers are applied to fabricate hepatic models consisting of HepG2 and 3T3 fibroblasts, and vascular models consisting of human umbilical vein endothelial cells and 3T3 fibroblasts. To evaluate the effect of co-culture, albumin secretion, and angiogenesis are evaluated. Beaded hydrogel fibers are used to fabricate many size-controlled spheroids for fiber and cloning applications. This method can be widely applied in tissue engineering and cell analysis.

Single-Cell RNA-seq Analysis of a Human Embryonic Stem Cell to Endothelial Cell System Based on Transcription Factor Overexpression

BACKGROUND

Human embryonic stem cell (hESC)-derived endothelial cells (ECs) possess therapeutic potential in many diseases. Cytokine supplementation induction and transcription factor overexpression have become two mainstream methods of hESC-EC induction. Single-cell RNA-seq technology has been widely used to analyse dynamic processes during hESC-EC induction and components of induced endothelial cells. However, studies that used single-cell RNA-seq are mainly based on cytokine supplementation methods. In this study, we used a high-efficiency human embryonic stem cell-endothelial cell line (hESC-EC) called the "FLI1-PKC system" as a research model and employed single-cell RNA sequencing (scRNA-seq) to investigate the transcriptional landscape and cellular dynamics.

METHODS

The high-efficiency hESC-EC induction (FLI1-PKC) system was established in our previous study. We applied single-cell RNA sequencing (scRNA-seq) of the differentiated cells at different time points and investigated the gene expression profiles.

RESULTS

The FLI1-PKC induction system can directionally differentiate hESCs into mature endothelial cells with all the requisite functions. Unlike other hES-EC induction protocols, the FLI1-PKC method follows a different induction route; nonetheless, the transcriptome of induced endothelial cells (iECs) remains the same. The elevated number of activated transcription factors may explain why the FLI1-PKC system is more effective than other hES-EC protocols.

CONCLUSION

Our study has presented a single-cell transcriptional overview of a high-efficiency hESC-EC induction system, which can be used as a model and reference for further improvement in other hESC induction systems.

Electrospinning and Cell Fibers in Biomedical Applications

Human body tissues such as muscle, blood vessels, tendon/ligaments, and nerves have fiber-like fascicle morphologies, where ordered organization of cells and extracellular matrix (ECM) within the bundles in specific 3D manners orchestrates cells and ECM to provide tissue functions. Through engineering cell fibers (which are fibers containing living cells) as living building blocks with the help of emerging "bottom-up" biomanufacturing technologies, it is now possible to reconstitute/recreate the fiber-like fascicle morphologies and their spatiotemporally specific cell-cell/cell-ECM interactions in vitro, thereby enabling the modeling, therapy, or repair of these fibrous tissues. In this article, a concise review is provided of the "bottom-up" biomanufacturing technologies and materials usable for fabricating cell fibers, with an emphasis on electrospinning that can effectively and efficiently produce thin cell fibers and with properly designed processes, 3D cell-laden structures that mimic those of native fibrous tissues. The importance and applications of cell fibers as models, therapeutic platforms, or analogs/replacements for tissues for areas such as drug testing, cell therapy, and tissue engineering are highlighted. Challenges, in terms of biomimicry of high-order hierarchical structures and complex dynamic cellular microenvironments of native tissues, as well as opportunities for cell fibers in a myriad of biomedical applications, are discussed.

Stem cell culture and differentiation in 3-D scaffolds

Conventional two-dimensional (2-D) cultivation are easy to utilize for human pluripotent stem (hPS) cell cultivation in standard techniques and are important for analysis or development of the signal pathways to keep pluripotent state of hPS cells cultivated on 2-D cell culture materials. However, the most efficient protocol to prepare hPS cells is the cell culture in a three dimensional (3-D) cultivation unit because huge numbers of hPS cells should be utilized in clinical treatment. Some 3-D cultivation strategies for hPS cells are considered: (a) microencapsulated cell cultivation in suspended hydrogels, (b) cell cultivation on microcarriers (MCs), (c) cell cultivation on self-aggregated spheroid [cell aggregates; embryoid bodies (EBs) and organoids], (d) cell cultivation on microfibers or nanofibers, and (e) cell cultivation in macroporous scaffolds. These cultivation ways are described in this chapter.

Development of an Induced Pluripotent Stem Cell-Based Liver-on-a-Chip Assessed with an Alzheimer's Disease Drug

Liver-related drug metabolism is a key aspect of pharmacokinetics and possible toxicity. From this perspective, the availability of advanced in vitro models for drug testing is still an open need, also to the end of reducing the burden of in vivo experiments. In this scenario, organ-on-a-chip is gaining attention as it couples a state-of-the art in vitro approach to the recapitulation of key in vivo physiological features such as fluidodynamics and a tri-dimensional cytoarchitecture. We implemented a novel liver-on-a-chip (LoC) device based on an innovative dynamic device (MINERVA 2.0) where functional hepatocytes (iHep) have been encapsulated into a 3D hydrogel matrix interfaced through a porous membrane with endothelial cells (iEndo)]. Both lines were derived from human-induced pluripotent stem cells (iPSCs), and the LoC was functionally assessed with donepezil, a drug approved for Alzheimer's disease therapy. The presence of iEndo and a 3D microenvironment enhanced the expression of liver-specific physiologic functions as in iHep, after 7 day perfusion, we noticed an increase of albumin, urea production, and cytochrome CYP3A4 expression compared to the iHep static culture. In particular, for donepezil kinetics, a computational fluid dynamic study conducted to assess the amount of donepezil diffused into the LoC indicated that the molecule should be able to pass through the iEndo and reach the target iHep construct. Then, we performed experiments of donepezil kinetics that confirmed the numerical simulations. Overall, our iPSC-based LoC reproduced the in vivo physiological microenvironment of the liver and was suitable for potential hepatotoxic screening studies.

Engineered human liver based on pullulan-dextran hydrogel promotes mice survival after liver failure

Liver tissue engineering approaches aim to support drug testing, assistance devices, or transplantation. However, their suitability for clinical application remains unsatisfactory. Herein, we demonstrate the beneficial and biocompatible use of porous pullulan-dextran hydrogel for the self-assembly of hepatocytes and biliary-like cells into functional 3D microtissues. Using HepaRG cells, we obtained 21 days maintenance of engineered liver polarity, functional detoxification and excretion systems, as well as glycogen storage in hydrogel. Implantation on two liver lobes in mice of hydrogels containing 3800 HepaRG 3D structures of 100 ​μm in diameter, indicated successful engraftment and no signs of liver toxicity after one month. Finally, after acetaminophen-induced liver failure, when mice were transplanted with engineered livers on left lobe and peritoneal cavity, the survival rate at 7 days significantly increased by 31.8% compared with mice without cell therapy. These findings support the clinical potential of pullulan-dextran hydrogel for liver failure management.

Natural Scaffolds Used for Liver Regeneration: A Narrative Update

Annually chronic liver diseases cause two million death worldwide. Although liver transplantation (LT) is still considered the best therapeutic option, the limited number of donated livers and lifelong side effects of LT has led researchers to seek alternative therapies. Tissue engineering (TE) as a promising method is considered for liver repair and regeneration. TE uses natural or synthetic scaffolds, functional somatic cells, multipotent stem cells, and growth factors to develop new organs. Biological scaffolds are notable in TE because of their capacity to mimic extracellular matrices, biodegradability, and biocompatibility. Moreover, natural scaffolds are classified based on their source and function in three separate groups. Hemostat-based scaffolds as the first group were reviewed for their application in coagulation in liver injury or surgery. Furthermore, recent studies showed improvement in the function of biological hydrogels in liver regeneration and vascularity. In addition, different applications of natural scaffolds were discussed and compared with synthetic scaffolds. Finally, we focused on the efforts to improve the performance of decellularized extracellular matrixes for liver implantation.

Laminin 411 mediates endothelial specification via multiple signaling axes that converge on β-catenin

The extracellular matrix (ECM) provides essential cues to promote endothelial specification during tissue development in vivo; correspondingly, ECM is considered essential for endothelial differentiation outside of the body. However, systematic studies to assess the precise contribution of individual ECM proteins to endothelial differentiation have not been conducted. Further, the multi-component nature of differentiation protocols makes it challenging to study the underlying mechanisms by which the ECM contributes to cell fate. In this study, we determined that Laminin 411 alone increases endothelial differentiation of induced pluripotent stem cells over collagen I or Matrigel. The effect of ECM was shown to be independent of vascular endothelial growth factor (VEGF) binding capacity. We also show that ECM-guided endothelial differentiation is dependent on activation of focal adhesion kinase (FAK), integrin-linked kinase (ILK), Notch, and β-catenin pathways. Our results indicate that ECM contributes to endothelial differentiation through multiple avenues, which converge at the expression of active β-catenin.

Controllable fabrication of alginate/poly-L-ornithine polyelectrolyte complex hydrogel networks as therapeutic drug and cell carriers

Polyelectrolyte complex (PEC) hydrogels are advantageous as therapeutic agent and cell carriers. However, due to the weak nature of physical crosslinking, PEC swelling and cargo burst release are easily initiated. Also, most current cell-laden PEC hydrogels are limited to fibers and microcapsules with unfavorable dimensions and structures for practical implantations. To overcome these drawbacks, alginate (Alg)/poly-L-ornithine (PLO) PEC hydrogels are fabricated into microcapsules, fibers, and bulk scaffolds to explore their feasibility as drug and cell carriers. Stable Alg/PLO microcapsules with controllable shapes are obtained through aqueous electrospraying technique, which avoids osmotic shock and prolongs the release time. Model enzyme and nanosized cargos are successfully encapsulated and continuously released for more than 21 days. Alg/PLO PEC fibers are then prepared to encapsulate brown adipose progenitors (BAPs) and Jurkat T cells. The electrostatic interactions between Alg and PLO are found to facilitate the printability and self-support ability of Alg/PLO bioinks. Alg/PLO PEC fibers and scaffolds support cell proliferation, differentiation, and functionalization. These results demonstrate new options for biocompatible PEC hydrogel preparation and improve the understanding of PEC hydrogels as drug and cell carriers. STATEMENT OF SIGNIFICANCE: In this study, the concept of polyelectrolyte complex hydrogel networks as drug and cell carriers has been demonstrated. Their feasibility to achieve sustained drug release and cell functionality was explored, from microcapsules to fibers to three-dimension printed scaffolds. PEC microcapsules with controllable shapes were obtained. Therapeutic drugs can be encapsulated and continuously release for more than 21 days. Benefiting from the dynamic interactions of physically crosslinked PEC, self-healing fibers were achieved. Besides, the electrostatic interactions between polyelectrolytes were found to facilitate the printability and self-support ability of PEC bioinks. The PEC fibers and scaffolds with controllable structure supported cell proliferation, differentiation, and function. The outcome of current research promotes design of new biocompatible PEC hydrogels and potential drug and cell carriers.

Recapitulating pancreatic cell-cell interactions through bioengineering approaches: the momentous role of non-epithelial cells for diabetes cell therapy

Over the past few years, extensive efforts have been made to generate in-vitro pancreatic micro-tissue, for disease modeling or cell replacement approaches in pancreatic related diseases such as diabetes mellitus. To obtain these goals, a closer look at the diverse cells participating in pancreatic development is necessary. Five major non-epithelial pancreatic (pN-Epi) cell populations namely, pancreatic endothelium, mesothelium, neural crests, pericytes, and stellate cells exist in pancreas throughout its development, and they are hypothesized to be endogenous inducers of the development. In this review, we discuss different pN-Epi cells migrating to and existing within the pancreas and their diverse effects on pancreatic epithelium during organ development mediated via associated signaling pathways, soluble factors or mechanical cell-cell interactions. In-vivo and in-vitro experiments, with a focus on N-Epi cells' impact on pancreas endocrine development, have also been considered. Pluripotent stem cell technology and multicellular three-dimensional organoids as new approaches to generate pancreatic micro-tissues have also been discussed. Main challenges for reaching a detailed understanding of the role of pN-Epi cells in pancreas development in utilizing for in-vitro recapitulation have been summarized. Finally, various novel and innovative large-scale bioengineering approaches which may help to recapitulate cell-cell interactions and are crucial for generation of large-scale in-vitro multicellular pancreatic micro-tissues, are discussed.

Regenerative Engineering: Current Applications and Future Perspectives

Many pathologies, congenital defects, and traumatic injuries are untreatable by conventional pharmacologic or surgical interventions. Regenerative engineering represents an ever-growing interdisciplinary field aimed at creating biological replacements for injured tissues and dysfunctional organs. The need for bioengineered replacement parts is ubiquitous among all surgical disciplines. However, to date, clinical translation has been limited to thin, small, and/or acellular structures. Development of thicker tissues continues to be limited by vascularization and other impediments. Nevertheless, currently available materials, methods, and technologies serve as robust platforms for more complex tissue fabrication in the future. This review article highlights the current methodologies, clinical achievements, tenacious barriers, and future perspectives of regenerative engineering.

Genetic Diagnosis in Hereditary Hemochromatosis: Discovering and Understanding the Biological Relevance of Variants

BACKGROUND

Hereditary hemochromatosis (HH) is a genetic disease, leading to iron accumulation and possible organ damage. Patients are usually homozygous for p. Cys282Tyr in the homeostatic iron regulator gene but may have mutations in other genes involved in the regulation of iron. Next-generation sequencing is increasingly being utilized for the diagnosis of patients, leading to the discovery of novel genetic variants. The clinical significance of these variants is often unknown.

CONTENT

Determining the pathogenicity of such variants of unknown significance is important for diagnostics and genetic counseling. Predictions can be made using in silico computational tools and population data, but additional evidence is required for a conclusive pathogenicity classification. Genetic disease models, such as in vitro models using cellular overexpression, induced pluripotent stem cells or organoids, and in vivo models using mice or zebrafish all have their own challenges and opportunities when used to model HH and other iron disorders. Recent developments in gene-editing technologies are transforming the field of genetic disease modeling.

SUMMARY

In summary, this review addresses methods and developments regarding the discovery and classification of genetic variants, from in silico tools to in vitro and in vivo models, and presents them in the context of HH. It also explores recent gene-editing developments and how they can be applied to the discussed models of genetic disease.

Challenges for the Applications of Human Pluripotent Stem Cell-Derived Liver Organoids

The current organoid culture systems allow pluripotent and adult stem cells to self-organize to form three-dimensional (3D) structures that provide a faithful recapitulation of the architecture and function of in vivo organs. In particular, human pluripotent stem cell-derived liver organoids (PSC-LOs) can be used in regenerative medicine and preclinical applications, such as disease modeling and drug discovery. New bioengineering tools, such as microfluidics, biomaterial scaffolds, and 3D bioprinting, are combined with organoid technologies to increase the efficiency of hepatic differentiation and enhance the functional maturity of human PSC-LOs by precise control of cellular microenvironment. Long-term stabilization of hepatocellular functions of in vitro liver organoids requires the combination of hepatic endodermal, endothelial, and mesenchymal cells. To improve the biological function and scalability of human PSC-LOs, bioengineering methods have been used to identify diverse and zonal hepatocyte populations in liver organoids for capturing heterogeneous pathologies. Therefore, constructing engineered liver organoids generated from human PSCs will be an extremely versatile tool in in vitro disease models and regenerative medicine in future. In this review, we aim to discuss the recent advances in bioengineering technologies in liver organoid culture systems that provide a timely and necessary study to model disease pathology and support drug discovery in vitro and to generate cell therapy products for transplantation.

Quantitative Evaluation of Human Umbilical Vein and Induced Pluripotent Stem Cell-Derived Endothelial Cells as an Alternative Cell Source to Skin-Specific Endothelial Cells in Engineered Skin Grafts

Objective: We compared the capability of human umbilical vein endothelial cells (HUVECs), induced pluripotent stem cell (iPSC)-derived endothelial cells (iECs), and human dermal blood endothelial cells (HDBECs) to effectively vascularize engineered human skin constructs (HSCs) in vitro and on immunodeficient mice. Approach: We quantified the angiogenesis within HSCs both in vitro and in vivo through computational analyses of immunofluorescent (IF) staining. We assayed with real-time quantitative PCR (RT-qPCR) the expression of key endothelial, dermal, and epidermal genes in 2D culture and HSCs. Epidermal integrity and proliferation were also evaluated through haematoxylin and eosin staining, and IF staining. Results: IF confirmed iEC commitment to endothelial phenotype. RT-qPCR showed HUVECs and iECs immaturity compared with HDBECs. In vitro, the vascular network extension was comparable for HDBECs and HUVECs despite differences in vascular diameter, whereas iECs formed unorganized rudimentary tubular structures. In vivo, all ECs produced discrete vascular networks of varying dimensions. HUVECs and HDBECs maintained a higher proliferation of basal keratinocytes. HDBECs had the best impact on extracellular matrix expression, and epidermal proliferation and differentiation. Innovation: To our knowledge, this study represents the first direct and quantitative comparison of HDBECs, HUVECs, and iECs angiogenic performance in HSCs. Conclusions: Our data indicate that HUVECs and iECs can be an alternative cell source to HDBEC to promote the short-term viability of prevascularized engineered grafts. Nevertheless, HDBECs maintain their capillary identity and outperform other EC types in promoting the maturation of the dermis and epidermis. These intrinsic characteristics of HDBECs may influence the long-term function of skin grafts.

Cardiac fibrosis models using human induced pluripotent stem cell-derived cardiac tissues allow anti-fibrotic drug screening in vitro

Drug efficacy assessment without using animals is important for development of cardiac fibrosis treatment. In this study, potential anti-fibrotic drugs were screened in a model of diseased myocardium using human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CM) and non-CM in in vitro and in vivo heart failure models. Cardiomyogenic differentiation was induced in hiPSC to generate cardiac tissue, including both iPSC-CM and non-CM expressing fibroblast markers. Stimulation with TGF-β significantly increased cardiac fibrotic extracellular matrix (ECM) gene expression, and decreased cardiac contractile/relaxation velocity. Anti-fibrotic HGF significantly decreased fibrotic changes induced by TGF-β. A prostacyclin agonist, ONO-1301 (ONO), camostat mesilate (Cs), and pirfenidone (Pf) significantly decreased fibrotic ECM expression, and improved contraction/relaxation in the model stimulated with TGF-β. Consistent with the in vitro assay, the administration of ONO, Cs, or Pf for 8 weeks in J2N-k hamsters preserved the left ventricular ejection fraction and decreased cardiac fibrosis compared with the controls. The in vitro model simulating fibrotic cardiac tissue showed precise screening of anti-fibrotic drugs which indicated the expected therapeutic response in an in vivo heart failure model, suggesting that the in vitro model presented in this study is a useful tool for the screening of anti-fibrotic drugs.

Tissue-Specific Hydrogels Ameliorate Hepatic Ischemia/Reperfusion Injury in Rats by Regulating Macrophage Polarization via TLR4/NF-κB Signaling

Injectable acellular matrix hydrogels are proven to be potential translational materials to facilitate the repairment in various tissues. However, their potential to repair hepatic ischemia/reperfusion injury (IRI) has not been explored. In this work, we made hepatic acellular matrix (HAM) hydrogels based on the decellularized process and evaluated the biocompatibility and hepatoprotective effects in a rat IRI model. HAM hydrogels supported viability, proliferation, and attachment of hepatocytes in vitro. Treatment with HAM hydrogels significantly attenuated hepatic damage caused by IRI, as evidenced by hepatic biochemistry, histology, and inflammatory responses. Importantly, HAM hydrogels inhibited macrophage M1 (CD68/CCR7) differentiation but promoted M2 (CD68/CD206) differentiation. Additionally, TLR4/NF-κB signaling was found to be involved in the hepatoprotective effect of HAM hydrogels. Collectively, our study reveals that HAM hydrogels ameliorate hepatic IRI by facilitating M2 polarization via TLR4/NF-κB signaling.

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.

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.

Regenerative medicine for the hepatobiliary system: A review

Liver transplantation, the only proven treatment for end-stage liver disease and acute liver failure, is hampered by the scarcity of donors. Regenerative medicine provides an alternative therapeutic approach. Tremendous efforts dedicated to liver regenerative medicine include the delivery of transplantable cells, microtissues, and bioengineered whole livers via tissue engineering and the maintenance of partial liver function via extracorporeal support. This brief review summarizes the current status of regenerative medicine for the hepatobiliary system. For liver regenerative medicine, the focus is on strategies for expansion of transplantable hepatocytes, generation of hepatocyte-like cells, and therapeutic potential of engineered tissues in liver disease models. For biliary regenerative medicine, the discussion concentrates on the methods for generation of cholangiocyte-like cells and strategies in the treatment of biliary disease. Significant advances have been made in large-scale and long-term expansion of liver cells. The development of tissue engineering and stem cell induction technology holds great promise for the future treatment of hepatobiliary diseases. The application of regenerative medicine in liver still lacks extensive animal experiments. Therefore, a large number of preclinical studies are necessary to provide sufficient evidence for their therapeutic effectiveness. Much remains to be done for the treatment of hepatobiliary diseases with regenerative medicine.

Engineered Liver Tissue Culture in an In Vitro Tubular Perfusion System

Liver disease and the subsequent loss of liver function is an enormous clinical challenge. A severe shortage of donor liver tissue greatly limits patients' options for a timely transplantation. Tissue engineering approaches offer a promising alternative to organ transplantation by engineering artificial implantable tissues. We have established a platform of cell-laden microbeads as basic building blocks to assemble macroscopic tissues via different mechanisms. This modular fabrication strategy possesses great potential for liver tissue engineering in a bottom-up manner. In this study, we encapsulated human hepatocytes into microbeads presenting a favorable microenvironment consisting of collagen and mesenchymal stem cells, and then we perfused the beads in a three-dimensional printed tubular perfusion bioreactor that promoted oxygen and medium diffusion to the impregnated cells. We noted high cell vitality and retention of parenchymal cell functionality for up to 30 days in this culture system. Our engineering-based approach led to the advancement in tissue size and long-term functionality of an artificial liver tissue in vitro.

Engineering liver microtissues for disease modeling and regenerative medicine

The burden of liver diseases is increasing worldwide, accounting for two million deaths annually. In the past decade, tremendous progress has been made in the basic and translational research of liver tissue engineering. Liver microtissues are small, three-dimensional hepatocyte cultures that recapitulate liver physiology and have been used in biomedical research and regenerative medicine. This review summarizes recent advances, challenges, and future directions in liver microtissue research. Cellular engineering approaches are used to sustain primary hepatocytes or produce hepatocytes derived from pluripotent stem cells and other adult tissues. Three-dimensional microtissues are generated by scaffold-free assembly or scaffold-assisted methods such as macroencapsulation, droplet microfluidics, and bioprinting. Optimization of the hepatic microenvironment entails incorporating the appropriate cell composition for enhanced cell-cell interactions and niche-specific signals, and creating scaffolds with desired chemical, mechanical and physical properties. Perfusion-based culture systems such as bioreactors and microfluidic systems are used to achieve efficient exchange of nutrients and soluble factors. Taken together, systematic optimization of liver microtissues is a multidisciplinary effort focused on creating liver cultures and on-chip models with greater structural complexity and physiological relevance for use in liver disease research, therapeutic development, and regenerative medicine.

Liver regeneration and alcoholic liver disease

Alcoholic liver diseases (ALD) are a wide spectrum of liver diseases caused by excessive alcohol consumption, from steatosis to cirrhosis. The pathogenesis of ALD is insufficiently understood, but mainly involves oxidative stress, inflammation, bacterial translocation, cell death, and impaired regeneration. Despite numerous attempts to improve patient prognosis, the treatment of advanced ALD is still based on abstinence, brief exposure to corticosteroids, or liver transplantation. However, poor response to corticosteroids and the shortage of liver donors leaves patients helpless towards the end stages. Advances in basic research have contributed to a better understanding of ALD pathophysiology, which offers new options for treatment. In recent years, several therapies related to liver regeneration have been tested with promising prospects, including molecule-induced liver regeneration, stem cell transplantation, and full-function 3D artificial liver assembly. This review discusses mechanisms underlying ALD that can be considered therapeutic targets for regeneration-based treatments.

Regulated differentiation of stem cells into an artificial 3D liver as a transplantable source

End-stage liver disease is one of the leading causes of death around the world. Since insufficient sources of transplantable liver and possible immune rejection severely hinder the wide application of conventional liver transplantation therapy, artificial three-dimensional (3D) liver culture and assembly from stem cells have become a new hope for patients with end-stage liver diseases, such as cirrhosis and liver cancer. However, the induced differentiation of single-layer or 3D-structured hepatocytes from stem cells cannot physiologically support essential liver functions due to the lack of formation of blood vessels, immune regulation, storage of vitamins, and other vital hepatic activities. Thus, there is emerging evidence showing that 3D organogenesis of artificial vascularized liver tissue from combined hepatic cell types derived from differentiated stem cells is practical for the treatment of end-stage liver diseases. The optimization of novel biomaterials, such as decellularized matrices and natural macromolecules, also strongly supports the organogenesis of 3D tissue with the desired complex structure. This review summarizes new research updates on novel differentiation protocols of stem cell-derived major hepatic cell types and the application of new supportive biomaterials. Future biological and clinical challenges of this concept are also discussed.

Liver Tissue Engineering as an Emerging Alternative for Liver Disease Treatment

Chronic liver diseases affect thousands of lives throughout the world every year. The shortage of liver donors for transplantation has been the main driving force to employ alternative methods such as liver tissue engineering (LTE) in fabricating a three-dimensional transplantable liver tissue or enhancing cell delivery techniques alleviating the need for liver donors. LTE consists of three components, cells, ECM (extracellular matrix), and signaling molecules, which we discuss the first and second. The three most common cell sources used in LTE are human and animal primary hepatocytes, and stem cells for different applications. Two major categories of ECM are used to mimic the microenvironment of these cells, named scaffolds and microbeads. Scaffolds have been made by numerous methods with a wide range of synthetic and natural biomaterials. Cell encapsulation has also been utilized by many polymeric biomaterials. To investigate their functions, many properties have been discussed in the literature, such as biochemical, geometrical, and mechanical properties, in both of these categories. Overall, LTE shows excellent potential in assisting hepatic disorders. However, some challenges exist that prevent the practical use of it clinically, making LTE an ongoing research subject in the scientific society.

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.

Impaired albumin function: a novel potential indicator for liver function damage?

Albumin is the most abundant plasma protein and albumin infusion is commonly used. Conventionally, the biologic and therapeutic effects of albumin have been thought to be due to its oncotic properties. However, albumin has a variety of biologic functions, including molecular transport, anti-oxidation, anti-inflammation, endothelial stabilisation, anti-thrombotic effects, and the adjustment of capillary permeability. Despite this, the functions of albumin have not been thoroughly investigated. Recent studies have shown non-alcoholic fatty liver disease (NAFLD), viral hepatitis, cirrhosis, and liver failure to be associated with impairments in albumin function, which are associated with impairments in liver function and disease prognosis. Post-translational modifications of albumin cause structural modifications that affect protein function. Recently, the concentration of albumin associated with normal function, the 'efficient albumin concentration', has been attracting more interest. In addition, although many biologic markers, including albumin concentration, are widely used for the assessment of early liver dysfunction in patients with liver diseases, the predictive values are unsatisfactory. However, clinical evidence has suggested that albumin function may represent a novel biomarker of early impairment in liver function. In this review, we summarise the factors affecting albumin function and discuss the clinical significance of impairments in albumin function in various liver diseases.Key messagesThe importance of albumin depends not only on its concentration, but also on its various physiological functions.Impaired albumin function has been reported in a variety of liver diseases, and is associated with disease severity and prognosis, thereby proposing the concept of 'effective albumin concentration'.Albumin dysfunction occurs earlier than other conventional indicators, and albumin dysfunction may be a new biomarker of early impairment in liver function.Many exogenous and endogenous factors lead to post-translational modifications of albumin, which alters the three-dimensional structure of albumin, resulting in a decrease in its biological activity.

Upgrading prevascularization in tissue engineering: A review of strategies for promoting highly organized microvascular network formation

Functional and perfusable vascular network formation is critical to ensure the long-term survival and functionality of engineered tissues after their transplantation. Although several vascularization strategies have been reviewed in past, the significance of microvessel organization in three-dimensional (3D) scaffolds has been largely ignored. Advances in high-resolution microscopy and image processing have revealed that the majority of tissues including cardiac, skeletal muscle, bone, and skin contain highly organized microvessels that orient themselves to align with tissue architecture for optimum molecular exchange and functional performance. Here, we review strategies to develop highly organized and mature vascular networks in engineered tissues, with a focus on electromechanical stimulation, surface topography, micro scaffolding, surface-patterning, microfluidics and 3D printing. This review will provide researchers with state of the art approaches to engineer vascularized functional tissues for diverse applications. STATEMENT OF SIGNIFICANCE: Vascularization is one of the critical challenges facing tissue engineering. Recent technological advances have enabled researchers to develop microvascular networks in engineered tissues. Although far from translational applications, current vascularization strategies have shown promising outcomes. This review emphasizes the most recent technological advances and future challenges for developing organized microvascular networks in vitro. The next critical step is to achieve highly perfusable, dense, mature and organized microvascular networks representative of native tissues.

Advances in Human Induced Pluripotent Stem Cell-Derived Hepatocytes for Use in Toxicity Testing

Induced pluripotent stem cells (iPSCs) can be differentiated into multiple cell types in the body while maintaining proliferative capabilities. The generation of hepatocyte-like cells (HLCs) from iPSCs has resulted in a new source for liver cells. Since healthy primary human hepatocytes and hepatic cells are difficult to obtain, HLCs are gaining attention. HLCs can be obtained from a continuous, stable source while maintaining their original donor genotype, which opens new avenues into patient-specific testing and therapeutics. Studies have utilized HLCs for toxicity testing to further understand their drug metabolizing capabilities. This review focuses on advances being made to achieve hepatic functions from HLCs, their current use in hepatotoxicity testing, and their potential for future liver-related toxicity evaluations.

Generation of fully functional hepatocyte-like organoids from human induced pluripotent stem cells mixed with Endothelial Cells

Despite advances in stem cell research, cell transplantation therapy for liver failure is impeded by a shortage of human primary hepatocytes (HPH), along with current differentiation protocol limitations. Several studies have examined the concept of co-culture of human induced pluripotent cells (hiPSCs) with various types of supporting non-parenchymal cells to attain a higher differentiation yield and to improve hepatocyte-like cell functions both in vitro and in vivo. Co-culturing hiPSCs with human endothelial cells (hECs) is a relatively new technique that requires more detailed studies. Using our 3D human embryoid bodies (hEBs) formation technology, we interlaced Human Adipose Microvascular Endothelial Cells (HAMEC) with hiPSCs, leading to a higher differentiation yield and notable improvements across a wide range of hepatic functions. We conducted a comprehensive gene and protein secretion analysis of our HLCs coagulation factors profile, showing promising results in comparison with HPH. Furthermore, a stage-specific glycomic analysis revealed that the differentiated hepatocyte-like clusters (HLCs) resemble the glycan features of a mature tissue rather than cells in culture. We tested our HLCs in animal models, where the presence of HAMEC in the clusters showed a consistently better performance compared to the hiPSCs only group in regard to persistent albumin secretion post-transplantation.

Three-dimensional bioprinting of stem-cell derived tissues for human regenerative medicine

Stem cell technology in regenerative medicine has the potential to provide an unlimited supply of cells for drug testing, medical transplantation and academic research. In order to engineer a realistic tissue model using stem cells as an alternative to human tissue, it is essential to create artificial stem cell microenvironment or niches. Three-dimensional (3D) bioprinting is a promising tissue engineering field that offers new opportunities to precisely place stem cells within their niches layer-by-layer. This review covers bioprinting technologies, the current development of 'bio-inks' and how bioprinting has already been applied to stem-cell culture, as well as their applications for human regenerative medicine. The key considerations for bioink properties such as stiffness, stability and biodegradation, biocompatibility and printability are highlighted. Bioprinting of both adult and pluriopotent stem cells for various types of artificial tissues from liver to brain has been reviewed. 3D bioprinting of stem-cell derived tissues for human regenerative medicine is an exciting emerging area that represents opportunities for new research, industries and products as well as future challenges in clinical translation.This article is part of the theme issue 'Designer human tissue: coming to a lab near you'.

Microphysiological systems meet hiPSC technology - New tools for disease modeling of liver infections in basic research and drug development

Complex cell culture models such as microphysiological models (MPS) mimicking human liver functionality in vitro are in the spotlight as alternative to conventional cell culture and animal models. Promising techniques like microfluidic cell culture or micropatterning by 3D bioprinting are gaining increasing importance for the development of MPS to address the needs for more predictivity and cost efficiency. In this context, human induced pluripotent stem cells (hiPSCs) offer new perspectives for the development of advanced liver-on-chip systems by recreating an in vivo like microenvironment that supports the reliable differentiation of hiPSCs to hepatocyte-like cells (HLC). In this review we will summarize current protocols of HLC generation and highlight recently established MPS suitable to resemble physiological hepatocyte function in vitro. In addition, we are discussing potential applications of liver MPS for disease modeling related to systemic or direct liver infections and the use of MPS in testing of new drug candidates.

Bone Tissue Engineering Using Human Cells: A Comprehensive Review on Recent Trends, Current Prospects, and Recommendations

The use of proper cells for bone tissue engineering remains a major challenge worldwide. Cells play a pivotal role in the repair and regeneration of the bone tissue in vitro and in vivo. Currently, a large number of differentiated (somatic) and undifferentiated (stem) cells have been used for bone reconstruction alone or in combination with different biomaterials and constructs (e.g., scaffolds). Although the results of the cell transplantation without any supporting or adjuvant material have been very effective with regard to bone healing. Recent advances in bone scaffolding are now becoming new players affecting the osteogenic potential of cells. In the present study, we have critically reviewed all the currently used cell sources for bone reconstruction and discussed the new horizons that are opening up in the context of cell-based bone tissue engineering strategies.

Bone physiology as inspiration for tissue regenerative therapies

The development, maintenance of healthy bone and regeneration of injured tissue in the human body comprise a set of intricate and finely coordinated processes. However, an analysis of current bone regeneration strategies shows that only a small fraction of well-reported bone biology aspects has been used as inspiration and transposed into the development of therapeutic products. Specific topics that include inter-scale bone structural organization, developmental aspects of bone morphogenesis, bone repair mechanisms, role of specific cells and heterotypic cell contact in the bone niche (including vascularization networks and immune system cells), cell-cell direct and soluble-mediated contact, extracellular matrix composition (with particular focus on the non-soluble fraction of proteins), as well as mechanical aspects of native bone will be the main reviewed topics. In this Review we suggest a systematic parallelization of (i) fundamental well-established biology of bone, (ii) updated and recent advances on the understanding of biological phenomena occurring in native and injured tissue, and (iii) critical discussion of how those individual aspects have been translated into tissue regeneration strategies using biomaterials and other tissue engineering approaches. We aim at presenting a perspective on unexplored aspects of bone physiology and how they could be translated into innovative regeneration-driven concepts.

Impact of Three-Dimentional Culture Systems on Hepatic Differentiation of Puripotent Stem Cells and Beyond

Generation of functional hepatocytes from human pluripotent stem cells (hPSCs) is a vital tool to produce large amounts of human hepatocytes, which hold a great promise for biomedical and regenerative medicine applications. Despite a tremendous progress in developing the differentiation protocols recapitulating the developmental signalling and stages, these resulting hepatocytes from hPSCs yet achieve maturation and functionality comparable to those primary hepatocytes. The absence of 3D milieu in the culture and differentiation of these hepatocytes may account for this, at least partly, thus developing an optimal 3D culture could be a step forward to achieve this aim. Hence, review focuses on current development of 3D culture systems for hepatic differentiation and maturation and the future perspectives of its application.

The contributions of mesoderm-derived cells in liver development

The liver is an indispensable organ for metabolism and drug detoxification. The liver consists of endoderm-derived hepatobiliary lineages and various mesoderm-derived cells, and interacts with the surrounding tissues and organs through the ventral mesentery. Liver development, from hepatic specification to liver maturation, requires close interactions with mesoderm-derived cells, such as mesothelial cells, hepatic stellate cells, mesenchymal cells, liver sinusoidal endothelial cells and hematopoietic cells. These cells affect liver development through precise signaling events and even direct physical contact. Through the use of new techniques, emerging studies have recently led to a deeper understanding of liver development and its related mechanisms, especially the roles of mesodermal cells in liver development. Based on these developments, the current protocols for in vitro hepatocyte-like cell induction and liver-like tissue construction have been optimized and are of great importance for the treatment of liver diseases. Here, we review the roles of mesoderm-derived cells in the processes of liver development, hepatocyte-like cell induction and liver-like tissue construction.

It's All in the Delivery: Designing Hydrogels for Cell and Non-viral Gene Therapies

Hydrogels provide a regenerative medicine platform with their ability to create an environment that supports transplanted or endogenous infiltrating cells and enables these cells to restore or replace the function of tissues lost to disease or trauma. Furthermore, these systems have been employed as delivery vehicles for therapeutic genes, which can direct and/or enhance the function of the transplanted or endogenous cells. Herein, we review recent advances in the development of hydrogels for cell and non-viral gene delivery through understanding the design parameters, including both physical and biological components, on promoting transgene expression, cell engraftment, and ultimately cell function. Furthermore, this review identifies emerging opportunities for combining cell and gene delivery approaches to overcome challenges to the field.

In Vitro Pluripotent Stem Cell Differentiation to Hepatocyte Ceases Further Maturation at an Equivalent Stage of E15 in Mouse Embryonic Liver Development

Hepatocyte-like cells (HLCs) can be derived from pluripotent stem cells (PSCs) by sequential treatment of chemical cues to mimic the microenvironment of embryonic liver development. However, these HLCs do not reach the full maturity level of primary hepatocytes. In this study, we carried out a meta-analysis of cross-species transcriptome data of in vitro differentiation of human PSCs to HLCs and in vivo mouse embryonic liver development to identify the developmental stage at which HLC maturation was blocked at. Systematic variations were found associated with the data source and removed by batch correction. Using principal component analysis, HLCs from different stages of differentiation were aligned with mouse embryonic liver development chronologically. A "unified developmental time" (DT) scale was developed after aligning in vitro HLC differentiation and in vivo embryonic liver development. HLCs were found to cease further maturation at an equivalent stage of mouse embryonic day (E)13-15. Genes with discordant time dynamics were identified by aligning in vivo and in vitro data set onto a common DT scale. These genes may be targets of genetic intervention for enhancing the maturity of PSC-derived HLCs.

Expansion of human primary hepatocytes in vitro through their amplification as liver progenitors in a 3D organoid system

Despite decades of investigation on the proliferation of adult human primary hepatocytes, their expansion in vitro still remains challenging. To later be able to consider hepatocytes as a cell therapy alternative or bridge to liver transplantation, dramatically impeded by a shortage in liver donors, the first step is having an almost unlimited source of these cells. The banking of transplantable hepatocytes also implies a protocol for their expansion that can be compatible with large-scale production. We show that adult human primary hepatocytes when grown in 3D organoids are easily amplified, providing a substantial source of functional hepatocytes ready for transplantation. Following their plating, differentiated human hepatocytes are amplified during a transient and reversible step as liver progenitors, and can subsequently be converted back to mature differentiated hepatocytes. The protocol we propose is not only compatible with automated and high-throughput cell culture systems, thanks to the expansion of hepatocytes in suspension, but also guarantees the generation of a high number of functional cells from the same patient sample, with a relatively easy set up.

In Situ Organ-Specific Vascularization in Tissue Engineering

Other than a few avascular tissues, almost all human tissues are connected to the systemic circulation via blood vessels that promote metabolism and function. Accordingly, engineered vascularization is a vital goal in tissue engineering for regenerative medicine. Endothelial cells (ECs) play a central role in vascularization with two significant specificities: physical interfaces between vascular stroma and blood, and phenotypic organ-specificity. Biomaterial scaffolding technologies that address these unique properties of ECs have been developed to promote the vascularization of various engineered tissues, and these have advanced from mimicking vascular architectures ex situ towards promoting spontaneous angiogenic remodeling in situ. Simultaneously, endothelial progenitor cells (EPCs) and organ-specific ECs are attracting more and more attention with the increasing awareness of the diversity of ECs in different organs.

Three-dimensional hydrogel culture conditions promote the differentiation of human induced pluripotent stem cells into hepatocytes

BACKGROUND AIMS

Human induced pluripotent stem cells (hiPSCs) are becoming increasingly popular in research endeavors due to their potential for clinical application; however, such application is challenging due to limitations such as inferior function and low induction efficiency. In this study, we aimed to establish a three-dimensional (3D) culture condition to mimic the environment in which hepatogenesis occurs in vivo to enhance the differentiation of hiPSCs for large-scale culture and high throughput BAL application.

METHODS

We used hydrogel to create hepatocyte-like cell (HLC) spheroids in a 3D culture condition and analyzed the cell-behavior and differentiation properties of hiPSCs in a synthetic nanofiber scaffold.

RESULTS

We found that treating cells with Y-27632 promoted the formation of spheroids, and the cells aggregated more rapidly in a 3D culture condition. The ALB secretion, urea production and glycogen synthesis by HLCs in 3D were significantly higher than those grown in a 2-dimensional culture condition. In addition, the metabolic activities of the CYP450 enzymes were also higher in cells differentiated in the 3D culture condition.

CONCLUSIONS

3D hydrogel culture condition can promote differentiation of hiPSCs into hepatocytes. The 3D culture approach could be applied to the differentiation of hiPSCs into hepatocytes for bioartificial liver.

Organ-On-A-Chip Platforms: A Convergence of Advanced Materials, Cells, and Microscale Technologies

Significant advances in biomaterials, stem cell biology, and microscale technologies have enabled the fabrication of biologically relevant tissues and organs. Such tissues and organs, referred to as organ-on-a-chip (OOC) platforms, have emerged as a powerful tool in tissue analysis and disease modeling for biological and pharmacological applications. A variety of biomaterials are used in tissue fabrication providing multiple biological, structural, and mechanical cues in the regulation of cell behavior and tissue morphogenesis. Cells derived from humans enable the fabrication of personalized OOC platforms. Microscale technologies are specifically helpful in providing physiological microenvironments for tissues and organs. In this review, biomaterials, cells, and microscale technologies are described as essential components to construct OOC platforms. The latest developments in OOC platforms (e.g., liver, skeletal muscle, cardiac, cancer, lung, skin, bone, and brain) are then discussed as functional tools in simulating human physiology and metabolism. Future perspectives and major challenges in the development of OOC platforms toward accelerating clinical studies of drug discovery are finally highlighted.

A decade of progress in liver regenerative medicine

Liver diseases can be caused by viral infection, metabolic disorder, alcohol consumption, carcinoma or injury, chronically progressing to end-stage liver disease or rapidly resulting in acute liver failure. In either situation, liver transplantation is most often sought for life saving, which is, however, significantly limited by severe shortage of organ donors. Until now, tremendous multi-disciplinary efforts have been dedicated to liver regenerative medicine, aiming at providing transplantable cells, microtissues, or bioengineered whole liver via tissue engineering, or maintaining partial liver functions via extracorporeal support. In both directions, new compatible biomaterials, stem cell sources, and bioengineering approaches have fast-forwarded liver regenerative medicine towards potential clinical applications. Another important progress in this field is the development of liver-on-a-chip technologies, which enable tissue engineering, disease modeling, and drug testing under biomimetic extracellular conditions. In this review, we aim to highlight the last decade's progress in liver regenerative medicine from liver tissue engineering, bioartificial liver devices (BAL), to liver-on-a-chip platforms, and then to present challenges ahead for further advancement.

Differentiation, Evaluation, and Application of Human Induced Pluripotent Stem Cell-Derived Endothelial Cells

The emergence of induced pluripotent stem cell (iPSC) technology paves the way to generate large numbers of patient-specific endothelial cells (ECs) that can be potentially delivered for regenerative medicine in patients with cardiovascular disease. In the last decade, numerous protocols that differentiate EC from iPSC have been developed by many groups. In this review, we will discuss several common strategies that have been optimized for human iPSC-EC differentiation and subsequent studies that have evaluated the potential of human iPSC-EC as a cell therapy or as a tool in disease modeling. In addition, we will emphasize the importance of using in vivo vessel-forming ability and in vitro clonogenic colony-forming potential as a gold standard with which to evaluate the quality of human iPSC-EC derived from various protocols.