Biomaterial–Related Cell Microenvironment in Tissue Engineering and Regenerative Medicine

An appropriate cell microenvironment is key to tissue engineering and regenerative medicine.Revealing the factors that influence the cell microenvironment is a fundamental research topic in the fields of cell biology,biomaterials,tissue engineering,and regenerative medicine.The cell microenvironment consists of not only its surrounding cells and soluble factors,but also its extracellular matrix(ECM)or nearby external biomaterials in tissue engineering and regeneration.This review focuses on six aspects of bioma-terial-related cell microenvironments:①chemical composition of materials,②material dimensions and architecture,③material-controlled cell geometry,④effects of material charges on cells,⑤matrix stiff-ness and biomechanical microenvironment,and⑥surface modification of materials.The present chal-lenges in tissue engineering are also mentioned,and eight perspectives are predicted.

How regenerative medicine and tissue engineering may complement the available armamentarium in gastroenterology?

The increasing shortage of donors and the adverse effects of immunosuppression have restricted the impact of solid organ transplantation.Despite the initial promising developments in xenotransplantation,roadblocks still need to be overcome and this form of organ support remains a long way from clinical practice.While hepatocyte transplantation may be effectively correct metabolic defects,it is far less effective in restoring liver function than liver transplantation.Tissue engineering,using extracellular matrix scaffolds with an intact but decellularized vascular network that is repopulated with autologous or allogeneic stem cells and/or adult cells,holds great promise for the treatment of failure of organs within gastrointestinal tract,such as endstage liver disease,pancreatic insufficiency,bowel failure and type 1 diabetes.Particularly in the liver field,where there is a significant mortality of patients awaiting transplant,human bioengineering may offer a source of readily available organs for transplantation.The use of autologous cells will mitigate the need for long term immunosuppression thus removing a major hurdle in transplantation.

Novel advancements in threedimensional neural tissue engineering and regenerative medicine

Neurological diseases and injuries present some of the great- est challenges in modern medicine, often causing irrevers- ible and lifelong burdens in the people whom they afflict. Conditions of stroke, traumatic brain injury, spinal cord injury, and neurodegenerative diseases have devastating con- sequences on millions of people each year, and yet there are currently no therapies or interventions that can repair the structure of neural circuits and restore neural tissue function in the brain and spinal cord. Despite the challenges of over- coming these limitations, there are many new approaches under development that hold much promise. Neural tissue engineering aims to restore and influence the function of damaged or diseased neural tissue generally through the use of stem cells and biomaterials. Many types of biomaterials may be implemented in various designs to influence the survival, differentiation, and function of developing stem cells, as well as to guide neurite extension and morphological architecture of cell cultures. Such designs may aim to reca- pitulate the cellular interactions, extracellular matrix char- acteristics, biochemical factors, and sequences of events that occur in neurodevelopment, in addition to supporting cell survival, differentiation, and integration into innate neural tissue.

Engineering vascularized organotypic tissues via module assembly

Adequate vascularization is a critical determinant for the successful construction and clinical implementation of complex organotypic tissue models. Currently, low cell and vessel density and insufficient vascular maturation make vascularized organotypic tissue construction difficult,greatly limiting its use in tissue engineering and regenerative medicine. To address these limitations, recent studies have adopted pre-vascularized microtissue assembly for the rapid generation of functional tissue analogs with dense vascular networks and high cell density. In this article, we summarize the development of module assembly-based vascularized organotypic tissue construction and its application in tissue repair and regeneration, organ-scale tissue biomanufacturing, as well as advanced tissue modeling.

Bone Regeneration Based on Tissue Engineering Conceptions – A 21st Century Perspective

The role of Bone Tissue Engineering in the field of Regenerative Medicine has been the topic of substantial research over the past two decades. Technological advances have improved orthopaedic implants and surgical techniques for bone reconstruction. However, improvements in surgical techniques to reconstruct bone have been limited by the paucity of autologous materials available and donor site morbidity. Recent advances in the development of biomaterials have provided attractive alternatives to bone grafting expanding the surgical options for restoring the form and function of injured bone. Specifically, novel bioactive （second generation） biomaterials have been developed that are characterised by controlled action and reaction to the host tissue environment, whilst exhibiting controlled chemical breakdown and resorption with an ultimate replacement by regenerating tissue. Future generations of biomaterials （third generation） are designed to be not only osteo- conductive but also osteoinductive, i.e. to stimulate regeneration of host tissues by combining tissue engineer- ing and in situ tissue regeneration methods with a focus on novel applications. These techniques will lead to novel possibilities for tissue regeneration and repair. At present, tissue engineered constructs that may find future use as bone grafts for complex skeletal defects, whether from post-traumatic, degenerative, neoplastic or congenital/developmental ＂origin＂ require osseous reconstruction to ensure structural and functional integrity. Engineering functional bone using combinations of cells, scaffolds and bioactive factors is a promising strategy and a particular feature for future development in the area of hybrid materials which are able to exhibit suitable biomimetic and mechanical properties. This review will discuss the state of the art in this field and what we can expect from future generations of bone regeneration concepts.

WJSC 6^(th) Anniversary Special Issues(2):Mesenchymal stem cellsAdipose mesenchymal stem cells in the field of bone tissue engineering

Bone tissue engineering represents one of the most challenging emergent fields for scientists and clinicians.Current failures of autografts and allografts in many pathological conditions have prompted researchers to find new biomaterials able to promote bone repair or regeneration with specific characteristics of biocompatibility,biodegradability and osteoinductivity.Recent advancements for tissue regeneration in bone defects have occurred by following the diamond concept and combining the use of growth factors and mesenchymal stem cells(MSCs).In particular,a more abundant and easily accessible source of MSCs was recently discovered in adipose tissue.These adipose stem cells(ASCs)can be obtained in large quantities with little donor site morbidity or patient discomfort,in contrast to the invasive and painful isolation of bone marrow MSCs.The osteogenic potential of ASCs on scaffolds has been examined in cell cultures and animal models,with only a few cases reporting the use of ASCs for successful reconstruction or accelerated healing of defects of the skull and jaw in patients.Although these reports extend our limited knowledge concerning the use of ASCs for osseous tissue repair and regeneration,the lack of standardization in applied techniques makes the comparison between studies difficult.Additional clinical trials are needed to assess ASC therapy and address potential ethical and safety concerns,which must be resolved to permit application in regenerative medicine.

WJSC 6^(th) Anniversary Special Issues(2):Mesenchymal stem cells “Ins” and “Outs” of mesenchymal stem cell osteogenesis in regenerative medicine

Repair and regeneration of bone requires mesenchymal stem cells that by self-renewal,are able to generate a critical mass of cells with the ability to differentiate into osteoblasts that can produce bone protein matrix(osteoid)and enable its mineralization.The number of human mesenchymal stem cells(hMSCs)diminishes with age and ex vivo replication of hMSCs has limited potential.While propagating hMSCs under hypoxic conditions may maintain their ability to self-renew,the strategy of using human telomerase reverse transcriptase(hTERT)to allow for hMSCs to prolong their replicative lifespan is an attractive means of ensuring a critical mass of cells with the potential to differentiate into various mesodermal structural tissues including bone.However,this strategy must be tempered by the oncogenic potential of TERT-transformed cells,or their ability to enhance already established cancers,the unknown differentiating potential of high population doubling hMSCs and the source of hMSCs(e.g.,bone marrow,adipose-derived,muscle-derived,umbilical cord blood,etc.)that may provide peculiarities to self-renewal,differentiation,and physiologic function that may differ from non-transformed native cells.Tissue engineering approaches to use hMSCs to repair bone defects utilize the growth of hMSCs on three-dimensional scaffolds that can either be a base on which hMSCs can attach and grow or as a means of sequestering growth factors to assist in the chemoattraction and differentiation of native hMSCs.The use of whole native extracellular matrix(ECM)produced by hMSCs,rather than individual ECM components,appear to be advantageous in not only being utilized as a three-dimensional attachment base but also in appropriate orientation of cells and their differentiation through the growth factors that native ECM harbor or in simulating growth factor motifs.The origin of native ECM,whether from hMSCs from young or old individuals is a critical factor in"rejuvenating"hMSCs from older individuals grown on ECM from younger individuals.

Adipose stromal/stem cells in regenerative medicine:Potentials and limitations

This article presents the stem and progenitor cells from subcutaneous adipose tissue,briefly comparing them with their bone marrow counterparts,and discussing their potential for use in regenerative medicine.Subcutaneous adipose tissue differs from other mesenchymal stromal/stem cells(MSCs)sources in that it contains a pre-adipocyte population that dwells in the adventitia of robust blood vessels.Pre-adipocytes are present both in the stromal-vascular fraction(SVF;freshly isolated cells)and in the adherent fraction of adipose stromal/stem cells(ASCs;in vitro expanded cells),and have an active role on the chronic inflammation environment established in obesity,likely due their monocyticmacrophage lineage identity.The SVF and ASCs have been explored in cell therapy protocols with relative success,given their paracrine and immunomodulatory effects.Importantly,the widely explored multipotentiality of ASCs has direct application in bone,cartilage and adipose tissue engineering.The aim of this editorial is to reinforce the peculiarities of the stem and progenitor cells from subcutaneous adipose tissue,revealing the spheroids as a recently described biotechnological tool for cell therapy and tissue engineering.Innovative cell culture techniques,in particular 3D scaffold-free cultures such as spheroids,are now available to increase the potential for regeneration and differentiation of mesenchymal lineages.Spheroids are being explored not only as a model for cell differentiation,but also as powerful 3D cell culture tools to maintain the stemness and expand the regenerative and differentiation capacities of mesenchymal cell lineages.

Can the venerated silk be the next generation nanobiomaterialfor biomedical device designing, regenerative medicine and drugdelivery? Prospects and hitches

Of late, the relevance of silk in a myriad of material science and biotechnological realms has been realized, as attested by the incessantly clambering number of reports and patents in the scienti fic repositories. The write-up is geared off with a scrutiny into the pertinence of the basic nano-structural features of silk, christened as the ‘queen of textile’ for exemplary bioengi- neering applications including designing and fabrication of devices for micro fluidics, opto fluidics, chemo/bio sensing, etc. Then, the major thrust of this short review is directed towards comprehending the prospects of using silk-based biomaterials (e.g. sca ffolds, electrospun membranes, films, hydrogels, bioinks) for tissue engineering and regenerative medicine as well as targeted delivery of various biomolecular cargoes/therapeutic agents, etc., as vouched by few avant-garde endeavours of the recent years. The write-up is entwined with a discussion on the various factors that could plausibly hinder the realization of silk as the next-generation nanobiomaterial, suggestions for some approaches to dodge and deal with the practical snags and what lies ahead!

Regenerative Medicine in Orthopaedics: Microsurgery Achievements for Translational Animal Model

Purpose: Despite many scientific advances, Regenerative Medicine is still in the preclinical stages in many areas. In this article, we intend to discuss the role of microsurgery in the bench-to-bedside transition of such primary findings. Method: By searching the papers related to the history of Regenerative Medicine (RM) and the news of Tissue Engineering (TE) in orthopedics in Pubmed, Scopus, and Google Scholar databases, we accessed a complete archive of various topics related to this field. Result: We first assessed the history and achievements of regenerative medicine, then we realized the importance of translational medical sciences and the role of animal models in this incipient phenomenon. Finally, after mastering the capabilities of microsurgery and the useful contribution of this technique to the advancement of clinical applications of regenerative medicine in various branches such as skin, skeletal system, nerves, and blood vessels, we decided to express the gist of our studies through this article. Conclusion: Considering the widespread use of small animals in regenerative medicine projects and the inevitable role of microsurgery in performing the best intervention on these animal models, the significant progress of regenerative medicine clinical application requires special attention to microsurgery in associated research.

Modifying oxygen tension affects bone marrow stromal cell osteogenesis for regenerative medicine

AIM To establish a hypoxic environment for promoting osteogenesis in rat marrow stromal cells(MSCs) using osteogenic matrix cell sheets(OMCSs).METHODS Rat MSCs were cultured in osteogenic media under one of four varying oxygen conditions: Normoxia(21% O_2) for 14 d(NN), normoxia for 7 d followed by hypoxia(5% O_2) for 7 d(NH), hypoxia for 7 d followed by normoxia for 7 d(HN), or hypoxia for 14 d(HH). Osteogenesis was evaluated by observing changes in cell morphology and calcium deposition, and by measuring osteocalcin secretion(ELISA) and calcium content. In vivo syngeneic transplantation using OMCSs and β-tricalcium phosphate discs, preconditioned under NN or HN conditions, was also evaluated by histology, calcium content measurements,and real-time quantitative PCR.RESULTS In the NN and HN groups, differentiated, cuboidal-shaped cells were readily observed, along with calcium deposits. In the HN group, the levels of secreted osteocalcin increased rapidly from day 10 as compared with the other groups, and plateaued at day 12(P < 0.05). At day 14, the HN group showed the highest amount of calcium deposition. In vivo, the HN group showed histologically prominent new bone formation, increased calcium deposition, and higher collagen type Ⅰ?messenger RNA expression as compared with the NN group.CONCLUSION The results of this study indicate that modifying oxygen tension is an effective method to enhance the osteogenic ability of MSCs used for OMCSs.

Bioreactor design and validation for manufacturing strategies in tissue engineering

The fields of regenerative medicine and tissue engineering offer new therapeutic options to restore,maintain or improve tissue function following disease or injury.To maximize the biological function of a tissue-engineered clinical product,specific conditions must be maintained within a bioreactor to allow the maturation of the product in preparation for implantation.Specifically,the bioreactor should be designed to mimic the mechanical,electrochemical and biochemical environment that the product will be exposed to in vivo.Real-time monitoring of the functional capacity of tissue-engineered products during manufacturing is a critical component of the quality management process.The present review provides a brief overview of bioreactor engineering considerations.In addition,strategies for bioreactor automation,in-line product monitoring and quality assurance are discussed.

Stem cells therapy for regenerative medicine:Principles of present and future practice

The characterization and isolation of various stem cell populations, from embryonic to tissue-derived stem cells and induced pluripotent stem cells (iPSCs), have led to a rapid growth in the field of stem cell research and its potentially clinical application in the field of regenerative medicine and tissue repair. Stem cell therapy has recently progressed from the preclinical to the early clinical trial arena for a variety of diseases states, although further knowledge on action mechanisms, long-term safety issues, and standardization and characterization of the therapeutic cell products remains to be thoroughly elucidated. In this paper we summarize the current state of the art of basic and clinical research that were highlighted at the 2012 meeting of the Spanish Cell Therapy Network. This includes the current research involving in genomic and transcriptomic characterization of selected stem cell populations, studies of the role of resident and transplanted stem cells during tissue regeneration and their mechanism of action, improved new strategies of tissue engineering, transplantation of mesenchymal stem cells (MSCs) in different animal models of disease, disease correction by iPSCs, and preliminary results of cell therapy in human clinical trials.

Esophageal tissue engineering:A new approach for esophageal replacement

A number of congenital and acquired disorders require esophageal tissue replacement.Various surgical techniques,such as gastric and colonic interposition,are standards of treatment,but frequently complicated by stenosis and other problems.Regenerative medicine approaches facilitate the use of biological constructs to replace or regenerate normal tissue function.We review the literature of esophageal tissue engineering,discuss its implications,compare the methodologies that have been employed and suggest possible directions for the future.Medline,Embase,the Cochrane Library,National Research Register and ClinicalTrials.gov databases were searched with the following search terms:stem cell and esophagus,esophageal replacement,esophageal tissue engineering,esophageal substitution.Reference lists of papers identified were also examined and experts in this field contacted for further information.All full-text articles in English of all potentially relevant abstracts were reviewed.Tissue engineering has involved acellular scaffolds that were either transplanted with the aim of being repopulated by host cells or seeded prior to transplantation.When acellular scaffolds were used to replace patch and short tubular defects they allowed epithelial and partial muscular migration whereas when employed for long tubular defects the results were poor leading to an increased rate of stenosis and mortality.Stenting has been shown as an effective means to reduce stenotic changes and promote cell migration,whilst omental wrapping to induce vascularization of the construct has an uncertain benefit.Decellularized matrices have been recently suggested as the optimal choice for scaffolds,but smart polymers that will incorporate signalling to promote cell-scaffold interaction may provide a more reproducible and available solution.Results in animal models that have used seeded scaffolds strongly suggest that seeding of both muscle and epithelial cells on scaffolds prior to implantation is a prerequisite for complete esophageal replacement.Novel approaches need to be designed to allow for peristalsis and vascularization in the engineered esophagus.Although esophageal tissue engineering potentially offers a real alternative to conventional treatments for severe esophageal disease,important barriers remain that need to be addressed.

Regenerative medicine in China:main progress in different fields

Regenerative medicine(RM) is an emerging interdisciplinary field of research and China has developed the research quickly and impressed the world with numerous research findings in stem cells,tissue engineering,active molecules and gene therapy.Important directions are induced differentiation of induced pluripotent stem and embryo stem cells as well as somatic stem cell differentiation potential and their application in trauma,burns,diseases of aging and nerve regeneration.The products Activ Skin and bone repair scaffolds have been approved and are applied in the clinic,and similar products are being studied.About 10 engineered growth-factor drugs for repair and regeneration have been approved and are used in the clinic.Gene therapy,therapeutic cloning and xenotransplantation are some of the strategies being studied.However,China needs to develop standards,regulations and management practices suitable for the healthy development of RM.Aspects that should be strengthened include sound administrative systems,laws,and technical specifications and guidelines;conservation of stem cell resources;emphasis on training and retention of talented stem cell researchers;and reasonable allocation of resources,diversification of investment and breakthroughs in key areas.Finally,broad and deep international cooperation is necessary.

Repair cell first,then regenerate the tissues and organs

Wound healing,tissue repair and regenerative medicine are in great demand,and great achievements in these fields have been made.The traditional strategy of tissue repair and regeneration has focused on the level of tissues and organs directly;however,the basic process of repair at the cell level is often neglected.Because the cell is the basic unit of organism structure and function;cell damage is caused first by ischemia or ischemia-reperfusion after severe trauma and injury.Then,damage to tissues and organs occurs with massive cell damage,apoptosis and even cell death.Thus,how to achieve the aim of perfect repair and regeneration?The basic process of tissue or organ repair and regeneration should involve repair of cells first,then tissues and organs.In this manuscript,it is my consideration about how to repair the cell first,then regenerate the tissues and organs.

Liver bioengineering:Current status and future perspectives

The present review aims to illustrate the strategies that are being implemented to regenerate or bioengineer livers for clinical purposes.There are two general pathways to liver bioengineering and regeneration.The first consists of creating a supporting scaffold,either synthetically or by decellularization of human or animal organs,and seeding cells on the scaffold,where they will mature either in bioreactors or in vivo.This strategy seems to offer the quickest route to clinical translation,as demonstrated by the development of liver organoids from rodent livers which were repopulated with organ specific cells of animal and/or human origin.Liver bioengineering has potential for transplantation and for toxicity testing during preclinical drug development.The second possibility is to induce liver regeneration of dead or resected tissue by manipulating cell pathways.In fact,it is well known that the liver has peculiar regenerative potential which allows hepatocyte hyperplasia after amputation of liver volume.Infusion of autologous bone marrow cells,which aids in liver regeneration,into patients was shown to be safe and to improve their clinical condition,but the specific cells responsible for liver regeneration have not yet been determined and the underlying mechanisms remain largely unknown.A complete understanding of the cell pathways and dynamics and of the functioning of liver stem cell niche is necessary for the clinical translation of regenerative medicine strategies.As well,it will be crucial to elucidate the mechanisms through which cells interact with the extracellular matrix,and how this latter supports and drives cell fate.

New paradigms in regenerative engineering:Emerging role of extracellular vesicles paired with instructive biomaterials

Mesenchymal stem cells(MSCs)have long been regarded as critical components of regenerative medicine strategies,given their multipotency and persistence in a variety of tissues.Recently,the specific role of MSCs in mediating regenerative outcomes has been attributed(in part)to secreted factors from transplanted cells,namely extracellular vesicles.This viewpoint manuscript highlights the promise of cell-derived extracellular vesicles as agents of regeneration,enhanced by synergy with appropriate biomaterials platforms.Extracellular vesicles are a potentially interesting regenerative tool to enhance the synergy between MSCs and biomaterials.As a result,we believe these technologies will improve patient outcomes through efficient therapeutic strategies resulting in predictable patient outcomes.

中药诱导干细胞在骨组织工程学中应用的研究概况

随着组织工程学的飞速发展以及科研人员的不断深入研究发现,用中药诱导干细胞在组织工程学中修复骨缺损具有一定的优势。骨组织工程学因具有良好的组织相容性、生物降解材料降解性而成为了研究的热点。近年来,骨组织工程技术的不断发展为修复骨缺损带来了希望,而干细胞移植是组织工程学中非常重要的环节之一,因其具有能够在特定条件下分化为不同类型细胞的能力,所以对于组织器官的损伤修复具有重要意义。而单纯利用干细胞治疗骨缺损疾病则存在治疗周期延长、靶向性不高等缺陷。中药参与治疗可促进干细胞增殖、诱导干细胞定向分化、促进移植后的干细胞定向迁移,从而缩短治疗周期并增强靶向性,可大大提高临床治疗效果。基于此,文章以中药诱导干细胞在骨组织工程学中应用的最新研究进展为契入点进行综述,探讨中药诱导干细胞在骨组织工程学中发挥的重要作用。

Microgel assembly:Fabrication,characteristics and application in tissue engineering and regenerative medicine

Microgel assembly,a macroscopic aggregate formed by bottom-up assembly of microgels,is now emerging as prospective biomaterials for applications in tissue engineering and regenerative medicine(TERM).This mini-review first summarizes the fabrication strategies available for microgel assembly,including chemical reaction,physical reaction,cell-cell interaction and external driving force,then highlights its unique characteristics,such as microporosity,injectability and heterogeneity,and finally itemizes its applications in the fields of cell culture,tissue regeneration and biofabrication,especially 3D printing.The problems to be addressed for further applications of microgel assembly are also discussed.