Next-generation regenerative medicine: organogenesis from stem cells in 3D culture

Modeling psychiatric disorders: from genomic findings to cellular phenotypes

Major programs in psychiatric genetics have identified >150 risk loci for psychiatric disorders. These loci converge on a small number of functional pathways, which span conventional diagnostic criteria, suggesting a partly common biology underlying schizophrenia, autism and other psychiatric disorders. Nevertheless, the cellular phenotypes that capture the fundamental features of psychiatric disorders have not yet been determined. Recent advances in genetics and stem cell biology offer new prospects for cell-based modeling of psychiatric disorders. The advent of cell reprogramming and induced pluripotent stem cells (iPSC) provides an opportunity to translate genetic findings into patient-specific in vitro models. iPSC technology is less than a decade old but holds great promise for bridging the gaps between patients, genetics and biology. Despite many obvious advantages, iPSC studies still present multiple challenges. In this expert review, we critically review the challenges for modeling of psychiatric disorders, potential solutions and how iPSC technology can be used to develop an analytical framework for the evaluation and therapeutic manipulation of fundamental disease processes.

A small molecule-based strategy for endothelial differentiation and three-dimensional morphogenesis from human embryonic stem cells

The emerging models of human embryonic stem cell (hESC) self-organizing organoids provide a valuable in vitro platform for studying self-organizing processes that presumably mimic in vivo human developmental events. Here we report that through a chemical screen, we identified two novel and structurally similar small molecules BIR1 and BIR2 which robustly induced the self-organization of a balloon-shaped three-dimensional structure when applied to two-dimensional adherent hESC cultures in the absence of growth factors. Gene expression analyses and functional assays demonstrated an endothelial identity of this balloon-like structure, while cell surface marker analyses revealed a VE-cadherin(+)CD31(+)CD34(+)KDR(+)CD43(-) putative endothelial progenitor population. Furthermore, molecular marker labeling and morphological examinations characterized several other distinct DiI-Ac-LDL(+) multi-cellular modules and a VEGFR3(+) sprouting structure in the balloon cultures that likely represented intermediate structures of balloon-formation.

Building brains in a dish: Prospects for growing cerebral organoids from stem cells

The recent development of organoid techniques, in which embryonic brain-like tissue can be grown from human or mouse stem cells in vitro offers the potential to transform the way in which brain development is studied. In this review, we summarize key aspects of the embryonic development of mammalian forebrains, focussing in particular on the cerebral cortex and highlight significant differences between mouse and primates, including human. We discuss recent work using cerebral organoids that has revealed key similarities and differences between their development and that of the brain in vivo. Finally, we outline the ways in which cerebral organoids can be used in combination with CRISPR/Cas9 genome editing to unravel genetic mechanisms that control embryonic development of the cerebral cortex, how this can help us understand the causes of neurodevelopmental disorders and some of the key challenges which will have to be resolved before organoids can become a mainstream tool to study brain development.

Cell-based therapy technology classifications and translational challenges

Cell therapies offer the promise of treating and altering the course of diseases which cannot be addressed adequately by existing pharmaceuticals. Cell therapies are a diverse group across cell types and therapeutic indications and have been an active area of research for many years but are now strongly emerging through translation and towards successful commercial development and patient access. In this article, we present a description of a classification of cell therapies on the basis of their underlying technologies rather than the more commonly used classification by cell type because the regulatory path and manufacturing solutions are often similar within a technology area due to the nature of the methods used. We analyse the progress of new cell therapies towards clinical translation, examine how they are addressing the clinical, regulatory, manufacturing and reimbursement requirements, describe some of the remaining challenges and provide perspectives on how the field may progress for the future.

WNT/β-catenin signaling mediates human neural crest induction via a pre-neural border intermediate

Neural crest (NC) cells arise early in vertebrate development, migrate extensively and contribute to a diverse array of ectodermal and mesenchymal derivatives. Previous models of NC formation suggested derivation from neuralized ectoderm, via meso-ectodermal, or neural-non-neural ectoderm interactions. Recent studies using bird and amphibian embryos suggest an earlier origin of NC, independent of neural and mesodermal tissues. Here, we set out to generate a model in which to decipher signaling and tissue interactions involved in human NC induction. Our novel human embryonic stem cell (ESC)-based model yields high proportions of multipotent NC cells (expressing SOX10, PAX7 and TFAP2A) in 5 days. We demonstrate a crucial role for WNT/β-catenin signaling in launching NC development, while blocking placodal and surface ectoderm fates. We provide evidence of the delicate temporal effects of BMP and FGF signaling, and find that NC development is separable from neural and/or mesodermal contributions. We further substantiate the notion of a neural-independent origin of NC through PAX6 expression and knockdown studies. Finally, we identify a novel pre-neural border state characterized by early WNT/β-catenin signaling targets that displays distinct responses to BMP and FGF signaling from the traditional neural border genes. In summary, our work provides a fast and efficient protocol for human NC differentiation under signaling constraints similar to those identified in vivo in model organisms, and strengthens a framework for neural crest ontogeny that is separable from neural and mesodermal fates.

Towards Organs on Demand: Breakthroughs and Challenges in Models of Organogenesis

In recent years, functional three-dimensional (3D) tissue generation in vitro has been significantly advanced by tissue-engineering methods, achieving better reproduction of complex native organs compared to conventional culture systems. This review will discuss traditional 3D cell culture techniques as well as newly developed technology platforms. These recent techniques provide new possibilities in the creation of human body parts and provide more accurate predictions of tissue response to drug and chemical challenges. Given the rapid advancement in the human induced pluripotent stem cell (iPSC) field, these platforms also hold great promise in the development of patient-specific, transplantable tissues and organs on demand.

Three-dimensional models for studying development and disease: moving on from organisms to organs-on-a-chip and organoids

Human development and disease are challenging to study because of lack of experimental accessibility to in vivo systems and the complex nature of biological processes. For these reasons researchers turn to the use of model systems, ranging in complexity and scale from single cells to model organisms. While the use of model organisms is valuable for studying physiology and pathophysiology in an in vivo context and for aiding pre-clinical development of therapeutics, animal models are costly, difficult to interrogate, and not always equivalent to human biology. For these reasons, three-dimensional (3D) cell cultures have emerged as an attractive model system that contains key aspects of in vivo tissue and organ complexity while being more experimentally tractable than model organisms. In particular, organ-on-a-chip and organoid models represent orthogonal approaches that have been able to recapitulate characteristics of physiology and disease. Here, we review advances in these two categories of 3D cultures and applications in studying development and disease. Additionally, we discuss development of key technologies that facilitate the generation of 3D cultures, including microfluidics, biomaterials, genome editing, and imaging technologies.

Establishment of Functional Genomics Pipeline in Mouse Epiblast-Like Tissue by Combining Transcriptomic Analysis and Gene Knockdown/Knockin/Knockout, Using RNA Interference and CRISPR/Cas9

The epiblast (foremost embryonic ectoderm) generates all three germ layers and therefore has crucial roles in the formation of all mammalian body cells. However, regulation of epiblast gene expression is poorly understood because of the difficulty of manipulating epiblast tissues in vivo. In the present study, using the self-organizing properties of mouse embryonic stem cell (ESC), we generated and characterized epiblast-like tissue in three-dimensional culture. We identified significant genome-wide gene expression changes in this epiblast-like tissue by transcriptomic analysis. In addition, we identified the particular significance of the Erk/Mapk and integrin-linked kinase pathways, and genes related to ectoderm/epithelial formation, using the bioinformatics resources IPA and DAVID. Here, we focused on Fgf5, which ranked in the top 10 among the discovered genes. To develop a functional analysis of Fgf5, we created an efficient method combining CRISPR/Cas9-mediated genome engineering and RNA interference (RNAi). Notably, we show one-step generation of various Fgf5 reporter lines including heterozygous and homozygous knockins (the GET method). For time- and dose-dependent depletion of fgf5 over the course of development, we generated an ESC line harboring Tol2 transposon-mediated integration of an inducible short hairpin RNA interference system (pdiRNAi). Our findings raised the possibility that Fgf/Erk signaling and apicobasal epithelial integrity are important factors in epiblast development. In addition, our methods provide a framework for a broad array of applications in the areas of mammalian genetics and molecular biology to understand development and to improve future therapeutics.

Whole Tooth Regeneration as a Future Dental Treatment

Dental problems caused by dental caries, periodontal disease and tooth injury compromise the oral and general health issues. Current advances for the development of regenerative therapy have been influenced by our understanding of embryonic development, stem cell biology, and tissue engineering technology. Tooth regenerative therapy for tooth tissue repair and whole tooth replacement is currently expected a novel therapeutic concept with the full recovery of tooth physiological functions. Dental stem cells and cell-activating cytokines are thought to be candidate approach for tooth tissue regeneration because they have the potential to differentiate into tooth tissues in vitro and in vivo. Whole tooth replacement therapy is considered to be an attractive concept for next generation regenerative therapy as a form of bioengineered organ replacement. For realization of whole tooth regeneration, we have developed a novel three-dimensional cell manipulation method designated the "organ germ method". This method involves compartmentalisation of epithelial and mesenchymal cells at a high cell density to mimic multicellular assembly conditions and epithelial-mesenchymal interactions in organogenesis. The bioengineered tooth germ generates a structurally correct tooth in vitro, and erupted successfully with correct tooth structure when transplanted into the oral cavity. We have ectopically generated a bioengineered tooth unit composed of a mature tooth, periodontal ligament and alveolar bone, and that tooth unit was engrafted into an adult jawbone through bone integration. Bioengineered teeth were also able to perform physiological tooth functions such as mastication, periodontal ligament function and response to noxious stimuli. In this review, we describe recent findings and technologies underpinning whole tooth regenerative therapy.

Bioengineering a 3D integumentary organ system from iPS cells using an in vivo transplantation model

The integumentary organ system is a complex system that plays important roles in waterproofing, cushioning, protecting deeper tissues, excreting waste, and thermoregulation. We developed a novel in vivo transplantation model designated as a clustering-dependent embryoid body transplantation method and generated a bioengineered three-dimensional (3D) integumentary organ system, including appendage organs such as hair follicles and sebaceous glands, from induced pluripotent stem cells. This bioengineered 3D integumentary organ system was fully functional following transplantation into nude mice and could be properly connected to surrounding host tissues, such as the epidermis, arrector pili muscles, and nerve fibers, without tumorigenesis. The bioengineered hair follicles in the 3D integumentary organ system also showed proper hair eruption and hair cycles, including the rearrangement of follicular stem cells and their niches. Potential applications of the 3D integumentary organ system include an in vitro assay system, an animal model alternative, and a bioengineered organ replacement therapy.

Long term morphological characterization of mesenchymal stromal cells 3D spheroids built with a rapid method based on entry-level equipment

Three-dimensional (3D) spheroids of mesenchymal stromal cells (MSC) have been demonstrated to improve a wide range of MSC features, such as multilineage potential, secretion of therapeutic factors, and resistance against hypoxic condition. Accordingly, they represent a promising tool in regenerative medicine for several biological and clinical applications. Many approaches have been proposed to generate MSC spheroids. They usually require specific generation systems, such as rotatory bioreactors or low-attachment plates, and each approach has its own disadvantages. Furthermore, an over-time analysis of morphological homogeneity and architectural stability of the spheroids generated is rarely provided. In this work we adapted the "pellet culture" method to obtain homogenous spheroids of MSC and maintain them in vitro for long term studies. We analysed their outer and inner structure over a 2-month period to provide morphological and architectural information regarding the spheroids generated. Quantitative and qualitative data were obtained using brightfield and confocal microscope imaging coupled to a computational analysis to estimate volume, sphericity, and jagging degree. In addition, histological evaluation was performed to more thoroughly assess the cellular composition and the internal architecture of the 3D spheroids. The results provided show that MSC spheroids generated with the proposed approach are homogeneous and stable, from both morphological and architectural points of view, for a period of at least 15 days, approximately between day 15 and day 30 after their generation. Accordingly, the approach proposed serves as a rapid, cost-effective, and efficient method to generate and maintain MSC spheroids using common entry-level laboratory equipment only.

Self-patterning of adipose-derived mesenchymal stem cells and chondrocytes cocultured on hyaluronan-grafted chitosan surface

The articular cartilage, once injured, has a limited capacity for intrinsic repair. Preparation of functionally biocartilage substitutes in vitro for cartilage repair is an attractive concept with the recent advances in tissue engineering. In this study, adipose-derived adult stem cells (ADAS) and chondrocytes (Ch) were cocultured in different population ratios on the surface of hyaluronan-grafted chitosan (CS-HA) membranes. The two types of cells could self-assemble into cospheroids with different morphologies. In particular, when ADAS and Ch were cocultured at an initial ratio of 7:3 on CS-HA surface, the expression of chondrogenic markers was upregulated, leading to preferred chondrogenesis of the cospheroids. Therefore, using the ADAS/Ch 7:3 cospheroids derived on CS-HA surface instead of using only a single type of cells may be favorable for future therapeutic applications.

Three-dimensional Organotypic Culture Models of Human Hepatocellular Carcinoma

Three-dimensional cell culture methods are viable in vitro approaches that facilitate the examination of biological features cancer cells present in vivo. In this study, we demonstrate that hepatocellular carcinoma (HCC) cells in porous alginate scaffolds can generate organoid-like spheroids that mimic numerous features of glandular epithelium in vivo, such as acinar morphogenesis and apical expression patterns of EpCAM, a hepatic stem/progenitor cell marker highly expressed in a subset of HCC with stemness features. We show that the activation of Wnt/β-catenin signaling, an essential pathway for maintaining HCC stemness, is required for EpCAM⁺ HCC spheroid formation as well as the maintenance of the acinous structure. Furthermore, we demonstrate that EpCAM⁺ HCC cells cultured as spheroids are more sensitive to TGF/β-induced epithelial-mesenchymal transition with highly tumorigenic and metastatic potential in vivo compared to conventional two-dimensional (2D) culture. In addition, HCC cells in EpCAM⁺ spheroids are more resistant to chemotherapeutic agents than 2D-cultured cells. The alginate scaffold-based organotypic culture system is a promising, reliable, and easy system that can be configured into a high throughput fashion for the identification of critical signaling pathways and screening of molecular drug targets specific for HCC.

Self-Organization of Spatial Patterning in Human Embryonic Stem Cells

The developing embryo is a remarkable example of self-organization, where functional units are created in a complex spatiotemporal choreography. Recently, human embryonic stem cells (ESCs) have been used to recapitulate in vitro the self-organization programs that are executed in the embryo in vivo. This represents an unique opportunity to address self-organization in humans that is otherwise not addressable with current technologies. In this chapter, we review the recent literature on self-organization of human ESCs, with a particular focus on two examples: formation of embryonic germ layers and neural rosettes. Intriguingly, both activation and elimination of TGFβ signaling can initiate self-organization, albeit with different molecular underpinnings. We discuss the mechanisms underlying the formation of these structures in vitro and explore future challenges in the field.

Modeling psychiatric disorders with patient-derived iPSCs

Psychiatric disorders are heterogeneous disorders characterized by complex genetics, variable symptomatology, and anatomically distributed pathology, all of which present challenges for effective treatment. Current treatments are often blunt tools used to ameliorate the most severe symptoms, often at the risk of disrupting functional neural systems, thus there is a pressing need to develop rational therapeutics. Induced pluripotent stem cells (iPSCs) reprogrammed from patient somatic cells offer an unprecedented opportunity to recapitulate both normal and pathologic human tissue and organ development, and provides new approaches for understanding disease mechanisms and for drug discovery with higher predictability of their effects in humans. Here we review recent progress and challenges in using human iPSCs for modeling neuropsychiatric disorders and developing novel therapeutic strategies.

Neural Substrate Expansion for the Restoration of Brain Function

Restoring neurological and cognitive function in individuals who have suffered brain damage is one of the principal objectives of modern translational neuroscience. Electrical stimulation approaches, such as deep-brain stimulation, have achieved the most clinical success, but they ultimately may be limited by the computational capacity of the residual cerebral circuitry. An alternative strategy is brain substrate expansion, in which the computational capacity of the brain is augmented through the addition of new processing units and the reconstitution of network connectivity. This latter approach has been explored to some degree using both biological and electronic means but thus far has not demonstrated the ability to reestablish the function of large-scale neuronal networks. In this review, we contend that fulfilling the potential of brain substrate expansion will require a significant shift from current methods that emphasize direct manipulations of the brain (e.g., injections of cellular suspensions and the implantation of multi-electrode arrays) to the generation of more sophisticated neural tissues and neural-electric hybrids in vitro that are subsequently transplanted into the brain. Drawing from neural tissue engineering, stem cell biology, and neural interface technologies, this strategy makes greater use of the manifold techniques available in the laboratory to create biocompatible constructs that recapitulate brain architecture and thus are more easily recognized and utilized by brain networks.

Human pancreatic beta-like cells converted from fibroblasts

Pancreatic beta cells are of great interest for biomedical research and regenerative medicine. Here we show the conversion of human fibroblasts towards an endodermal cell fate by employing non-integrative episomal reprogramming factors in combination with specific growth factors and chemical compounds. On initial culture, converted definitive endodermal progenitor cells (cDE cells) are specified into posterior foregut-like progenitor cells (cPF cells). The cPF cells and their derivatives, pancreatic endodermal progenitor cells (cPE cells), can be greatly expanded. A screening approach identified chemical compounds that promote the differentiation and maturation of cPE cells into functional pancreatic beta-like cells (cPB cells) in vitro. Transplanted cPB cells exhibit glucose-stimulated insulin secretion in vivo and protect mice from chemically induced diabetes. In summary, our study has important implications for future strategies aimed at generating high numbers of functional beta cells, which may help restoring normoglycemia in patients suffering from diabetes.

The Application of Human iPSCs in Neurological Diseases: From Bench to Bedside

In principle, induced pluripotent stem cells (iPSCs) are generated from somatic cells by reprogramming and gaining the capacity to self-renew indefinitely as well as the ability to differentiate into cells of different lineages. Human iPSCs have absolute advantages over human embryonic stem cells (ESCs) and animal models in disease modeling, drug screening, and cell replacement therapy. Since Takahashi and Yamanaka first described in 2007 that iPSCs can be generated from human adult somatic cells by retroviral transduction of the four transcription factors, Oct3/4, Sox2, Klf4, and c-Myc, disease specific iPSC lines have sprung up worldwide like bamboo shoots after a spring rain, making iPSC one of the hottest and fastest moving topics in modern science. The craze for iPSCs has spread throughout main branches of clinical medicine, covering neurology, hematology, cardiology, endocrinology, hepatology, ophthalmology, and so on. Here in this paper, we will focus on the clinical application of human iPSCs in disease modeling, drug screening, and cell replacement therapy for neurological diseases.

Bridging the Gap: From 2D Cell Culture to 3D Microengineered Extracellular Matrices

Historically the culture of mammalian cells in the laboratory has been performed on planar substrates with media cocktails that are optimized to maintain phenotype. However, it is becoming increasingly clear that much of biology discerned from 2D studies does not translate well to the 3D microenvironment. Over the last several decades, 2D and 3D microengineering approaches have been developed that better recapitulate the complex architecture and properties of in vivo tissue. Inspired by the infrastructure of the microelectronics industry, lithographic patterning approaches have taken center stage because of the ease in which cell-sized features can be engineered on surfaces and within a broad range of biocompatible materials. Patterning and templating techniques enable precise control over extracellular matrix properties including: composition, mechanics, geometry, cell-cell contact, and diffusion. In this review article we explore how the field of engineered extracellular matrices has evolved with the development of new hydrogel chemistry and the maturation of micro- and nano- fabrication. Guided by the spatiotemporal regulation of cell state in developing tissues, techniques for micropatterning in 2D, pseudo-3D systems, and patterning within 3D hydrogels will be discussed in the context of translating the information gained from 2D systems to synthetic engineered 3D tissues.

Regenerative medicine: Current therapies and future directions

Organ and tissue loss through disease and injury motivate the development of therapies that can regenerate tissues and decrease reliance on transplantations. Regenerative medicine, an interdisciplinary field that applies engineering and life science principles to promote regeneration, can potentially restore diseased and injured tissues and whole organs. Since the inception of the field several decades ago, a number of regenerative medicine therapies, including those designed for wound healing and orthopedics applications, have received Food and Drug Administration (FDA) approval and are now commercially available. These therapies and other regenerative medicine approaches currently being studied in preclinical and clinical settings will be covered in this review. Specifically, developments in fabricating sophisticated grafts and tissue mimics and technologies for integrating grafts with host vasculature will be discussed. Enhancing the intrinsic regenerative capacity of the host by altering its environment, whether with cell injections or immune modulation, will be addressed, as well as methods for exploiting recently developed cell sources. Finally, we propose directions for current and future regenerative medicine therapies.

Generation of functional hippocampal neurons from self-organizing human embryonic stem cell-derived dorsomedial telencephalic tissue

The developing dorsomedial telencephalon includes the medial pallium, which goes on to form the hippocampus. Generating a reliable source of human hippocampal tissue is an important step for cell-based research into hippocampus-related diseases. Here we show the generation of functional hippocampal granule- and pyramidal-like neurons from self-organizing dorsomedial telencephalic tissue using human embryonic stem cells (hESCs). First, we develop a hESC culture method that utilizes bone morphogenetic protein (BMP) and Wnt signalling to induce choroid plexus, the most dorsomedial portion of the telencephalon. Then, we find that titrating BMP and Wnt exposure allowed the self-organization of medial pallium tissues. Following long-term dissociation culture, these dorsomedial telencephalic tissues give rise to Zbtb20⁺/Prox1⁺ granule neurons and Zbtb20⁺/KA1⁺ pyramidal neurons, both of which were electrically functional with network formation. Thus, we have developed an in vitro model that recapitulates human hippocampus development, allowing the generation of functional hippocampal granule- and pyramidal-like neurons.

In vitro differentiation of human pluripotent stem cells (hPSCs) has enabled the generation of neuroectodermal tissues. Here, Sakaguchi et al. use a modified neocortical induction method to generate functional hippocampal granule and pyramidal-like neurons, as well as dorsomedial telencephalic tissues from hPSCs.

Spheroid Culture of Mesenchymal Stem Cells

Compared with traditional 2D adherent cell culture, 3D spheroidal cell aggregates, or spheroids, are regarded as more physiological, and this technique has been exploited in the field of oncology, stem cell biology, and tissue engineering. Mesenchymal stem cells (MSCs) cultured in spheroids have enhanced anti-inflammatory, angiogenic, and tissue reparative/regenerative effects with improved cell survival after transplantation. Cytoskeletal reorganization and drastic changes in cell morphology in MSC spheroids indicate a major difference in mechanophysical properties compared with 2D culture. Enhanced multidifferentiation potential, upregulated expression of pluripotency marker genes, and delayed replicative senescence indicate enhanced stemness in MSC spheroids. Furthermore, spheroid formation causes drastic changes in the gene expression profile of MSC in microarray analyses. In spite of these significant changes, underlying molecular mechanisms and signaling pathways triggering and sustaining these changes are largely unknown.

Scaffold and scaffold-free self-assembled systems in regenerative medicine

Self-assembly in tissue engineering refers to the spontaneous chemical or biological association of components to form a distinct functional construct, reminiscent of native tissue. Such self-assembled systems have been widely used to develop platforms for the delivery of therapeutic and/or bioactive molecules and various cell populations. Tissue morphology and functional characteristics have been recapitulated in several self-assembled constructs, designed to incorporate stimuli responsiveness and controlled architecture through spatial confinement or field manipulation. In parallel, owing to substantial functional properties, scaffold-free cell-assembled devices have aided in the development of functional neotissues for various clinical targets. Herein, we discuss recent advancements and future aspirations in scaffold and scaffold-free self-assembled devices for regenerative medicine purposes. Biotechnol. Bioeng. 2016;113: 1155-1163. © 2015 Wiley Periodicals, Inc.

Cell-Penetrating Peptide as a Means of Directing the Differentiation of Induced-Pluripotent Stem Cells

Protein transduction using cell-penetrating peptides (CPPs) is useful for the delivery of large protein molecules, including some transcription factors. This method is safer than gene transfection methods with a viral vector because there is no risk of genomic integration of the exogenous DNA. Recently, this method was reported as a means for the induction of induced pluripotent stem (iPS) cells, directing the differentiation into specific cell types and supporting gene editing/correction. Furthermore, we developed a direct differentiation method to obtain a pancreatic lineage from mouse and human pluripotent stem cells via the protein transduction of three transcription factors, Pdx1, NeuroD, and MafA. Here, we discuss the possibility of using CPPs as a means of directing the differentiation of iPS cells and other stem cell technologies.

Extracellular Matrix and Integrins in Embryonic Stem Cell Differentiation

Embryonic stem cells (ESCs) are pluripotent cells with great therapeutic potentials. The in vitro differentiation of ESC was designed by recapitulating embryogenesis. Significant progress has been made to improve the in vitro differentiation protocols by toning soluble maintenance factors. However, more robust methods for lineage-specific differentiation and maturation are still under development. Considering the complexity of in vivo embryogenesis environment, extracellular matrix (ECM) cues should be considered besides growth factor cues. ECM proteins bind to cells and act as ligands of integrin receptors on cell surfaces. Here, we summarize the role of the ECM and integrins in the formation of three germ layer progenies. Various ECM–integrin interactions were found, facilitating differentiation toward definitive endoderm, hepatocyte-like cells, pancreatic beta cells, early mesodermal progenitors, cardiomyocytes, neuroectoderm lineages, and epidermal cells, such as keratinocytes and melanocytes. In the future, ECM combinations for the optimal ESC differentiation environment will require substantial study.

Modeling mouse and human development using organoid cultures

In vitro three-dimensional (3D) cultures are emerging as novel systems with which to study tissue development, organogenesis and stem cell behavior ex vivo. When grown in a 3D environment, embryonic stem cells (ESCs) self-organize into organoids and acquire the right tissue patterning to develop into several endoderm- and ectoderm-derived tissues, mimicking their in vivo counterparts. Tissue-resident adult stem cells (AdSCs) also form organoids when grown in 3D and can be propagated in vitro for long periods of time. In this Review, we discuss recent advances in the generation of pluripotent stem cell- and AdSC-derived organoids, highlighting their potential for enhancing our understanding of human development. We will also explore how this new culture system allows disease modeling and gene repair for a personalized regenerative medicine approach.

Vascularized and Complex Organ Buds from Diverse Tissues via Mesenchymal Cell-Driven Condensation

Transplantation of in-vitro-generated organ buds is a promising approach toward regenerating functional and vascularized organs. Though it has been recently shown in the context of liver models, demonstrating the applicability of this approach to other systems by delineating the molecular mechanisms guiding organ bud formation is critical. Here, we demonstrate a generalized method for organ bud formation from diverse tissues by combining pluripotent stem cell-derived tissue-specific progenitors or relevant tissue samples with endothelial cells and mesenchymal stem cells (MSCs). The MSCs initiated condensation within these heterotypic cell mixtures, which was dependent upon substrate matrix stiffness. Defining optimal mechanical properties promoted formation of 3D, transplantable organ buds from tissues including kidney, pancreas, intestine, heart, lung, and brain. Transplanted pancreatic and renal buds were rapidly vascularized and self-organized into functional, tissue-specific structures. These findings provide a general platform for harnessing mechanical properties to generate vascularized, complex organ buds with broad applications for regenerative medicine.

Development by three-dimensional approaches and four-dimensional imaging: to the knowledge frontier and beyond

Many advances have been taken on elucidating embryonic development and tissue homeostasis and repair by the use of experimental strategies that preserve the three-dimensional (3D) organization and allow quantitative analysis of images over time (four-dimensional). Ranging from the understanding about the relationship between blastomeres and the events that take place during gastrulation by the use of time-lapse imaging through 3D cultures that mimic organogenesis, the advances in this area are of critical value. The studies on embryonic development without disrupting the original architecture and the development of 3D organoid cultures pave a new avenue for unprecedented experimental advances that will positively impact the emergence of new treatments applying regenerative principles for both tissue repair and organ transplant.

From classical to current: analyzing peripheral nervous system and spinal cord lineage and fate

During vertebrate development, the central (CNS) and peripheral nervous systems (PNS) arise from the neural plate. Cells at the margin of the neural plate give rise to neural crest cells, which migrate extensively throughout the embryo, contributing to the majority of neurons and all of the glia of the PNS. The rest of the neural plate invaginates to form the neural tube, which expands to form the brain and spinal cord. The emergence of molecular cloning techniques and identification of fluorophores like Green Fluorescent Protein (GFP), together with transgenic and electroporation technologies, have made it possible to easily visualize the cellular and molecular events in play during nervous system formation. These lineage-tracing techniques have precisely demonstrated the migratory pathways followed by neural crest cells and increased knowledge about their differentiation into PNS derivatives. Similarly, in the spinal cord, lineage-tracing techniques have led to a greater understanding of the regional organization of multiple classes of neural progenitor and post-mitotic neurons along the different axes of the spinal cord and how these distinct classes of neurons assemble into the specific neural circuits required to realize their various functions. Here, we review how both classical and modern lineage and marker analyses have expanded our knowledge of early peripheral nervous system and spinal cord development.

Concise review: tailoring bioengineered scaffolds for stem cell applications in tissue engineering and regenerative medicine

The potential for the clinical application of stem cells in tissue regeneration is clearly significant. However, this potential has remained largely unrealized owing to the persistent challenges in reproducibly, with tight quality criteria, and expanding and controlling the fate of stem cells in vitro and in vivo. Tissue engineering approaches that rely on reformatting traditional Food and Drug Administration-approved biomedical polymers from fixation devices to porous scaffolds have been shown to lack the complexity required for in vitro stem cell culture models or translation to in vivo applications with high efficacy. This realization has spurred the development of advanced mimetic biomaterials and scaffolds to increasingly enhance our ability to control the cellular microenvironment and, consequently, stem cell fate. New insights into the biology of stem cells are expected to eventuate from these advances in material science, in particular, from synthetic hydrogels that display physicochemical properties reminiscent of the natural cell microenvironment and that can be engineered to display or encode essential biological cues. Merging these advanced biomaterials with high-throughput methods to systematically, and in an unbiased manner, probe the role of scaffold biophysical and biochemical elements on stem cell fate will permit the identification of novel key stem cell behavioral effectors, allow improved in vitro replication of requisite in vivo niche functions, and, ultimately, have a profound impact on our understanding of stem cell biology and unlock their clinical potential in tissue engineering and regenerative medicine.

Symmetry breaking, germ layer specification and axial organisation in aggregates of mouse embryonic stem cells

Mouse embryonic stem cells (mESCs) are clonal populations derived from preimplantation mouse embryos that can be propagated in vitro and, when placed into blastocysts, contribute to all tissues of the embryo and integrate into the normal morphogenetic processes, i.e. they are pluripotent. However, although they can be steered to differentiate in vitro into all cell types of the organism, they cannot organise themselves into structures that resemble embryos. When aggregated into embryoid bodies they develop disorganised masses of different cell types with little spatial coherence. An exception to this rule is the emergence of retinas and anterior cortex-like structures under minimal culture conditions. These structures emerge from the cultures without any axial organisation. Here, we report that small aggregates of mESCs, of about 300 cells, self-organise into polarised structures that exhibit collective behaviours reminiscent of those that cells exhibit in early mouse embryos, including symmetry breaking, axial organisation, germ layer specification and cell behaviour, as well as axis elongation. The responses are signal specific and uncouple processes that in the embryo are tightly associated, such as specification of the anteroposterior axis and anterior neural development, or endoderm specification and axial elongation. We discuss the meaning and implications of these observations and the potential uses of these structures which, because of their behaviour, we suggest to call ‘gastruloids’.

Wnt/β-catenin and FGF signalling direct the specification and maintenance of a neuromesodermal axial progenitor in ensembles of mouse embryonic stem cells

The development of the central nervous system is known to result from two sequential events. First, an inductive event of the mesoderm on the overlying ectoderm that generates a neural plate that, after rolling into a neural tube, acts as the main source of neural progenitors. Second, the axial regionalization of the neural plate that will result in the specification of neurons with different anteroposterior identities. Although this description of the process applies with ease to amphibians and fish, it is more difficult to confirm in amniote embryos. Here, a specialized population of cells emerges at the end of gastrulation that, under the influence of Wnt and FGF signalling, expands and generates the spinal cord and the paraxial mesoderm. This population is known as the long-term neuromesodermal precursor (NMp). Here, we show that controlled increases of Wnt/β-catenin and FGF signalling during adherent culture differentiation of mouse embryonic stem cells (mESCs) generates a population with many of the properties of the NMp. A single-cell analysis of gene expression within this population reveals signatures that are characteristic of stem cell populations. Furthermore, when this activation is triggered in three-dimensional aggregates of mESCs, the population self-organizes macroscopically and undergoes growth and axial elongation that mimics some of the features of the embryonic spinal cord and paraxial mesoderm. We use both adherent and three-dimensional cultures of mESCs to probe the establishment and maintenance of NMps and their differentiation.

Salivary Secretory Disorders, Inducing Drugs, and Clinical Management

BACKGROUND

Salivary secretory disorders can be the result of a wide range of factors. Their prevalence and negative effects on the patient's quality of life oblige the clinician to confront the issue.

AIM

To review the salivary secretory disorders, inducing drugs and their clinical management.

METHODS

In this article, a literature search of these dysfunctions was conducted with the assistance of a research librarian in the MEDLINE/PubMed Database.

RESULTS

Xerostomia, or dry mouth syndrome, can be caused by medication, systemic diseases such as Sjögren's Syndrome, glandular pathologies, and radiotherapy of the head and neck. Treatment of dry mouth is aimed at both minimizing its symptoms and preventing oral complications with the employment of sialogogues and topical acting substances. Sialorrhea and drooling, are mainly due to medication or neurological systemic disease. There are various therapeutic, pharmacologic, and surgical alternatives for its management. The pharmacology of most of the substances employed for the treatment of salivary disorders is well-known. Nevertheless, in some cases a significant improvement in salivary function has not been observed after their administration.

CONCLUSION

At present, there are numerous frequently prescribed drugs whose unwanted effects include some kind of salivary disorder. In addition, the differing pathologic mechanisms, and the great variety of existing treatments hinder the clinical management of these patients. The authors have designed an algorithm to facilitate the decision making process when physicians, oral surgeons, or dentists face these salivary dysfunctions.

Build-a-brain

A major barrier in understanding nervous system development is modeling the cellular interactions that form the human brain. Recently, in the journal Nature, Lancaster et al. (2013) established a protocol for culturing pluripotent stem cell (PSC)-derived "cerebral organoids" that mimics the developing human brain's cellular organization, segregates into distinct brain regions, and models microcephaly.

Three-dimensional organotypic culture: experimental models of mammalian biology and disease

Mammalian organs are challenging to study as they are fairly inaccessible to experimental manipulation and optical observation. Recent advances in three-dimensional (3D) culture techniques, coupled with the ability to independently manipulate genetic and microenvironmental factors, have enabled the real-time study of mammalian tissues. These systems have been used to visualize the cellular basis of epithelial morphogenesis, to test the roles of specific genes in regulating cell behaviours within epithelial tissues and to elucidate the contribution of microenvironmental factors to normal and disease processes. Collectively, these novel models can be used to answer fundamental biological questions and generate replacement human tissues, and they enable testing of novel therapeutic approaches, often using patient-derived cells.

Single luminal epithelial progenitors can generate prostate organoids in culture

The intrinsic ability to display self-organizing morphogenetic properties in ex vivo culture may represent a general property of tissue stem cells. Here we show that single luminal stem/progenitor cells can generate prostate organoids in a three-dimensional culture system in the absence of stroma. Organoids generated from CARNs (castration-resistant Nkx3.1-expressing cells) or normal prostate epithelium exhibit tissue architecture containing luminal and basal cells, undergo long-term expansion in culture, and display functional androgen receptor signaling. Lineage-tracing demonstrates that luminal cells are favored for organoid formation, and generate basal cells in culture. Furthermore, tumor organoids can initiate from CARNs after oncogenic transformation, and from mouse models of prostate cancer, and can facilitate analyses of drug response. Finally, we provide evidence supporting the feasibility of organoid studies of human prostate tissue. Our studies underscore the progenitor properties of luminal cells, and identify in vitro approaches for studying prostate biology.

Transient vascularization of transplanted human adult-derived progenitors promotes self-organizing cartilage

Millions of patients worldwide are affected by craniofacial deformations caused by congenital defects or trauma. Current surgical interventions have limited therapeutic outcomes; therefore, methods that would allow cartilage restoration are of great interest. A number of studies on embryonic limb development have shown that chondrogenesis is initiated by cellular condensation, during which mesenchymal progenitors aggregate and form 3D structures. Here, we demonstrated efficient regeneration of avascular elastic cartilage from in vitro-grown mesenchymal condensation, which recapitulated the early stages of chondrogenesis, including transient vascularization. After transplantation of vascularized condensed progenitors into immunodeficient mice, we used an intravital imaging approach to follow cartilage maturation. We determined that endothelial cells are present inside rudimentary cartilage (mesenchymal condensation) prior to cartilage maturation. Recreation of endothelial interactions in culture enabled a recently identified population of adult elastic cartilage progenitors to generate mesenchymal condensation in a self-driven manner, without requiring the support of exogenous inductive factors or scaffold materials. Moreover, the culture-grown 3D condensed adult-derived progenitors were amenable to storage via simple freezing methods and efficiently reconstructed 3D elastic cartilage upon transplantation. Together, our results indicate that transplantation of endothelialized and condensed progenitors represents a promising approach to realizing a regenerative medicine treatment for craniofacial deformations.

Yoshiki Sasai: stem cell Sensei

Stefano Piccolo looks back at the life and research of his friend and colleague Yoshiki Sasai.

Functional tooth restoration by next-generation bio-hybrid implant as a bio-hybrid artificial organ replacement therapy

Bio-hybrid artificial organs are an attractive concept to restore organ function through precise biological cooperation with surrounding tissues in vivo. However, in bio-hybrid artificial organs, an artificial organ with fibrous connective tissues, including muscles, tendons and ligaments, has not been developed. Here, we have enveloped with embryonic dental follicle tissue around a HA-coated dental implant, and transplanted into the lower first molar region of a murine tooth-loss model. We successfully developed a novel fibrous connected tooth implant using a HA-coated dental implant and dental follicle stem cells as a bio-hybrid organ. This bio-hybrid implant restored physiological functions, including bone remodelling, regeneration of severe bone-defect and responsiveness to noxious stimuli, through regeneration with periodontal tissues, such as periodontal ligament and cementum. Thus, this study represents the potential for a next-generation bio-hybrid implant for tooth loss as a future bio-hybrid artificial organ replacement therapy.

Ex vivo culture of the intestinal epithelium: strategies and applications

Limited pools of resident adult stem cells are critical effectors of epithelial renewal in the intestine throughout life. Recently, significant progress has been made regarding the isolation and in vitro propagation of fetal and adult intestinal stem cells in mammals. It is now possible to generate ever-expanding, three-dimensional epithelial structures in culture that closely parallel the in vivo epithelium of the intestine. Growing such organotypic epithelium ex vivo facilitates a detailed description of endogenous niche factors or stem-cell characteristics, as they can be monitored in real time. Accordingly, this technology has already greatly contributed to our understanding of intestinal adult stem-cell renewal and differentiation. Transplanted organoids have also been proven to readily integrate into, and effect the long-term repair of, mouse colonic epithelia in vivo, establishing the organoid culture as a promising tool for adult stem cell/gene therapy. In another exciting development, novel genome-editing techniques have been successfully employed to functionally repair disease loci in cultured intestinal stem cells from human patients with a hereditary defect. It is anticipated that this technology will be instrumental in exploiting the regenerative medicine potential of human intestinal stem cells for treating human disorders in the intestinal tract and for creating near-physiological ex vivo models of human gastrointestinal disease.

Functional tooth regenerative therapy: tooth tissue regeneration and whole-tooth replacement

Oral and general health is compromised by irreversible dental problems, including dental caries, periodontal disease and tooth injury. Regenerative therapy for tooth tissue repair and whole-tooth replacement is currently considered a novel therapeutic concept with the potential for the full recovery of tooth function. Several types of stem cells and cell-activating cytokines have been identified in oral tissues. These cells are thought to be candidate cell sources for tooth tissue regenerative therapies because they have the ability to differentiate into tooth tissues in vitro and in vivo. Whole-tooth replacement therapy is regarded as an important model for the development of an organ regenerative concept. A novel three-dimensional cell-manipulation method, designated the organ germ method, has been developed to recapitulate organogenesis. This method involves compartmentalisation of epithelial and mesenchymal cells at a high cell density to mimic multicellular assembly conditions and epithelial-mesenchymal interactions. A bioengineered tooth germ can generate a structurally correct tooth in vitro and erupt successfully with the correct tooth structure when transplanted into the oral cavity. We have ectopically generated a bioengineered tooth unit composed of a mature tooth, periodontal ligament and alveolar bone, and that tooth unit was successfully engrafted into an adult jawbone through bone integration. Such bioengineered teeth were able to perform normal physiological tooth functions, such as developing a masticatory potential in response to mechanical stress and a perceptive potential for noxious stimuli. In this review, we describe recent findings and technologies underpinning tooth regenerative therapy.