Brain organoids: advances, applications and challenges

Generation of Skin Organoids: Potential Opportunities and Challenges

Although several types of human skin substitutes are currently available, they usually do not include important skin appendages such as hair follicles and sweat glands, or various skin-related cells, such as dermal adipocytes and sensory neurons. This highlights the need to improve the in vitro human skin generation model for use as a tool for investigating skin diseases and as a source of cells or tissues for skin regeneration. Skin organoids are generated from stem cells and are expected to possess the complexity and function of natural skin. Here, we summarize the current literatures relating to the "niches" of the local skin stem cell microenvironment and the formation of skin organoids, and then discuss the opportunities and challenges associated with multifunctional skin organoids.

Recent Advances in Three-Dimensional Stem Cell Culture Systems and Applications

Cell culture is one of the most core and fundamental techniques employed in the fields of biology and medicine. At present, although the two-dimensional cell culture method is commonly used in vitro, it is quite different from the cell growth microenvironment in vivo. In recent years, the limitations of two-dimensional culture and the advantages of three-dimensional culture have increasingly attracted more and more attentions. Compared to two-dimensional culture, three-dimensional culture system is better to realistically simulate the local microenvironment of cells, promote the exchange of information among cells and the extracellular matrix (ECM), and retain the original biological characteristics of stem cells. In this review, we first present three-dimensional cell culture methods from two aspects: a scaffold-free culture system and a scaffold-based culture system. The culture method and cell characterizations will be summarized. Then the application of three-dimensional cell culture system is further explored, such as in the fields of drug screening, organoids and assembloids. Finally, the directions for future research of three-dimensional cell culture are stated briefly.

Heart organoids and tissue models for modeling development and disease

Organoids, or miniaturized organs formed in vitro, hold potential to revolutionize how researchers approach and answer fundamental biological and pathological questions. In the context of cardiac biology, development of a bona fide cardiac organoid enables study of heart development, function, and pathogenesis in a dish, providing insight into the nature of congenital heart disease and offering the opportunity for high-throughput probing of adult heart disease and drug discovery. Recently, multiple groups have reported novel methods for generating in vitro models of the heart; however, there are substantial conceptual and methodological differences. In this review we will evaluate recent cardiac organoid studies through the lens of the core principles of organoid technology: patterned self-organization of multiple cell types resembling the in vivo organ. Based on this, we will classify systems into the following related types of tissues: developmental cardiac organoids, chamber cardiac organoids, microtissues, and engineered heart tissues. Furthermore, we highlight the interventions which allow for organoid formation, such as modulation of highly conserved cardiogenic signaling pathways mediated by developmental morphogens. We expect that consolidation and categorization of existing organoid models will help eliminate confusion in the field and facilitate progress towards creation of an ideal cardiac organoid.

Advancing models of neural development with biomaterials

Human pluripotent stem cells have emerged as a promising in vitro model system for studying the brain. Two-dimensional and three-dimensional cell culture paradigms have provided valuable insights into the pathogenesis of neuropsychiatric disorders, but they remain limited in their capacity to model certain features of human neural development. Specifically, current models do not efficiently incorporate extracellular matrix-derived biochemical and biophysical cues, facilitate multicellular spatio-temporal patterning, or achieve advanced functional maturation. Engineered biomaterials have the capacity to create increasingly biomimetic neural microenvironments, yet further refinement is needed before these approaches are widely implemented. This Review therefore highlights how continued progression and increased integration of engineered biomaterials may be well poised to address intractable challenges in recapitulating human neural development.

Human neural organoids: Models for developmental neurobiology and disease

Human organoids stand at the forefront of basic and translational research, providing experimentally tractable systems to study human development and disease. These stem cell-derived, in vitro cultures can generate a multitude of tissue and organ types, including distinct brain regions and sensory systems. Neural organoid systems have provided fundamental insights into molecular mechanisms governing cell fate specification and neural circuit assembly and serve as promising tools for drug discovery and understanding disease pathogenesis. In this review, we discuss several human neural organoid systems, how they are generated, advances in 3D imaging and bioengineering, and the impact of organoid studies on our understanding of the human nervous system.

Identification of neural oscillations and epileptiform changes in human brain organoids

Brain organoids represent a powerful tool for studying human neurological diseases, particularly those that affect brain growth and structure. However, many diseases manifest with clear evidence of physiological and network abnormality in the absence of anatomical changes, raising the question of whether organoids possess sufficient neural network complexity to model these conditions. Here, we explore the network-level functions of brain organoids using calcium sensor imaging and extracellular recording approaches that together reveal the existence of complex network dynamics reminiscent of intact brain preparations. We demonstrate highly abnormal and epileptiform-like activity in organoids derived from induced pluripotent stem cells from individuals with Rett syndrome, accompanied by transcriptomic differences revealed by single-cell analyses. We also rescue key physiological activities with an unconventional neuroregulatory drug, pifithrin-α. Together, these findings provide an essential foundation for the utilization of brain organoids to study intact and disordered human brain network formation and illustrate their utility in therapeutic discovery.

Advances in neural organoid systems and their application in neurotoxicity testing of environmental chemicals

Due to the complex structure and function of central nervous system (CNS), human CNS in vitro modeling is still a great challenge. Neurotoxicity testing of environmental chemicals mainly depends on the traditional animal models, which have various limitations such as species differences, expensive and time-consuming. Meanwhile, in vitro two-dimensional (2D) cultured cells or three-dimensional (3D) cultured neurospheres cannot fully simulate complex 3D structure of neural tissues. Recent advancements in neural organoid systems provides excellent models for the testing of environmental chemicals that affect the development of human CNS. Neural organoids derived from hPSCs not only can simulate the process of CNS development, including early stage neural tube formation, neuroepithelium differentiation and regional specification, but also its 3D structure, thus can be used to evaluate the effect of chemicals on differentiation and morphogenesis. Here, we provide a review of recent progress in the methods of culturing neural organoids and their applications in neurotoxicity testing of environmental chemicals. We conclude by highlighting challenge and future directions in neurotoxicity testing based on neural organoids.

Generation of hypothalamic arcuate organoids from human induced pluripotent stem cells

Human brain organoids represent remarkable platforms for recapitulating features of human brain development and diseases. Existing organoid models do not resolve fine brain subregions, such as different nuclei in the hypothalamus. We report the generation of arcuate organoids (ARCOs) from human induced pluripotent stem cells (iPSCs) to model the development of the human hypothalamic arcuate nucleus. Single-cell RNA sequencing of ARCOs revealed significant molecular heterogeneity underlying different arcuate cell types, and machine learning-aided analysis based on the neonatal human hypothalamus single-nucleus transcriptome further showed a human arcuate nucleus molecular signature. We also explored ARCOs generated from Prader-Willi syndrome (PWS) patient iPSCs. These organoids exhibit aberrant differentiation and transcriptomic dysregulation similar to postnatal hypothalamus of PWS patients, indicative of cellular differentiation deficits and exacerbated inflammatory responses. Thus, patient iPSC-derived ARCOs represent a promising experimental model for investigating nucleus-specific features and disease-relevant mechanisms during early human arcuate development.

Organoid technology for tissue engineering

For centuries, attempts have been continuously made to artificially reconstitute counterparts of in vivo organs from their tissues or cells. Only in the recent decade has organoid technology as a whole technological field systematically emerged and been shown to play important roles in tissue engineering. Based on their self-organizing capacities, stem cells of versatile organs, both harvested and induced, can form 3D structures that are structurally and functionally similar to their in vivo counterparts. These organoid models provide a powerful platform for elucidating the development mechanisms, modeling diseases, and screening drug candidates. In this review, we will summarize the advances of this technology for generating various organoids of tissues from the three germ layers and discuss their drawbacks and prospects for tissue engineering.

Progress in human liver organoids

Understanding the development, regeneration, and disorders of the liver is the major goal in liver biology. Current mechanistic knowledge of human livers has been largely derived from mouse models and cell lines, which fall short in recapitulating the features of human liver cells or the structures and functions of human livers. Organoids as an in vitro system hold the promise to generate organ-like tissues in a dish. Recent advances in human liver organoids also facilitate the understanding of the biology and diseases in this complex organ. Here we review the progress in human liver organoids, mainly focusing on the methods to generate liver organoids, their applications, and possible future directions.

Modeling schizophrenia with iPS cell technology and disease mouse models

Induced pluripotent stem cell (iPSC) technology, which enables the direct analysis of neuronal cells with the same genetic background as patients, has recently garnered significant attention in schizophrenia research. This technology is important because it enables a comprehensive interpretation using mice and human clinical research and cross-species verification. Here I review recent advances in modeling schizophrenia using iPSC technology, alongside the utility of disease mouse models.

O-GlcNAc transferase Ogt regulates embryonic neuronal development through modulating Wnt/β-catenin signaling

Ogt-mediated O-GlcNAcylation is enriched in the nervous system, and involves in neuronal development, brain function and neurological diseases. However, the roles of Ogt and O-GlcNAcylation in embryonic neurogenesis has remained largely unknown. Here, we show that Ogt is highly expressed in embryonic brain, and Ogt depletion reduces the proliferation of embryonic neural stem cells and migration of new born neurons. Furthermore, Ogt in cultured hippocampal neurons impaires neuronal maturation including reduced dendritic numbers and length, and immature development of spines. Mechanistically, Ogt depletion decreases the activity of Wnt/β-catenin signaling. Ectopic β-catenin rescues neuronal developmental deficits caused by Ogt depletion. Ogt also regulates human cortical neurogenesis in forebrain organoids derived from induced pluripotent stem cells. Our findings reveal the essential roles and mechanisms of Ogt-mediated O-GlcNAc modification in regulating mammalian neuronal development.

Rapid Generation of Neurospheres from Hippocampal Neurons Using Extracellular-Matrix-Mimetic Scaffolds

3D models of brain organoids represent an innovative and promising tool in neuroscience studies. However, the process of neurosphere formation in vitro remains complicated and is not always very effective. This is largely due to the lack of growth factors, guidance cues, and scaffold structures commonly found in tissues. Here we present a new, simple, and efficient method for generating neurospheres using scaffolds composed of electrospun nylon fibers with a diameter of 40-180 nm, which makes them similar to the brain extracellular matrix (ECM) components. Several main advantages of the proposed method should be highlighted. The method is fast, and the biomaterial consumption is low. Also, the resulting neurospheres are attached to the scaffold nanofibers. This not only provides the experimental convenience but also suggests that the resulting organoid models can potentially demonstrate fundamentally new properties, being closer to the nervous tissue in vivo. We demonstrate the influence of the fibrous scaffold structure on the formation, morphology, and composition of neurospheres and confirm adequate functional activity of the cellular components of these spheroids. The proposed approach can be further used for drug screening, modeling of neurodevelopmental, neurodegenerative disorders, and, potentially, therapeutic tissue engineering.

Klotho inhibits neuronal senescence in human brain organoids

Aging is a major risk factor for many neurodegenerative diseases. Klotho (KL) is a glycosylated transmembrane protein that is expressed in the choroid plexus and neurons of the brain. KL exerts potent anti-aging effects on multiple cell types in the body but its role in human brain cells remains largely unclear. Here we show that human cortical neurons, derived from human pluripotent stem cells in 2D cultures or in cortical organoids, develop the typical hallmarks of senescent cells when maintained in vitro for prolonged periods of time, and that moderate upregulation or repression of endogenous KL expression in cortical organoids inhibits and accelerates senescence, respectively. We further demonstrate that KL expression alters the expression of senescence-associated genes including, extracellular matrix genes, and proteoglycans, and can act in a paracrine fashion to inhibit neuronal senescence. In summary, our results establish an important role for KL in the regulation of human neuronal senescence and offer new mechanistic insight into its role in human brain aging.

Brain Organoids: Filling the Need for a Human Model of Neurological Disorder

Neurological disorders are among the leading causes of death worldwide, accounting for almost all onsets of dementia in the elderly, and are known to negatively affect motor ability, mental and cognitive performance, as well as overall wellbeing and happiness. Currently, most neurological disorders go untreated due to a lack of viable treatment options. The reason for this lack of options is s poor understanding of the disorders, primarily due to research models that do not translate well into the human in vivo system. Current models for researching neurological disorders, neurodevelopment, and drug interactions in the central nervous system include in vitro monolayer cell cultures, and in vivo animal models. These models have shortcomings when it comes to translating research about disorder pathology, development, and treatment to humans. Brain organoids are three-dimensional (3D) cultures of stem cell-derived neural cells that mimic the development of the in vivo human brain with high degrees of accuracy. Researchers have started developing these miniature brains to model neurodevelopment, and neuropathology. Brain organoids have been used to model a wide range of neurological disorders, including the complex and poorly understood neurodevelopmental and neurodegenerative disorders. In this review, we discuss the brain organoid technology, placing special focus on the different brain organoid models that have been developed, discussing their strengths, weaknesses, and uses in neurological disease modeling.

Brain organoid: a 3D technology for investigating cellular composition and interactions in human neurological development and disease models in vitro

Abstract

The study of human brain physiology, including cellular interactions in normal and disease conditions, has been a challenge due to its complexity and unavailability. Induced pluripotent stem cell (iPSC) study is indispensable in the study of the pathophysiology of neurological disorders. Nevertheless, monolayer systems lack the cytoarchitecture necessary for cellular interactions and neurological disease modeling. Brain organoids generated from human pluripotent stem cells supply an ideal environment to model both cellular interactions and pathophysiology of the human brain. This review article discusses the composition and interactions among neural lineage and non-central nervous system cell types in brain organoids, current studies, and future perspectives in brain organoid research. Ultimately, the promise of brain organoids is to unveil previously inaccessible features of neurobiology that emerge from complex cellular interactions and to improve our mechanistic understanding of neural development and diseases.

Graphical abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-021-02369-8.

Exposure to cadmium induces neuroinflammation and impairs ciliogenesis in hESC-derived 3D cerebral organoids

Cadmium (Cd) is an environmental heavy metal toxicant with central nervous system toxicity and has a greater negative impact on fetal neurodevelopment. However, the causative mechanisms for the neurodevelopmental toxicity of Cd have remained unclear. The human cerebral organoids can better mimic the three-dimensional structure of the early fetal nerve tissue, which can be used to study the developmental neurotoxicity under the condition of maternal exposure to Cd. Our study identified that Cd exposure specifically induced apoptosis in neurons and inhibited the proliferation of neural progenitor cells, but neural differentiation was not significantly affected in cerebral organoids. Cd exposure also elicited overexpression of GFAP, a marker of astrocytes and resulted in IL-6 release. This study revealed that mineral absorption was significantly disturbed with metallothioneins expression up-regulation. Moreover, we found Cd exposure inhibited cilium-related gene expression and reduced ciliary length with increasing dose. In conclusion, our study has shown that Cd exposure regulated neural cell proliferation and death, induced neuroinflammation, enhanced metal ion absorption, and impaired ciliogenesis, which hinder the normal development of the fetal brain.

The Translational Application of Hydrogel for Organoid Technology: Challenges and Future Perspectives

Human organoids mimic the physiology and tissue architecture of organs and are of great significance for promoting the study of human diseases. Traditionally, organoid cultures rely predominantly on animal or tumor-derived extracellular matrix (ECM), resulting in poor reproducibility. This limits their utility in for large-scale drug screening and application for regenerative medicine. Recently, synthetic polymeric hydrogels, with high biocompatibility and biodegradability, stability, uniformity of compositions, and high throughput properties, have emerged as potential materials for achieving 3D architectures for organoid cultures. Compared to conventional animal or tumor-derived organoids, these newly engineered hydrogel-based organoids more closely resemble human organs, as they are able to mimic native structural and functional properties observed in-situ. In this review, recent developments in hydrogel-based organoid culture will be summarized, emergent hydrogel technology will be highlighted, and future challenges in applying them to organoid culture will be discussed.

Defense of COVID-19 by Human Organoids

Coronavirus disease 2019 (COVID-19), caused by severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2), has created an immense menace to public health worldwide, exerting huge effects on global economic and political conditions. Understanding the biology and pathogenesis mechanisms of this novel virus, in large parts, relies on optimal physiological models that allow replication and propagation of SARS-CoV-2. Human organoids, derived from stem cells, are three-dimensional cell cultures that recapitulate the cellular organization, transcriptional and epigenetic signatures of their counterpart organs. Recent studies have indicated their great values as experimental virology platforms, making human organoid an ideal tool for investigating host-pathogen interactions. Here, we summarize research developments for SARS-CoV-2 infection of various human organoids involved in multiple systems, including lung, liver, brain, intestine, kidney and blood vessel organoids. These studies help us reveal the pathogenesis mechanism of COVID-19, and facilitate the development of effective vaccines and drugs as well as other therapeutic regimes.

Biology and Models of the Blood-Brain Barrier

The blood-brain barrier (BBB) is one of the most selective endothelial barriers. An understanding of its cellular, morphological, and biological properties in health and disease is necessary to develop therapeutics that can be transported from blood to brain. In vivo models have provided some insight into these features and transport mechanisms adopted at the brain, yet they have failed as a robust platform for the translation of results into clinical outcomes. In this article, we provide a general overview of major BBB features and describe various models that have been designed to replicate this barrier and neurological pathologies linked with the BBB. We propose several key parameters and design characteristics that can be employed to engineer physiologically relevant models of the blood-brain interface and highlight the need for a consensus in the measurement of fundamental properties of this barrier.

Next-Generation Human Cerebral Organoids as Powerful Tools To Advance NeuroHIV Research

Long-term effective use of antiretroviral therapy (ART) among people with HIV (PWH) has significantly reduced the burden of disease, yet a cure for HIV has not been universally achieved, likely due to the persistence of an HIV reservoir. The central nervous system (CNS) is an understudied HIV sanctuary. Importantly, due to viral persistence in the brain, cognitive disturbances persist to various degrees at high rates in PWH despite suppressive ART. Given the complexity and accessibility of the CNS compartment and that it is a physiologically and anatomically unique immune site, human studies to reveal molecular mechanisms of viral entry, reservoir establishment, and the cellular and structural interactions leading to viral persistence and brain injury to advance a cure and either prevent or limit cognitive impairments in PWH remain challenging. Recent advances in human brain organoids show that they can mimic the intercellular dynamics of the human brain and may recapitulate many of the events involved in HIV infection of the brain (neuroHIV). Human brain organoids can be produced, spontaneously or with addition of growth factors and at immature or mature states, and have become stronger models to study neurovirulent viral infections of the CNS. While organoids provide opportunities to study neuroHIV, obstacles such as the need to incorporate microglia need to be overcome to fully utilize this model. Here, we review the current achievements in brain organoid biology and their relevance to neuroHIV research efforts.

[Pharmacological studies using iPSC-derived neurons from patients with schizophrenia]

Schizophrenia is characterized by positive symptoms, negative symptoms and cognitive dysfunction. Although the abnormal neuronal development, impaired synaptic functions and impaired neural circuit functions are suggested to be the causes of psychiatric disorders, the molecular and cellular etiology of schizophrenia remains largely unclear. iPS-related technologies can be powerful for not only understanding the molecular and cellular etiology of schizophrenia but also drug discovery research. In 2011, the first iPS cells derived from patients with schizophrenia harboring a DISC1 mutation were generated. Subsequently, many iPS cells from patients with schizophrenia were established for understanding the molecular and cellular disease phenotypes of the differentiated neuronal cells. For replicating disease phenotypes with iPSC-derived neuronal cells, it is important to develop the differentiation strategies for generating cell-type specific cultures of various types of neurons, astrocytes and oligodendrocytes. Especially, scalable cultures of iPSC-derived neuronal cells are valuable platforms for drug discovery research. In this review, the focus has been made on the iPSC differentiation technology, pharmacological and drug discovery studies with iPSC-derived neurons from patients with schizophrenia. Continued advancement of the iPSC-related technologies and research will help the success in central nervous system drug discovery and development.

Advanced Bio-Based Polymers for Astrocyte Cell Models

The development of in vitro neural tissue analogs is of great interest for many biomedical engineering applications, including the tissue engineering of neural interfaces, treatment of neurodegenerative diseases, and in vitro evaluation of cell-material interactions. Since astrocytes play a crucial role in the regenerative processes of the central nervous system, the development of biomaterials that interact favorably with astrocytes is of great research interest. The sources of human astrocytes, suitable natural biomaterials, guidance scaffolds, and ligand patterned surfaces are discussed in the article. New findings in this field are essential for the future treatment of spinal cord and brain injuries.

The Effects of Environmental Adversities on Human Neocortical Neurogenesis Modeled in Brain Organoids

Over the past decades, a growing body of evidence has demonstrated the impact of prenatal environmental adversity on the development of the human embryonic and fetal brain. Prenatal environmental adversity includes infectious agents, medication, and substances of use as well as inherently maternal factors, such as diabetes and stress. These adversities may cause long-lasting effects if occurring in sensitive time windows and, therefore, have high clinical relevance. However, our knowledge of their influence on specific cellular and molecular processes of in utero brain development remains scarce. This gap of knowledge can be partially explained by the restricted experimental access to the human embryonic and fetal brain and limited recapitulation of human-specific neurodevelopmental events in model organisms. In the past years, novel 3D human stem cell-based in vitro modeling systems, so-called brain organoids, have proven their applicability for modeling early events of human brain development in health and disease. Since their emergence, brain organoids have been successfully employed to study molecular mechanisms of Zika and Herpes simplex virus-associated microcephaly, as well as more subtle events happening upon maternal alcohol and nicotine consumption. These studies converge on pathological mechanisms targeting neural stem cells. In this review, we discuss how brain organoids have recently revealed commonalities and differences in the effects of environmental adversities on human neurogenesis. We highlight both the breakthroughs in understanding the molecular consequences of environmental exposures achieved using organoids as well as the on-going challenges in the field related to variability in protocols and a lack of benchmarking, which make cross-study comparisons difficult.

Automation of Organoid Cultures: Current Protocols and Applications

GRAPHICAL ABSTRACT

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Development of 3D Cerebral Aggregates in the Brain Ventricles of Adult Mice

Abstract

The cerebral organoids are three-dimensional cell cultures formed from brain-specific cell types arising from embryonic or pluripotent stem cells. Organoids provide an opportunity to study the early stages of brain development and diseases of the central nervous system. However, the modeling of organoids is associated with a number of unsolved problems. Organoid production techniques involve a complex cell culture process that requires special media, growth factors, and often the use of a bioreactor. Even under standardized conditions, structures of different morphology are formed: from disorganized cell aggregates to structured minibrains, which are selected for study. For natural reasons, organoids grown in vitro do not have a blood supply, which limits their development. We tried to obtain cerebral aggregates similar to organoids in an in vivo model, where vascular growth and tissue blood supply are provided, for which we transplanted a cell suspension from the mouse embryonic neocortex into the lateral ventricles of the brain of adult mice. Therefore, the medium for cultivation was the cerebrospinal fluid, and the lateral ventricles of the brain, where it circulates, served as a bioreactor. The results showed that the neocortex from E14.5 is a suitable source of stem/progenitor cells that self-assemble into three-dimensional aggregates and vascularized in vivo. The aggregates consisted of a central layer of mature neurons, the marginal zone free of cells and a glia limitans, which resembled cerebral organoids. Thus, the lateral ventricles of the adult mouse brain can be used to obtain vascularized cell aggregates resembling cerebral organoids.

From Brain Organoids to Networking Assembloids: Implications for Neuroendocrinology and Stress Medicine

Brain organoids are three-dimensional cultures that contain multiple types of cells and cytoarchitectures, and resemble fetal human brain structurally and functionally. These organoids are being used increasingly to model brain development and disorders, however, they only partially recapitulate such processes, because of several limitations, including inability to mimic the distinct cortical layers, lack of functional neuronal circuitry as well as non-neural cells and gyrification, and increased cellular stress. Efforts to create improved brain organoid culture systems have led to region-specific organoids, vascularized organoids, glia-containing organoids, assembloids, sliced organoids and polarized organoids. Assembloids are fused region-specific organoids, which attempt to recapitulate inter-regional and inter-cellular interactions as well as neural circuitry development by combining multiple brain regions and/or cell lineages. As a result, assembloids can be used to model subtle functional aberrations that reflect complex neurodevelopmental, neuropsychiatric and neurodegenerative disorders. Mammalian organisms possess a highly complex neuroendocrine system, the stress system, whose main task is the preservation of systemic homeostasis, when the latter is threatened by adverse forces, the stressors. The main central parts of the stress system are the paraventricular nucleus of the hypothalamus and the locus caeruleus/norepinephrine-autonomic nervous system nuclei in the brainstem; these centers innervate each other and interact reciprocally as well as with various other CNS structures. Chronic dysregulation of the stress system has been implicated in major pathologies, the so-called chronic non-communicable diseases, including neuropsychiatric, neurodegenerative, cardiometabolic and autoimmune disorders, which lead to significant population morbidity and mortality. We speculate that brain organoids and/or assembloids could be used to model the development, regulation and dysregulation of the stress system and to better understand stress-related disorders. Novel brain organoid technologies, combined with high-throughput single-cell omics and gene editing, could, thus, have major implications for precision medicine.

Understanding, engineering, and modulating the growth of neural networks: An interdisciplinary approach

A deeper understanding of the brain and its function remains one of the most significant scientific challenges. It not only is required to find cures for a plethora of brain-related diseases and injuries but also opens up possibilities for achieving technological wonders, such as brain-machine interface and highly energy-efficient computing devices. Central to the brain's function is its basic functioning unit (i.e., the neuron). There has been a tremendous effort to understand the underlying mechanisms of neuronal growth on both biochemical and biophysical levels. In the past decade, this increased understanding has led to the possibility of controlling and modulating neuronal growth in vitro through external chemical and physical methods. We provide a detailed overview of the most fundamental aspects of neuronal growth and discuss how researchers are using interdisciplinary ideas to engineer neuronal networks in vitro. We first discuss the biochemical and biophysical mechanisms of neuronal growth as we stress the fact that the biochemical or biophysical processes during neuronal growth are not independent of each other but, rather, are complementary. Next, we discuss how utilizing these fundamental mechanisms can enable control over neuronal growth for advanced neuroengineering and biomedical applications. At the end of this review, we discuss some of the open questions and our perspectives on the challenges and possibilities related to controlling and engineering the growth of neuronal networks, specifically in relation to the materials, substrates, model systems, modulation techniques, data science, and artificial intelligence.

Advances in removing mass transport limitations for more physiologically relevant in vitro 3D cell constructs

Spheroids and organoids are promising models for biomedical applications ranging from human disease modeling to drug discovery. A main goal of these 3D cell-based platforms is to recapitulate important physiological parameters of their in vivo organ counterparts. One way to achieve improved biomimetic architectures and functions is to culture cells at higher density and larger total numbers. However, poor nutrient and waste transport lead to low stability, survival, and functionality over extended periods of time, presenting outstanding challenges in this field. Fortunately, important improvements in culture strategies have enhanced the survival and function of cells within engineered microtissues/organs. Here, we first discuss the challenges of growing large spheroids/organoids with a focus on mass transport limitations, then highlight recent tools and methodologies that are available for producing and sustaining functional 3D in vitro models. This information points toward the fact that there is a critical need for the continued development of novel cell culture strategies that address mass transport in a physiologically relevant human setting to generate long-lasting and large-sized spheroids/organoids.

Ethical Aspects of Brain Organoid Research in News Reports: An Exploratory Descriptive Analysis

Background and Objectives: Brain organoids are self-assembled, three-dimensional (3D) aggregates generated from pluripotent stem cells. These models are useful for experimental studies on human brain development and function and are therefore increasingly used for research worldwide. As their increasing use raises several ethical questions, we aimed to assess the current state of the press on brain organoid research using a cross-sectional database to understand the extent of discussion of this subject in the public. Materials and Methods: We conducted a descriptive analysis of news reports obtained from the Nexis Uni database, searched in April 2020. After extracting the news reports, the number of published reports in each year and the included terms were analyzed. Results: Up to April 2020, 332 news reports had been published, with over half of them published in the United States and the United Kingdom, with the numbers gradually increasing every year. In total, 113 (34.0%) news reports included ethics-related keywords, and the ratio of studies before and after the study-period midpoint was significantly increased (21.0% (2013–2016) vs. 38.2% (2017–2020); p = 0.0066, Chi-square test with Yates’ continuity correction). Conclusions: Although news reports on the ethical aspects of brain organoid research have been increasing gradually, there was a bias in the region of publication. Additional studies focusing on the ethical aspects of brain organoid research should strive to assess the public perception on the subject in different parts of the world.

The Potential of Induced Pluripotent Stem Cells to Treat and Model Alzheimer's Disease

An estimated 6.2 million Americans aged 65 or older are currently living with Alzheimer's disease (AD), a neurodegenerative disease that disrupts an individual's ability to function independently through the degeneration of key regions in the brain, including but not limited to the hippocampus, the prefrontal cortex, and the motor cortex. The cause of this degeneration is not known, but research has found two proteins that undergo posttranslational modifications: tau, a protein concentrated in the axons of neurons, and amyloid precursor protein (APP), a protein concentrated near the synapse. Through mechanisms that have yet to be elucidated, the accumulation of these two proteins in their abnormal aggregate forms leads to the neurodegeneration that is characteristic of AD. Until the invention of induced pluripotent stem cells (iPSCs) in 2006, the bulk of research was carried out using transgenic animal models that offered little promise in their ability to translate well from benchtop to bedside, creating a bottleneck in the development of therapeutics. However, with iPSC, patient-specific cell cultures can be utilized to create models based on human cells. These human cells have the potential to avoid issues in translatability that have plagued animal models by providing researchers with a model that closely resembles and mimics the neurons found in humans. By using human iPSC technology, researchers can create more accurate models of AD ex vivo while also focusing on regenerative medicine using iPSC in vivo. The following review focuses on the current uses of iPSC and how they have the potential to regenerate damaged neuronal tissue, in the hopes that these technologies can assist in getting through the bottleneck of AD therapeutic research.

Brain organoids: A promising model to assess oxidative stress-induced central nervous system damage

Oxidative stress (OS) is one of the most significant propagators of systemic damage with implications for widespread pathologies such as vascular disease, accelerated aging, degenerative disease, inflammation, and traumatic injury. OS can be induced by numerous factors such as environmental conditions, lifestyle choices, disease states, and genetic susceptibility. It is tied to the accumulation of free radicals, mitochondrial dysfunction, and insufficient antioxidant protection, which leads to cell aging and tissue degeneration over time. Unregulated systemic increase in reactive species, which contain harmful free radicals, can lead to diverse tissue-specific OS responses and disease. Studies of OS in the brain, for example, have demonstrated how this state contributes to neurodegeneration and altered neural plasticity. As the worldwide life expectancy has increased over the last few decades, the prevalence of OS-related diseases resulting from age-associated progressive tissue degeneration. Unfortunately, vital translational research studies designed to identify and target disease biomarkers in human patients have been impeded by many factors (e.g., limited access to human brain tissue for research purposes and poor translation of experimental models). In recent years, stem cell-derived three-dimensional tissue cultures known as "brain organoids" have taken the spotlight as a novel model for studying central nervous system (CNS) diseases. In this review, we discuss the potential of brain organoids to model the responses of human neural cells to OS, noting current and prospective limitations. Overall, brain organoids show promise as an innovative translational model to study CNS susceptibility to OS and elucidate the pathophysiology of the aging brain.

Running the full human developmental clock in interspecies chimeras using alternative human stem cells with expanded embryonic potential

Human pluripotent stem cells (hPSCs) can generate specialized cell lineages that have great potential for regenerative therapies and disease modeling. However, the developmental stage of the lineages generated from conventional hPSC cultures in vitro are embryonic in phenotype, and may not possess the cellular maturity necessary for corrective regenerative function in vivo in adult recipients. Here, we present the scientific evidence for how adult human tissues could generate human–animal interspecific chimeras to solve this problem. First, we review the phenotypes of the embryonic lineages differentiated from conventional hPSC in vitro and through organoid technologies and compare their functional relevance to the tissues generated during normal human in utero fetal and adult development. We hypothesize that the developmental incongruence of embryo-stage hPSC-differentiated cells transplanted into a recipient adult host niche is an important mechanism ultimately limiting their utility in cell therapies and adult disease modeling. We propose that this developmental obstacle can be overcome with optimized interspecies chimeras that permit the generation of adult-staged, patient-specific whole organs within animal hosts with human-compatible gestational time-frames. We suggest that achieving this goal may ultimately have to await the derivation of alternative, primitive totipotent-like stem cells with improved embryonic chimera capacities. We review the scientific challenges of deriving alternative human stem cell states with expanded embryonic potential, outline a path forward for conducting this emerging research with appropriate ethical and regulatory oversight, and defend the case of why current federal funding restrictions on this important category of biomedical research should be liberalized.

The Combination of Cell Cultured Technology and In Silico Model to Inform the Drug Development

Human-derived in vitro models can provide high-throughput efficacy and toxicity data without a species gap in drug development. Challenges are still encountered regarding the full utilisation of massive data in clinical settings. The lack of translated methods hinders the reliable prediction of clinical outcomes. Therefore, in this study, in silico models were proposed to tackle these obstacles from in vitro to in vivo translation, and the current major cell culture methods were introduced, such as human-induced pluripotent stem cells (hiPSCs), 3D cells, organoids, and microphysiological systems (MPS). Furthermore, the role and applications of several in silico models were summarised, including the physiologically based pharmacokinetic model (PBPK), pharmacokinetic/pharmacodynamic model (PK/PD), quantitative systems pharmacology model (QSP), and virtual clinical trials. These credible translation cases will provide templates for subsequent in vitro to in vivo translation. We believe that synergising high-quality in vitro data with existing models can better guide drug development and clinical use.

Potential of Induced Pluripotent Stem Cells for Use in Gene Therapy: History, Molecular Bases, and Medical Perspectives

Induced pluripotent stem cells (iPSCs) are defined as reprogrammed somatic cells exhibiting embryonic stem cell characteristics. Since their discovery in 2006, efforts have been made to utilize iPSCs in clinical settings. One of the promising fields of medicine, in which genetically patient-specific stem cells may prove themselves useful, is gene therapy. iPSCs technology holds potential in both creating models of genetic diseases and delivering therapeutic agents into the organism via auto-transplants, which reduces the risk of rejection compared to allotransplants. However, in order to safely administer genetically corrected stem cells into patients’ tissues, efforts must be made to establish stably pluripotent stem cells and reduce the risk of insertional tumorigenesis. In order to achieve this, optimal reprogramming factors and vectors must be considered. Therefore, in this review, the molecular bases of reprogramming safe iPSCs for clinical applications and recent attempts to translate iPSCs technology into the clinical setting are discussed.

Liver Organoids: Recent Developments, Limitations and Potential

Liver cell types derived from induced pluripotent stem cells (iPSCs) share the potential to investigate development, toxicity, as well as genetic and infectious disease in ways currently limited by the availability of primary tissue. With the added advantage of patient specificity, which can play a role in all of these areas. Many iPSC differentiation protocols focus on 3 dimensional (3D) or organotypic differentiation, as these offer the advantage of more closely mimicking in vivo systems including; the formation of tissue like architecture and interactions/crosstalk between different cell types. Ultimately such models have the potential to be used clinically and either with or more aptly, in place of animal models. Along with the development of organotypic and micro-tissue models, there will be a need to co-develop imaging technologies to enable their visualization. A variety of liver models termed "organoids" have been reported in the literature ranging from simple spheres or cysts of a single cell type, usually hepatocytes, to those containing multiple cell types combined during the differentiation process such as hepatic stellate cells, endothelial cells, and mesenchymal cells, often leading to an improved hepatic phenotype. These allow specific functions or readouts to be examined such as drug metabolism, protein secretion or an improved phenotype, but because of their relative simplicity they lack the flexibility and general applicability of ex vivo tissue culture. In the liver field these are more often constructed rather than developed together organotypically as seen in other organoid models such as brain, kidney, lung and intestine. Having access to organotypic liver like surrogates containing multiple cell types with in vivo like interactions/architecture, would provide vastly improved models for disease, toxicity and drug development, combining disciplines such as microfluidic chip technology with organoids and ultimately paving the way to new therapies.

Cerebral organoids and their potential for studies of brain diseases in domestic animals

The brain is a complex organ and any model for studying it in its normal and pathological aspects becomes a tool of choice for neuroscientists. The mastering and dissemination of protocols allowing brain organoids development have paved the way for a whole range of new studies in the field of brain development, modeling of neurodegenerative or neurodevelopmental diseases, understanding tumors as well as infectious diseases that affect the brain. While studies are so far limited to the use of human cerebral organoids, there is a growing interest in having similar models in other species. This review presents what is currently developed in this field, with a particular focus on the potential of cerebral organoids for studying neuro-infectious diseases in human and domestic animals.

Epigenetic regulation during human cortical development: Seq-ing answers from the brain to the organoid

Epigenetic regulation plays an important role in controlling gene expression during complex processes, such as development of the human brain. Mutations in genes encoding chromatin modifying proteins and in the non-protein coding sequences of the genome can potentially alter transcription factor binding or chromatin accessibility. Such mutations can frequently cause neurodevelopmental disorders, therefore understanding how epigenetic regulation shapes brain development is of particular interest. While epigenetic regulation of neural development has been extensively studied in murine models, significant species-specific differences in both the genome sequence and in brain development necessitate human models. However, access to human fetal material is limited and these tissues cannot be grown or experimentally manipulated ex vivo. Therefore, models that recapitulate particular aspects of human fetal brain development, such as the in vitro differentiation of human pluripotent stem cells (hPSCs), are instrumental for studying the epigenetic regulation of human neural development. Here, we examine recent studies that have defined changes in the epigenomic landscape during fetal brain development. We compare these studies with analogous data derived by in vitro differentiation of hPSCs into specific neuronal cell types or as three-dimensional cerebral organoids. Such comparisons can be informative regarding which aspects of fetal brain development are faithfully recapitulated by in vitro differentiation models and provide a foundation for using experimentally tractable in vitro models of human brain development to study neural gene regulation and the basis of its disruption to cause neurodevelopmental disorders.

Artificially Induced Pluripotent Stem Cell-Derived Whole-Brain Organoid for Modelling the Pathophysiology of Metachromatic Leukodystrophy and Drug Repurposing

Metachromatic leukodystrophy (MLD) is a rare neurodegenerative disease that results from a deficiency of the lysosomal enzyme arylsulfatase A (ARSA). Worldwide, there are between one in 40,000 and one in 160,000 people living with the disease. While there are currently no effective treatments for MLD, induced pluripotent stem cell-derived brain organoids have the potential to provide a better understanding of MLD pathogenesis. However, developing brain organoid models is expensive, time consuming and may not accurately reflect disease progression. Using accurate and inexpensive computer simulations of human brain organoids could overcome the current limitations. Artificially induced whole-brain organoids (aiWBO) have the potential to greatly expand our ability to model MLD and guide future wet lab research. In this study, we have upgraded and validated our artificially induced whole-brain organoid platform (NEUBOrg) using our previously validated machine learning platform, DeepNEU (v6.2). Using this upgraded NEUBorg, we have generated aiWBO simulations of MLD and provided a novel approach to evaluate factors associated with MLD pathogenesis, disease progression and new potential therapeutic options.

Functional 3-Dimensional Retinal Organoids: Technological Progress and Existing Challenges

Stem cell scientists have developed methods for the self-formation of artificial organs, often referred to as organoids. Organoids can be used as model systems for research in multiple biological disciplines. Yoshiki Sasai’s innovation for deriving mammalian retinal tissue from in vitro stem cells has had a large impact on the study of the biology of vision. New developments in retinal organoid technology provide avenues for in vitro models of human retinal diseases, studies of pathological mechanisms, and development of therapies for retinal degeneration, including electronic retinal implants and gene therapy. Moreover, these innovations have played key roles in establishing models for large-scale drug screening, studying the stages of retinal development, and providing a human model for personalized therapeutic approaches, like cell transplants to replace degenerated retinal cells. Here, we first discuss the importance of human retinal organoids to the biomedical sciences. Then, we review various functional features of retinal organoids that have been developed. Finally, we highlight the current limitations of retinal organoid technologies.

Human Cerebrospinal Fluid Promotes Neuronal Circuit Maturation of Human Induced Pluripotent Stem Cell-Derived 3D Neural Aggregates

Human induced pluripotent stem cell (hiPSC)-derived in vitro neural and organoid models resemble fetal, rather than adult brain properties, indicating that currently applied cultivation media and supplements are insufficient to achieve neural maturation beyond the fetal stage. In vivo, cerebrospinal fluid molecules are regulating the transition of the immature fetal human brain into a mature adult brain. By culturing hiPSC-3D neural aggregates in human cerebrospinal fluid (hCSF) obtained from healthy adult individuals, we demonstrate that hCSF rapidly triggers neurogenesis, gliogenesis, synapse formation, neurite outgrowth, suppresses proliferation of residing neural stem cells, and results in the formation of synchronously active neuronal circuits in vitro within 3 days. Thus, a physiologically relevant and adult brain-like milieu triggers maturation of hiPSC-3D neural aggregates into highly functional neuronal circuits in vitro. The approach presented here opens a new avenue to identify novel physiological factors for the improvement of hiPSC neural in vitro models.

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Human CSF triggers rapidly multiple maturation processes in human 3D neural models

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Human CSF triggers human neurogenesis and suppresses neural stem cell proliferation

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Human CSF triggers human astrocyte development, neurite growth, and synapse formation

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Human CSF triggers the maturation of neurons into highly functional neuronal circuits

Review of Design Considerations for Brain-on-a-Chip Models

In recent years, the need for sophisticated human in vitro models for integrative biology has motivated the development of organ-on-a-chip platforms. Organ-on-a-chip devices are engineered to mimic the mechanical, biochemical and physiological properties of human organs; however, there are many important considerations when selecting or designing an appropriate device for investigating a specific scientific question. Building microfluidic Brain-on-a-Chip (BoC) models from the ground-up will allow for research questions to be answered more thoroughly in the brain research field, but the design of these devices requires several choices to be made throughout the design development phase. These considerations include the cell types, extracellular matrix (ECM) material(s), and perfusion/flow considerations. Choices made early in the design cycle will dictate the limitations of the device and influence the end-point results such as the permeability of the endothelial cell monolayer, and the expression of cell type-specific markers. To better understand why the engineering aspects of a microfluidic BoC need to be influenced by the desired biological environment, recent progress in microfluidic BoC technology is compared. This review focuses on perfusable blood-brain barrier (BBB) and neurovascular unit (NVU) models with discussions about the chip architecture, the ECM used, and how they relate to the in vivo human brain. With increased knowledge on how to make informed choices when selecting or designing BoC models, the scientific community will benefit from shorter development phases and platforms curated for their application.

Pierre S, Hong K, Pavon N, Pak C, Sun Y. Patterning Neuroepithelial Cell Sheet via a Sustained Chemical Gradient Generated by Localized Passive Diffusion Devices

Recent advances in human pluripotent stem cells (hPSCs)-derived in vitro models open a new avenue for studying early stage human development. While current approaches leverage the self-organizing capability of hPSCs, it remains unclear whether extrinsic morphogen gradients are sufficient to pattern neuroectoderm tissues in vitro. While microfluidics or hydrogel-based approaches to generate chemical gradients are well-established, these systems either require continuous pumping or encapsulating cells in gels, making it difficult for adaptation in standard biology laboratories and downstream analysis. In this work, we report a new device design that leverages localized passive diffusion, or LPaD for short, to generate a stable chemical gradient in an open environment. As LPaD is operated simply by media changing, common issues for microfluidic systems such as leakage, bubble formation, and contamination can be avoided. The device contains a slit carved in a film filled with solid gelatin and connected to a static aqueous morphogen reservoir. Concentration gradients generated by the device were visualized via DAPI fluorescent intensity and were found to be stable for up to 168 h. Using this device, we successfully induced cellular response of Madin-Darby canine kidney (MDCK) cells to the concentration gradient of a small-molecule drug, cytochalasin D. Furthermore, we efficiently patterned the dorsal-ventral axis of hPSC-derived forebrain neuroepithelial cells with the sonic hedgehog (Shh) signal gradient generated by the LPaD devices. Together, LPaD devices are powerful tools to control the local chemical microenvironment for engineering organotypic structures in vitro.

Modeling Rett Syndrome with Human Pluripotent Stem Cells: Mechanistic Outcomes and Future Clinical Perspectives

Rett syndrome (RTT) is a neurodevelopmental disorder caused by mutations in the gene encoding the methyl-CpG-binding protein 2 (MeCP2). Among many different roles, MeCP2 has a high phenotypic impact during the different stages of brain development. Thus, it is essential to intensively investigate the function of MeCP2, and its regulated targets, to better understand the mechanisms of the disease and inspire the development of possible therapeutic strategies. Several animal models have greatly contributed to these studies, but more recently human pluripotent stem cells (hPSCs) have been providing a promising alternative for the study of RTT. The rapid evolution in the field of hPSC culture allowed first the development of 2D-based neuronal differentiation protocols, and more recently the generation of 3D human brain organoid models, a more complex approach that better recapitulates human neurodevelopment in vitro. Modeling RTT using these culture platforms, either with patient-specific human induced pluripotent stem cells (hiPSCs) or genetically-modified hPSCs, has certainly contributed to a better understanding of the onset of RTT and the disease phenotype, ultimately allowing the development of high throughput drugs screening tests for potential clinical translation. In this review, we first provide a brief summary of the main neurological features of RTT and the impact of MeCP2 mutations in the neuropathophysiology of this disease. Then, we provide a thorough revision of the more recent advances and future prospects of RTT modeling with human neural cells derived from hPSCs, obtained using both 2D and organoids culture systems, and its contribution for the current and future clinical trials for RTT.

Primate cell fusion disentangles gene regulatory divergence in neurodevelopment

Among primates, humans display a unique trajectory of development that is responsible for the many traits specific to our species. However, the inaccessibility of primary human and chimpanzee tissues has limited our ability to study human evolution. Comparative in vitro approaches using primate-derived induced pluripotent stem cells have begun to reveal species differences on the cellular and molecular levels1,2. In particular, brain organoids have emerged as a promising platform to study primate neural development in vitro3-5, although cross-species comparisons of organoids are complicated by differences in developmental timing and variability of differentiation6,7. Here we develop a new platform to address these limitations by fusing human and chimpanzee induced pluripotent stem cells to generate a panel of tetraploid hybrid stem cells. We applied this approach to study species divergence in cerebral cortical development by differentiating these cells into neural organoids. We found that hybrid organoids provide a controlled system for disentangling cis- and trans-acting gene-expression divergence across cell types and developmental stages, revealing a signature of selection on astrocyte-related genes. In addition, we identified an upregulation of the human somatostatin receptor 2 gene (SSTR2), which regulates neuronal calcium signalling and is associated with neuropsychiatric disorders8,9. We reveal a human-specific response to modulation of SSTR2 function in cortical neurons, underscoring the potential of this platform for elucidating the molecular basis of human evolution.

Toward Studying Cognition in a Dish

Bioengineered neural tissues help advance our understanding of neurodevelopment, regeneration, and neural disease; however, it remains unclear whether they can replicate higher-order functions including cognition. Building upon technical achievements in the fields of biomaterials, tissue engineering, and cell biology, investigators have generated an assortment of artificial brain structures and cocultured circuits. Though they have displayed basic electrochemical signaling, their capacities to generate minimal patterns of information processing suggestive of high-order cognitive analogues have not yet been explored. Here, we review the current state of neural tissue engineering and consider the possibility of a study of cognition in vitro. We adopt a practical definition of minimal cognition, anticipate problems of measurement, and discuss solutions toward a study of cognition in a dish.

Vascularization of human brain organoids

Human brain organoids are three-dimensional tissues that are generated in vitro from pluripotent stem cells and recapitulate the early development of the human brain. Brain organoids consist mainly of neural lineage cells, such as neural stem/precursor cells, neurons, astrocytes, and oligodendrocytes. However, all human brain organoids lack vasculature, which plays indispensable roles not only in brain homeostasis but also in brain development. In addition to the delivery of oxygen and nutrition, accumulating evidence suggests that the vascular system of the brain regulates neural differentiation, migration, and circuit formation during development. Therefore, vascularization of human brain organoids is of great importance. Current trials to vascularize various organoids include the adjustment of cultivation protocols, the introduction of microfluidic devices, and the transplantation of organoids into immunodeficient mice. In this review, we summarize the efforts to accomplish vascularization and perfusion of brain organoids, and we discuss these attempts from a forward-looking perspective.

Hippocampal regenerative medicine: neurogenic implications for addiction and mental disorders

Psychiatric illness is a prevalent and highly debilitating disorder, and more than 50% of the general population in both middle- and high-income countries experience at least one psychiatric disorder at some point in their lives. As we continue to learn how pervasive psychiatric episodes are in society, we must acknowledge that psychiatric disorders are not solely relegated to a small group of predisposed individuals but rather occur in significant portions of all societal groups. Several distinct brain regions have been implicated in neuropsychiatric disease. These brain regions include corticolimbic structures, which regulate executive function and decision making (e.g., the prefrontal cortex), as well as striatal subregions known to control motivated behavior under normal and stressful conditions. Importantly, the corticolimbic neural circuitry includes the hippocampus, a critical brain structure that sends projections to both the cortex and striatum to coordinate learning, memory, and mood. In this review, we will discuss past and recent discoveries of how neurobiological processes in the hippocampus and corticolimbic structures work in concert to control executive function, memory, and mood in the context of mental disorders.

A region of the brain called the hippocampus and its connections to other parts of the brain via what are called cortico-limbic structures are implicated in a variety of mental health disorders. These disorders can be accompanied by reduced hippocampal volume. Mi-Hyeon Jang, Doo-Sup Choi and colleagues at the Mayo Clinic College of Medicine and Science, Rochester, USA, review the role of hippocampal and cortico-limbic neurobiology in memory and mood in mental disorders. They focus particular attention on the role of neurogenesis, the production and growth of new nerve cells and connections. Disrupted neurogenesis in the adult hippocampus is implicated in conditions including addiction, depression, schizophrenia and related psychotic disorders. Learning more about neural regeneration in the hippocampus could yield insights into mental health conditions and open new avenues toward developing drug-based treatments.

Advanced Spheroid, Tumouroid and 3D Bioprinted In-Vitro Models of Adult and Paediatric Glioblastoma

The life expectancy of patients with high-grade glioma (HGG) has not improved in decades. One of the crucial tools to enable future improvement is advanced models that faithfully recapitulate the tumour microenvironment; they can be used for high-throughput screening that in future may enable accurate personalised drug screens. Currently, advanced models are crucial for identifying and understanding potential new targets, assessing new chemotherapeutic compounds or other treatment modalities. Recently, various methodologies have come into use that have allowed the validation of complex models—namely, spheroids, tumouroids, hydrogel-embedded cultures (matrix-supported) and advanced bioengineered cultures assembled with bioprinting and microfluidics. This review is designed to present the state of advanced models of HGG, whilst focusing as much as is possible on the paediatric form of the disease. The reality remains, however, that paediatric HGG (pHGG) models are years behind those of adult HGG. Our goal is to bring this to light in the hope that pGBM models can be improved upon.

Cryogel biomaterials for neuroscience applications

Biomaterials in the form of 3D polymeric scaffolds have been used to create structurally and functionally biomimetic constructs of nervous system tissue. Such constructs can be used to model defects and disease or can be used to supplement neuronal tissue regeneration and repair. One such group of biomaterial scaffolds are hydrogels, which have been widely investigated for cell/tissue culture and as cell or molecule delivery systems in the field of neurosciences. However, a subset of hydrogels called cryogels, have shown to possess several distinct structural advantages over conventional hydrogel networks. Their macroporous structure, created via the time and resource efficient fabrication process (cryogelation) not only allows mass fluid transport throughout the structure, but also creates a high surface area to volume ratio for cell growth or drug loading. In addition, the macroporous structure of cryogels is ideal for applications in the central nervous system as they are very soft and spongey, yet also robust, which makes them a user-friendly and reproducible tool to address neuroscience challenges. In this review, we aim to provide the neuroscience community, who may not be familiar with the fundamental concepts of cryogels, an accessible summary of the basic information that pertain to their use in the brain and nervous tissue. We hope that this review shall initiate creative ways that cryogels could be further adapted and employed to tackle unsolved neuroscience challenges.