Self-formation of optic cups and storable stratified neural retina from human ESCs

Timely neurogenesis drives the transition from nematic to crystalline nuclear packing during retinal morphogenesis

Correct organogenesis depends on the timely coordination of developmental processes, such as cell proliferation, differentiation, and migration. This coordination is particularly critical in crowded tissues, such as pseudostratified epithelia (PSE) that are often found as organ precursors. They are composed of elongated epithelial cells with densely packed nuclei aligned along the apicobasal axis. While cell cycle-dependent nuclear movements in PSE are well studied, less is known about how nuclear packing influences tissue morphogenesis. To investigate this, we analyzed nuclear shapes, sizes, and neighborhood statistics in zebrafish neuroepithelia, focusing on the retinal PSE. We found that nuclei exhibit elongated shapes and biaxial nematic-like orientational order but remain positionally disordered. During retinal development, nuclear packing density increases, approaching theoretical limits. This occurs when the tissue transitions to a laminated structure and nuclear shapes are remodeled. Timely neurogenesis is critical as failure to initiate neurogenesis leads to tissue deformations. These findings highlight the influence of nuclear shape and positioning for organ morphogenesis.

A new protocol for the development of organoids based on molecular mechanisms in the developing newborn rat brain: Prospective applications in the study of Alzheimer's disease

BACKGROUND

Brain organoids have emerged as powerful models for studying brain development and neurological disorders COMPARISON WITH EXISTING METHODS: Current models rely on stem cell isolation and differentiation using different growth factors. Thus, their composition varies according to the protocol followed.

NEW METHOD

We developed a simple protocol to generate organoids from newborn rat whole brain. It is a one-step procedure that yields organoids of consistent composition. The whole brains from 3-day old pups were digested enzymatically. All isolated cells were seeded in culture plates using a basement membrane extract (BME) matrix as a scaffold and cultured in the presence of the appropriate medium.

RESULTS

Hematoxylin-eosin staining of 28-day-old cultured domes revealed their structural integrity, while immunohistochemistry confirmed the presence of neurons, astrocytes, microglia, and progenitor stem cells in the structures. To assess whether these organoids can serve as a model to study brain physiopathology, and in particular neurodegenerative diseases such as Alzheimer's disease (AD), we determined how these organoids respond upon their exposure to lipopolysaccharides (LPS), a potent neuroinflammatory factor. LPS-induced amyloid precursor protein (APP), tau protein and glial fibrillary acidic protein (GFAP) expression. Moreover, the intracellular levels of IL-1β and the extracellular levels of amyloid-beta (Aβ) were also elevated.

CONCLUSIONS

Therefore, this simple protocol results in the generation of functional brain organoids with a consistent structure, that requires no use of varying factors that may affect the structure and function of the produced organoids, thus providing a valuable tool for the study of the physiopathology of neurodegenerative disorders.

Modelling human brain development and disease with organoids

Organoids are systems derived from pluripotent stem cells at the interface between traditional monolayer cultures and in vivo animal models. The structural and functional characteristics of organoids enable the modelling of early stages of brain development in a physiologically relevant 3D environment. Moreover, organoids constitute a tool with which to analyse how individual genetic variation contributes to the susceptibility and progression of neurodevelopmental disorders. This Roadmap article describes the features of brain organoids, focusing on the neocortex, and their advantages and limitations - in comparison with other model systems - for the study of brain development, evolution and disease. We highlight avenues for enhancing the physiological relevance of brain organoids by integrating bioengineering techniques and unbiased high-throughput analyses, and discuss future applications. As organoids advance in mimicking human brain functions, we address the ethical and societal implications of this technology.

Intelligent Manufacturing for Osteoarthritis Organoids

Osteoarthritis (OA) is the most prevalent degenerative joint disease worldwide, imposing a substantial global disease burden. However, its pathogenesis remains incompletely understood, and effective treatment strategies are still lacking. Organoid technology, in which stem cells or progenitor cells self-organise into miniature tissue structures under three-dimensional (3D) culture conditions, provides a promising in vitro platform for simulating the pathological microenvironment of OA. This approach can be employed to investigate disease mechanisms, carry out high-throughput drug screening and facilitate personalised therapies. This review summarises joint structure, OA pathogenesis and pathological manifestations, thereby establishing the disease context for the application of organoid technology. It then examines the components of the arthrosis organoid system, specifically addressing cartilage, subchondral bone, synovium, skeletal muscle and ligament organoids. Furthermore, it details various strategies for constructing OA organoids, including considerations of cell selection, pathological classification and fabrication techniques. Notably, this review introduces the concept of intelligent manufacturing of OA organoids by incorporating emerging engineering technologies such as artificial intelligence (AI) into the organoid fabrication process, thereby forming an innovative software and hardware cluster. Lastly, this review discusses the challenges currently facing intelligent OA organoid manufacturing and highlights future directions for this rapidly evolving field. By offering a comprehensive overview of state-of-the-art methodologies and challenges, this review anticipates that intelligent, automated fabrication of OA organoids will expedite fundamental research, drug discovery and personalised translational applications in the orthopaedic field.

Rekindling Vision: Innovative Strategies for Treating Retinal Degeneration

Retinal degeneration, characterized by the progressive loss of photoreceptors, retinal pigment epithelium cells, and/or ganglion cells, is a leading cause of vision impairment. These diseases are generally classified as inherited (e.g., retinitis pigmentosa, Stargardt disease) or acquired (e.g., age-related macular degeneration, diabetic retinopathy, glaucoma) ocular disorders that can lead to blindness. Available treatment options focus on managing symptoms or slowing disease progression and do not address the underlying causes of these diseases. However, recent advancements in regenerative medicine offer alternative solutions for repairing or protecting degenerated retinal tissue. Stem and progenitor cell therapies have shown great potential to differentiate into various retinal cell types and can be combined with gene editing, extracellular vesicles and exosomes, and bioactive molecules to modulate degenerative cellular pathways. Additionally, gene therapy and neuroprotective molecules play a crucial role in enhancing the efficacy of regenerative approaches. These innovative strategies hold the potential to halt the progression of retinal degenerative disorders, repair or replace damaged cells, and improve visual function, ultimately leading to a better quality of life for those affected.

Three-Dimensional Bioprinting of Astrocytes and Endothelial Cells to Direct Retinal Axon Growth and Vascularization

Retinal organoids (ROs) are currently used to study retinal development and diseases but cannot model glaucoma because they fail to form a nerve fiber layer (NFL) and optic nerve (ON). Utilizing three-dimensional bioprinting, ON head astrocytes (ONHAs) and vascular endothelial cells, both of which contribute to NFL development in vivo but are absent in ROs, were positioned at the center of scaffolds seeded with retinal ganglion cells (RGCs). In experiments using ONHAs isolated from developing retinas, polarization of RGC neurite growth increased by 43% while ONHA from adult retinas or astrocytes from the developing peripheral retina or developing cortex did not increase polarization above controls. Furthermore, RGC-seeded scaffolds increased both the number and rate of ONHAs migrating out from the printed center compared to scaffolds lacking RGCs, mimicking the migration pattern observed during retinal development. Finally, in scaffolds containing both ONHAs and endothelial cells, the endothelial cells preferentially migrate on and only form vascular tube structures on scaffolds also containing RGCs. These results suggest that recreating the developmental organization of the retina can recapitulate the mechanism of NFL development and retinal vascularization in vitro. This step is not only necessary for the development of retinal models of glaucoma but has the potential for translation to other parts of the central nervous system.

Current Development of iPSC-Based Modeling in Neurodegenerative Diseases

Over the past two decades, significant advancements have been made in the induced pluripotent stem cell (iPSC) technology. These developments have enabled the broader application of iPSCs in neuroscience, improved our understanding of disease pathogenesis, and advanced the investigation of therapeutic targets and methods. Specifically, optimizations in reprogramming protocols, coupled with improved neuronal differentiation and maturation techniques, have greatly facilitated the generation of iPSC-derived neural cells. The integration of the cerebral organoid technology and CRISPR/Cas9 genome editing has further propelled the application of iPSCs in neurodegenerative diseases to a new stage. Patient-derived or CRISPR-edited cerebral neurons and organoids now serve as ideal disease models, contributing to our understanding of disease pathophysiology and identifying novel therapeutic targets and candidates. In this review, we examine the development of iPSC-based models in neurodegenerative diseases, including Alzheimer's disease, Parkinson's disease, and Huntington's disease.

Retinal Organoids: Innovative Tools for Understanding Retinal Degeneration

Retinal degenerative diseases (RDDs) comprise diverse genetic and phenotypic conditions that cause progressive retinal dysfunction and cell loss, leading to vision impairment or blindness. Most RDDs lack appropriate animal models for their study, which affects understanding their disease mechanisms and delays the progress of new treatment development. Recent advances in stem cell engineering, omics, and organoid technology are facilitating research into diseases for which there are no previously existing models. The development of retinal organoids produced from human stem cells has impacted the study of retinal development as well as the development of in vitro models of diseases, opening possibilities for applications in regenerative medicine, drug discovery, and precision medicine. In this review, we recapitulate research in the retinal organoid models for RDD, mentioning some of the main pathways underlying retinal neurodegeneration that can be studied in these new models, as well as their limitations and future challenges in this rapidly advancing field.

Mechanical characterization of nonlinear elasticity of growing intestinal organoids with a microinjection method

Mechanical properties of intestinal organoids are crucial for intestinal development, homeostatic renewal, and pathogenesis. However, characterizing these properties remains challenging. Here, we developed a microinjection-based method to quantify the growth time-dependent nonlinear elasticity of intestinal organoids. With aid of the neo-Hookean hyperelastic constitutive model, we discovered that the global elastic modulus of intestinal organoids increased linearly during the early stages of culture, followed by a sharp rise, indicating a time-dependent nonlinear hardening behaviour during growth. The global modulus of intestinal organoids was found to correlate with the cell phenotype ratio, revealing a significant relationship between mechanical properties and biological phenotypes. Furthermore, we developed a biomechanical model on the basis of the unsteady Bernoulli equation to quantitatively explore the global mechanical responses of intestinal organoids, which showed good agreement with the experimental data. The work not only elucidated the mechanical response and modulus characteristics of small intestinal organoids from a biomechanical perspective, but also presented a new microinjection-based methodology for quantifying the mechanical properties of organoids, offering significant potential for various organoid-related applications. STATEMENT OF SIGNIFICANCE: Mechanical properties of intestinal organoids are essential for intestinal development, homeostatic renewal, and pathogenesis. However, how to quantitatively characterize their global mechanical properties remains challenging. Here, we developed a new microinjection-based experimental platform to quantify spatiotemporal dynamics of mechanical responses and global elasticity of intestinal organoids. Unlike traditional nanoindentation methods, the proposed characterization technique can quantitatively measure the global mechanical properties of organoids, which is crucial for detecting the inherent relationship between the global mechanical properties and the biological phenotypes of organoids. Likewise, it established a methodological foundation for revealing the mechanobiological characteristics associated with the growth and development of various organoids. This can enhance our understanding of mechanobiological mechanisms of organoids and is beneficial for various organoid-related applications.

Early spinal cord development: from neural tube formation to neurogenesis

As one of the simplest and most evolutionarily conserved parts of the vertebrate nervous system, the spinal cord serves as a key model for understanding the principles of nervous system construction. During embryonic development, the spinal cord originates from a population of bipotent stem cells termed neuromesodermal progenitors, which are organized within a transient embryonic structure known as the neural tube. Neural tube morphogenesis differs along its anterior-to-posterior axis: most of the neural tube (including the regions that will develop into the brain and the anterior spinal cord) forms via the bending and dorsal fusion of the neural groove, but the establishment of the posterior region of the neural tube involves de novo formation of a lumen within a solid medullary cord. The early spinal cord primordium consists of highly polarized neural progenitor cells organized into a pseudostratified epithelium. Tight regulation of the cell division modes of these progenitors drives the embryonic growth of the neural tube and initiates primary neurogenesis. A rich history of observational and functional studies across various vertebrate models has advanced our understanding of the cellular events underlying spinal cord development, and these foundational studies are beginning to inform our knowledge of human spinal cord development.

Identification and characterization of human retinal stem cells capable of retinal regeneration

Human retinal stem cells hold great promise in regenerative medicine, yet their existence and characteristics remain elusive. Here, we performed single-cell multiomics and spatial transcriptomics of human fetal retinas and uncovered a cell subpopulation, human neural retinal stem-like cells (hNRSCs), distinct from retinal pigment epithelium stem-like cells and traditional retinal progenitor cells. We found that these hNRSCs reside in the peripheral retina in the ciliary marginal zone, exhibiting substantial self-renewal and differentiation potential. We conducted single-cell and spatial transcriptomic analyses of human retinal organoids (hROs) and revealed that hROs contain a population of hNRSCs with similar transcriptional profiles and developmental trajectories to hNRSCs in the fetal retina potentially capable of regenerating all retinal cells. Furthermore, we identified crucial transcription factors, such as MECOM, governing hNRSC commitment to neural retinogenesis and regulating repair processes in hROs. hRO-derived hNRSCs transplanted into the rd10 mouse model of retinitis pigmentosa differentiated and were integrated into the retina, alleviated retinal degeneration, and improved visual function. Overall, our work identifies and characterizes a distinct category of retinal stem cells from human retinas, underscoring their regenerative potential and promise for transplantation therapy.

Human Retinal Organoid Model of Ocular Toxoplasmosis

The health burden of ocular toxoplasmosis is substantial, and there is an unmet need for safe and curative anti-microbial drugs. One major barrier to research on new therapeutics is the lack of in vitro human-based models beyond two-dimensional cultured cells and tissue explants. We aimed to address this research gap by establishing a human retinal organoid model of ocular toxoplasmosis. Retinal organoids, generated from human induced pluripotent stem cells and grown to two stages of organization, were incubated with a suspension of live or heat-killed GT-1 strain T. gondii tachyzoites, or medium without tachyzoites. Both developing (1 month post-isolation) and matured (6 months post-isolation) organoids were susceptible to infection. Spread of live parasites from the margin to the entire organoid over 1 week was indicated by immunolabelling for T. gondii surface antigen 1. This progression was accompanied by changes in the levels of selected tachyzoite transcripts-SAG1, GRA6, and ROP16-and human cytokine transcripts-CCL2, CXCL8, CXCL10, and IL6-in infected versus control conditions. Our human retinal organoid model of ocular toxoplasmosis offers the opportunity for many future lines of study, including tachyzoite interactions with retinal cell populations and leukocyte subsets, parasite stage progression, and disease processes of different T. gondii strains, as well as drug testing.

Inter-chromosomal insertions at Xq27.1 associated with retinal dystrophy induce dysregulation of LINC00632 and CDR1as/ciRS-7

In two unrelated families with X-linked inherited retinal dystrophy, identification of the causative variants was elusive. Interrogation of the next-generation sequencing (NGS) data revealed a "dark" intergenic region on Xq27.1 with poor coverage. Long-range PCR and DNA walking across this region revealed different inter-chromosomal insertions into the human-specific palindrome on Xq27.1: a 58 kb insertion of 9p24.3 [der(X)dir ins(X;9)(q27.1;p24.3)] in family 1 and a 169 kb insertion of 3p14.2 [der(X)inv ins(X;3)(q27.1;p14.2)] in family 2. To explore the functional consequence of these structural variants in genomic and cellular contexts, induced pluripotent stem cells were derived from affected and control fibroblasts and differentiated to retinal organoids (ROs) and retinal pigment epithelium. Transcriptional dysregulation was evaluated using RNA sequencing (RNA-seq) and RT-qPCR. A downstream long non-coding RNA, LINC00632 (Xq27.1), was upregulated in ROs from both families compared to control samples. In contrast, the circular RNA CDR1as/ciRS-7 (circular RNA sponge for miR-7), spliced from linear LINC00632, was downregulated. To investigate this tissue-specific dysregulation, we interrogated the landscape of the locus using Hi-C and cleavage under targets and tagmentation sequencing (CUT&Tag). This revealed active retinal enhancers within the insertions within a topologically associated domain that also contained the upstream promoter of LINC00632, permitting ectopic contact. Furthermore, CDR1as/ciRS-7 acts as a "sponge" for miR-7, and target genes of miR-7 were also dysregulated in ROs derived from both families. We describe a new genomic mechanism for retinal dystrophy, and our data support a convergent tissue-specific mechanism of altered regulation of LINC00632 and CDR1as/ciRS-7 as a consequence of the insertions within the palindrome on Xq27.1.

From morphology to single-cell molecules: high-resolution 3D histology in biomedicine

High-resolution three-dimensional (3D) tissue analysis has emerged as a transformative innovation in the life sciences, providing detailed insights into the spatial organization and molecular composition of biological tissues. This review begins by tracing the historical milestones that have shaped the development of high-resolution 3D histology, highlighting key breakthroughs that have facilitated the advancement of current technologies. We then systematically categorize the various families of high-resolution 3D histology techniques, discussing their core principles, capabilities, and inherent limitations. These 3D histology techniques include microscopy imaging, tomographic approaches, single-cell and spatial omics, computational methods and 3D tissue reconstruction (e.g. 3D cultures and spheroids). Additionally, we explore a wide range of applications for single-cell 3D histology, demonstrating how single-cell and spatial technologies are being utilized in the fields such as oncology, cardiology, neuroscience, immunology, developmental biology and regenerative medicine. Despite the remarkable progress made in recent years, the field still faces significant challenges, including high barriers to entry, issues with data robustness, ambiguous best practices for experimental design, and a lack of standardization across methodologies. This review offers a thorough analysis of these challenges and presents recommendations to surmount them, with the overarching goal of nurturing ongoing innovation and broader integration of cellular 3D tissue analysis in both biology research and clinical practice.

Retina-specific laminin-based generation of photoreceptor progenitors from human pluripotent stem cells under xeno-free and chemically defined conditions

Photoreceptor cell replacement therapy for retinal degenerative diseases is a promising approach. Presently, most protocols aimed at generating clinically safe and functional cells for retinal diseases face challenges such as low efficiency, poor reproducibility, and time-consuming and complex procedures. These could be due to the dependency on animal-derived components in cell culture media and substrates that support the cell differentiation process. Such conditions are poorly defined chemically, which could affect the robustness of the method and hinder clinical translation of cell therapy in retinal diseases. Here, we describe a simple protocol that is xenogen free and chemically defined to differentiate human embryonic stem cells to photoreceptor progenitors. Human recombinant extracellular matrix laminin 523 and 521 isoforms were used to mimic the inter-photoreceptor matrix niche environment to promote the retinal cell differentiation process. This was also accomplished by the unique combination of two cell differentiation media that recapitulates the retinal development signaling processes. In comparison to other protocols, our protocol does not require any mechanical dissection, which can be technically subjective and tedious. Our directed differentiation method generates photoreceptor progenitors that express ~17% cone-rod homeobox (CRX) transcript based on single-cell transcriptomic analyses by day 32. These day 32 photoreceptor progenitors can be cryopreserved and still maintain high cell viability after thawing for cell transplantation. This protocol can be easily reproduced and performed by researchers with basic cell culture experience, which is particularly important for retinal research progress and clinical cell manufacturing in a Good Manufacturing Practice facility.

Organoids from pluripotent stem cells and human tissues: When two cultures meet each other

Human organoids are a widely used tool in cell biology to study homeostatic processes, disease, and development. The term organoids covers a plethora of model systems from different cellular origins that each have unique features and applications but bring their own challenges. This review discusses the basic principles underlying organoids generated from pluripotent stem cells (PSCs) as well as those derived from tissue stem cells (TSCs). We consider how well PSC- and TSC-organoids mimic the different intended organs in terms of cellular complexity, maturity, functionality, and the ongoing efforts to constitute predictive complex models of in vivo situations. We discuss the advantages and limitations associated with each system to answer different biological questions including in the field of cancer and developmental biology, and with respect to implementing emerging advanced technologies, such as (spatial) -omics analyses, CRISPR screens, and high-content imaging screens. We postulate how the two fields may move forward together, integrating advantages of one to the other.

Unravelling genotype-phenotype correlations in Stargardt disease using patient-derived retinal organoids

Stargardt disease is an inherited retinopathy affecting approximately 1:8000 individuals. It is characterised by biallelic variants in ABCA4 which encodes a vital protein for the recycling of retinaldehydes in the retina. Despite its prevalence and impact, there are currently no treatments available for this condition. Furthermore, 35% of STGD1 cases remain genetically unsolved. To investigate the cellular and molecular characteristics associated with STGD1, we generated iPSCs from two monoallelic unresolved (PT1 & PT2), late-onset STGD1 cases with the heterozygous complex allele - c.[5461-10 T > C;5603 A > T]. Both patient iPSCs and those from a biallelic affected control (AC) carrying -c.4892 T > C and c.4539+2001G > A, were differentiated to retinal organoids, which developed all key retinal neurons and photoreceptors with outer segments positive for ABCA4 expression. We observed patient-specific disruption to lamination with OPN1MW/LW+ cone photoreceptor retention in the retinal organoid centre during differentiation. Photoreceptor retention was more severe in the AC case affecting both cones and rods, suggesting a genotype/phenotype correlation. scRNA-Seq suggests retention may be due to the induction of stress-related pathways in photoreceptors. Whole genome sequencing successfully identified the missing alleles in both cases; PT1 reported c.-5603A > T in homozygous state and PT2 uncovered a rare hypomorph - c.-4685T > C. Furthermore, retinal organoids were able to recapitulate the retina-specific splicing defect in PT1 as shown by long-read RNA-seq data. Collectively, these results highlight the suitability of retinal organoids in STGD1 modelling. Their ability to display genotype-phenotype correlations enhances their utility as a platform for therapeutic development.

Comparative models of uveitis

Several clinical subtypes of uveitis exist yet specific immunopathogenic mechanisms involved remain unclear. Ex vivo studies are limited by lack of fresh retinal biopsies and studies have relied on aqueous humour or peripheral blood, which may not directly reflect disease. The aim of this review is to compare the various in vivo models and review their contributions to our understanding of disease processes. These models, although unable to reflect all clinical signs, have provided insight into the contribution of genes and molecules, characterisation of effector T-cells, cell trafficking into retinal tissues, the contribution of tissue-resident myeloid cells and the mechanism(s) of action of several anti-inflammatory compounds. In vivo uveitis models have provided an excellent resource with which to study the molecular and cellular processes involved. Recent refinements in models, improved imaging, and the application of omics have greatly increased the number of readouts and translational opportunities. Future approaches with in vitro models will also be discussed.

Generation of a Double Reporter mES Cell Line to Simultaneously Trace the Generation of Retinal Progenitors and Photoreceptors

Three-dimensional retinal culture systems help to understand eye development and the pathology of disorders. There is a need for reporter stem cell lines to allow in vitro studies on retinal progenitors and photoreceptors and their developmental dynamics or properties and to test therapeutic approaches. The isolation of pure progenitor populations or photoreceptor precursors may serve for drug, gene, and cell therapy development. Here, we generated a dual-reporter mouse embryonic stem cell line Crx-GFP;Rax-mCherry enabling the visualization or isolation of photoreceptors and retinal progenitors from retinal organoid settings. From day 4 organoids, we isolated mCherry-positive cells to assess their early retinal progenitor identity with proliferation tests as well as transcriptomic and proteomic profiling. The timing of eye field transcription factor expression at the transcriptomic and protein levels is in accordance with mouse retinogenesis. This new line will be helpful for rapidly investigating biological questions or testing therapeutics before using human induced pluripotent stem cells (iPSCs), which require a much longer time for retinal organoid formation.

Reprogramming patient-induced pluripotent stem cell-specific retinal organoids for deciphering epigenetic modifications of RNA methylation

BACKGROUND

Induced pluripotent stem cell (iPSC) technology has emerged as a powerful tool for disease modeling, providing an innovative platform for investigating disease mechanisms. iPSC-derived organoids, including retinal organoids, offer patient-specific models that closely replicate in vivo cellular environments, making them ideal for studying retinal neurodegenerative diseases where retinal ganglion cells (RGCs) are impacted. N6-methyladenosine (m6A), a prevalent internal modification in eukaryotic mRNAs, plays a critical role in RNA metabolic processes such as splicing, stability, translation, and transport. Given the high energy demands of RGCs, mitochondrial dysfunction, which leads to impaired adenosine triphosphate (ATP) production and increased reactive oxygen species (ROS) levels, is often central to the progression of retinal neurodegenerative disorders. However, the epigenetic mechanisms underlying m6A modification and their contributions to these conditions remain unclear.

METHODS

Patient-specific iPSCs were generated from individuals with Leber hereditary optic neuropathy (LHON) and differentiated into RGCs within retinal organoids. To analyze m6A methylation, we used quantitative polymerase chain reaction (PCR) and focused on differential expression of key m6A-modifying enzymes.

RESULTS

iPSC-derived retinal organoids are adaptable for studying and investigating the epigenetic mechanisms of retinal neurodegenerative diseases. Our data demonstrated the profiling of global m6A-related gene expression levels in LHON patient-derived iPSC-RGCs compared with controls, highlighting specific disruptions in m6A modification pathways.

CONCLUSION

These findings suggest that differential m6A modifications may play pivotal roles in the pathogenesis of retinal neurodegenerative diseases and affect the progression of the disease in affected individuals.

Biomaterial-assisted organoid technology for disease modeling and drug screening

Developing disease models and screening for effective drugs are key areas of modern medical research. Traditional methodologies frequently fall short in precisely replicating the intricate architecture of bodily tissues and organs. Nevertheless, recent advancements in biomaterial-assisted organoid technology have ushered in a paradigm shift in biomedical research. This innovative approach enables the cultivation of three-dimensional cellular structures in vitro that closely emulate the structural and functional attributes of organs, offering physiologically superior models compared to conventional techniques. The evolution of biomaterials plays a pivotal role in supporting the culture and development of organ tissues, thereby facilitating more accurate disease state modeling and the rigorous evaluation of drug efficacy and safety profiles. In this review, we will explore the roles that various biomaterials play in organoid development, examine the fundamental principles and advantages of utilizing these technologies in constructing disease models, and highlight recent advances and practical applications in drug screening using disease-specific organoid models. Additionally, the challenges and future directions of organoid technology are discussed. Through continued research and innovation, we aim to make remarkable strides in disease treatment and drug development, ultimately enhancing patient quality of life.

Progress in photoreceptor replacement therapy for retinal degenerative diseases

Retinal degenerative diseases encompass a diverse range of eye conditions that result in blindness, many due to photoreceptor dysfunction and loss. Regrettably, current clinical treatments are frequently not overly effective. However, photoreceptor transplantation shows promise as a potential therapy for late-stage retinal degenerative diseases. This article will review the various donor cell sources for this transplantation, as well as the mechanisms and factors that impact donor cell integration and material transfer, donor cell maturation, and other auxiliary methods that can be combined with photoreceptor transplantation to treat these degenerative retinal diseases.

Advances in lacrimal gland organoid development: Techniques and therapeutic applications

The human lacrimal gland (LG), located above the outer orbital region within the frontal bone socket, is essential in maintaining eye surface health and lubrication. It is firmly anchored to the orbital periosteum by the connective tissue, and it is vital for protecting and lubricating the eye by secreting lacrimal fluid. Disruption in the production, composition, or secretion of lacrimal fluid can lead to dry eye syndrome, a condition characterized by ocular discomfort and potential eye surface damage. This review explores the recent advancements in LG organoid generation using tissues and stem cells, highlighting cutting-edge techniques in biomaterial-based and scaffold-free technologies. Additionally, we shed light on the complex pathophysiology of LG dysfunction, providing insights into the LG physiological roles while identifying strategies for generating LG organoids and exploring their potential clinical applications. Alterations in LG morphology or secretory function can affect the tear film stability and quality, leading to various ocular pathological conditions. This comprehensive review underlines the critical crosslink of LG organoid development with disease modeling and drug screening, underscoring their potential for advancing therapeutic applications.

Retinal Organoids from Induced Pluripotent Stem Cells of Patients with Inherited Retinal Diseases: A Systematic Review

BACKGROUND

Currently, most inherited retinal diseases lack curative interventions, and available treatment modalities are constrained to symptomatic approaches. Retinal organoid technology has emerged as a method for treating inherited retinal diseases, with growing academic interest in recent years. The purpose of this review was to systematically organize the current protocols for generating retinal organoids using induced pluripotent stem cells from patients with inherited retinal disease and to investigate the application of retinal organoids in inherited retinal disease research.

METHODS

Data were collected from the PubMed, Scopus, and Web of Science databases using a keyword search. The main search term used was "retinal organoid," accompanied by secondary keywords such as "optic cup," "three-dimensional," and "self-organizing." The final search was conducted on October 2, 2024.

RESULTS

Of the 2,129 studies retrieved, 130 were included in the qualitative synthesis. The protocols for the generation of retinal organoids in inherited retinal disease research use five major approaches, categorized into 3D and a combination of 2D/3D approaches, implemented with modifications. Disease phenotypes have been successfully reproduced via the generation of retinal organoids from the induced pluripotent stem cells of individuals with inherited retinal diseases, facilitating the progression of research into novel therapeutic developments. Cells have been obtained from retinal organoids for cell therapy, and progress toward their potential integration into clinical practice is underway. Considering their potential applications, retinal organoid technology has shown promise across various domains.

CONCLUSION

In this systematic review, we organized protocols for generating retinal organoids using induced pluripotent stem cells from patients with inherited retinal diseases. Retinal organoid technology has various applications including disease modeling, screening for novel therapies, and cell replacement therapy. Further advancements would make this technology a clinically significant tool for patients with inherited retinal diseases.

Generation of Induced-Primary Retinal Pigment Epithelium from Human Retinal Organoids

Retinal pigment epithelium (RPE) cells derived from induced pluripotent stem cells (iPSCs) serve multiple roles, including among others, modeling RPE development in normal and pathological conditions, investigating mechanisms of RPE physiology, modeling retinal diseases involving the RPE, and developing strategies for regenerative therapies. We have developed a simple and efficient protocol to generate RPE tissue from human iPSCs-derived retinal organoids. The RPE tissue present in the retinal organoids is analogous to the native human RPE in differentiation timeline, histological organization, and key features of functional maturation. Building upon this system, we established a method to generate functionally mature, polarized RPE monolayers comparable to human primary RPE. This comprehensive protocol outlines the steps for isolating and culturing RPE tissue using retinal organoids. The outcome is a pure population of cells expressing mature RPE signatures and organized in a characteristic cobblestone monolayer featuring robust ultrastructural polarization. These RPE monolayers also exhibit the functional hallmarks of bona fide mature RPE cells, providing a suitable system to mimic the biology and function of the native human RPE.

Central nervous system vascularization in human embryos and neural organoids

In recent years, neural organoids derived from human pluripotent stem cells (hPSCs) have offered a transformative pre-clinical platform for understanding central nervous system (CNS) development, disease, drug effects, and toxicology. CNS vasculature plays an important role in all these scenarios; however, most published studies describe CNS organoids that lack a functional vasculature or demonstrate rudimentary incorporation of endothelial cells or blood vessel networks. Here, we review the existing knowledge of vascularization during the development of different CNS regions, including the brain, spinal cord, and retina, and compare it to vascularized CNS organoid models. We highlight several areas of contrast where further bioengineering innovation is needed and discuss potential applications of vascularized neural organoids in modeling human CNS development, physiology, and disease.

A Large Model-Derived Algorithm for Complex Organoids with Internal Morphogenesis and Digital Marker Derivation

Automated segmentation and evaluation algorithms have been demonstrated to enhance the simplicity and translational utility of organoid technology. However, there is a pressing need for the development of complex organoids that possess epithelium environmental elements, dense regional cell aggregation, and intraorganoid morphologies. Nevertheless, there has been limited progress, including both the construction of data sets and the development of algorithms, in the use of user-friendly microscopy to address such complex organoids. In this study, a data set of bright-field and living cell fluorescence images in paired forms and with temporal variance was constructed using droplet-engineered lung organoids. Additionally, a large model-based algorithm was developed. Both the organoid contours and intraorganoid morphologies were included in the data set, and their physical parameters were included and screened to form multiplex digital markers for organoid evaluation. The algorithm has been demonstrated to outperform existing methods and is therefore suitable for the evaluation of complex organoids. It is expected that the algorithm will facilitate the successful demonstration of AI in organoid evaluation and decision-making regarding their status.

A deep learning approach to predict differentiation outcomes in hypothalamic-pituitary organoids

We use three-dimensional culture systems of human pluripotent stem cells for differentiation into pituitary organoids. Three-dimensional culture is inherently characterized by its ability to induce heterogeneous cell populations, making it difficult to maintain constant differentiation efficiency. That is why the culture process involves empirical aspects. In this study, we use deep-learning technology to create a model that can predict from images of organoids whether differentiation is progressing appropriately. Our models using EfficientNetV2-S or Vision Transformer, employing VENUS-coupled RAX expression, predictively class bright-field images of organoids into three categories with 70% accuracy, superior to expert-observer predictions. Furthermore, the model obtained by ensemble learning with the two algorithms can predict RAX expression in cells without RAX::VENUS, suggesting that our model can be deployed in clinical applications such as transplantation.

Integration and Differentiation of Transplanted Human iPSC-Derived Retinal Ganglion Cell Precursors in Murine Retinas

Optic neuropathy such as glaucoma, stemming from retinal ganglion cell (RGC) degeneration, is a leading cause of visual impairment. Given the substantial loss of RGCs preceding clinical detection of visual impairment, cell replacement therapy emerges as a compelling treatment strategy. Human-induced pluripotent stem cells (hiPSCs) serve as invaluable tools for exploring the developmental processes and pathological mechanisms associated with human RGCs. Utilizing a 3D stepwise differentiation protocol for retinal organoids, we successfully differentiated RGC precursors from hiPSCs harboring a BRN3B-GFP RGC reporter, verified by GFP expression. Intravitreal transplantation of enriched RGC precursors into healthy or N-methyl-D-aspartate (NMDA)-injured mice demonstrated their survival, migration, and integration into the proper retinal layer, the ganglion cell layer, after 3 weeks. Notably, these transplanted cells differentiated into marker-positive RGCs and extended neurites. Moreover, enhanced cell survival was observed with immunosuppressive and anti-inflammatory treatments of the host prior to transplantation. These data underscore the potential of transplanted RGC precursors as a promising therapeutic avenue for treating degenerative retinal diseases resulting from RGC dysfunction.

Unveiling the potential: implications of successful somatic cell-to-ganglion organoid reprogramming

Organoids have a wide range of potential applications in areas such as organ development, precision medicine, regenerative medicine, drug screening, disease modeling, and gene editing. Currently, most organoids are generated through three-dimensional (3D) in vitro culture of adult stem cells or pluripotent stem cells. However, this method of generating organoids still has several limitations and challenges, including complex manipulations, costly culturing materials, extended time requirements, and certain heterogeneity. Recently, we have found that fibroblasts, when overexpressing several key regulatory transcription factors, are able to directly and rapidly generate two types of ganglion organoids: sensory ganglion (SG) and autonomic ganglion (AG) organoids. They have structures and electrophysiological properties similar to those of endogenous organs in the body. Here, we provide a brief overview of organoid development, focusing on direct reprogramming of SG and AG organoids and their transplantation and regeneration. Finally, the advantages and prospects of direct reprogramming of organoids are discussed.

DNA microbeads for spatio-temporally controlled morphogen release within organoids

Organoids are transformative in vitro model systems that mimic features of the corresponding tissue in vivo. However, across tissue types and species, organoids still often fail to reach full maturity and function because biochemical cues cannot be provided from within the organoid to guide their development. Here we introduce nanoengineered DNA microbeads with tissue mimetic tunable stiffness for implementing spatio-temporally controlled morphogen gradients inside of organoids at any point in their development. Using medaka retinal organoids and early embryos, we show that DNA microbeads can be integrated into embryos and organoids by microinjection and erased in a non-invasive manner with light. Coupling a recombinant surrogate Wnt to the DNA microbeads, we demonstrate the spatio-temporally controlled morphogen release from the microinjection site, which leads to morphogen gradients resulting in the formation of retinal pigmented epithelium while maintaining neuroretinal cell types. Thus, we bioengineered retinal organoids to more closely mirror the cell type diversity of in vivo retinae. Owing to the facile, one-pot fabrication process, the DNA microbead technology can be adapted to other organoid systems for improved tissue mimicry.

Pluripotent stem cell-derived organoids: A brief history of curiosity-led discoveries

Organoids are quickly becoming an accepted model for understanding human biology and disease. Pluripotent stem cells (PSC) provide a starting point for many organs and enable modeling of the embryonic development and maturation of such organs. The foundation of PSC-derived organoids can be found in elegant developmental studies demonstrating the remarkable ability of immature cells to undergo histogenesis even when taken out of the embryo context. PSC-organoids are an evolution of earlier methods such as embryoid bodies, taken to a new level with finer control and in some cases going beyond tissue histogenesis to organ-like morphogenesis. But many of the discoveries that led to organoids were not necessarily planned, but rather the result of inquisitive minds with freedom to explore. Protecting such curiosity-led research through flexible funding will be important going forward if we are to see further ground-breaking discoveries.

Cell-cell interactions between transplanted retinal organoid cells and recipient tissues

The transplantation of human organoid-derived retinal cells is being studied as a potentially viable strategy to treat vision loss due to retinal degeneration. Experiments in animal models have demonstrated the feasibility of organoid-derived photoreceptor transplantation in various recipient contexts. In some cases, vision repair has been shown. However, recipient-donor cell-cell interactions are incompletely understood. This review briefly summarizes these interactions, categorizing them as synaptic structure formation, cellular component transfer, glial activation, immune cell infiltration, and cellular migration. Each of these interactions may affect the survival and functionality of the donor cells and, ultimately, their efficacy as a treatment substrate. Additionally, recipient characteristics, such as the cytoarchitecture of the retina and immune status, may also impact the type and frequency of cell-cell interactions. Despite the procedural challenges associated with culturing human retinal organoids and the technical difficulties in transplanting donor cells into the delicate recipient retina, transplantation of retinal organoid-derived cells is a promising tool for degenerative retinal disease treatment.

Cellular diversity of human inner ear organoids revealed by single-cell transcriptomics

Human inner ear organoids are three-dimensional tissular structures grown in vitro that recapitulate some aspects of the fetal inner ear and allow the differentiation of inner ear cell types. These organoids offer a system in which to study human inner ear development, mutations causing hearing loss and vertigo, and new therapeutic drugs. However, the extent to which such organoids mimic in vivo human inner ear development and cellular composition remains unclear. Several recent studies have performed single-cell transcriptomics on human inner ear organoids to interrogate cellular heterogeneity, reveal the developmental trajectories of sensory lineages and compare organoid-derived vesicles to the developing human inner ear. Here, we discuss the new insights provided by these analyses that help to define new paths of investigation to understand inner ear development.

Molecular characterization and sub-retinal transplantation of hypoimmunogenic human retinal sheets in a minipig model of severe photoreceptor degeneration

Retinal degenerative diseases affect millions of people worldwide, and legal blindness is generally associated with the loss of cone photoreceptors located in the central region of the retina called the macula. Currently, there is no treatment to replace the macula. Addressing this unmet need, we employed control isogenic and hypoimmunogenic induced pluripotent stem cell lines to generate spontaneously polarized retinal sheets (RSs). RSs were enriched in retinal progenitor and cone precursor cells, which could differentiate into mature S- and M/L-cones in long-term cultures. Single-cell RNA-seq analysis showed that RSs recapitulate the ontogeny of the developing human retina. Isolation of neural rosettes for sub-retinal transplantation effectively eliminated unwanted cells such as RPE cells. In a porcine model of chemically induced retinal degeneration, grafts integrated the host retina and formed a new, yet immature, photoreceptor layer. In one transplanted animal, functional and immunohistochemical assays suggest that grafts exhibited responsiveness to light stimuli and established putative synaptic connections with host bipolar neurons. This study underscores the potential and challenges of RSs for clinical applications.

Extrinsic electric field modulates neuronal development and increases photoreceptor population in retinal organoids

Introduction

Considering the significant role played by both intrinsic and extrinsic electric fields in the growth and maturation of the central nervous system, the impact of short exposure to external electric fields on the development and differentiation of retinal organoids was investigated.

Methods

Retinal organoids derived from human embryonic stem cells were used at day 80, a key stage in their differentiation. A single 60-minute exposure to a biphasic electrical field was administered to assess its influence on retinal cell populations and maturation markers. Immunohistochemistry, qPCR, and RNA sequencing were employed to evaluate cell type development and gene expression changes.

Results

Electrical stimulation significantly enhanced neuronal development and increased the population of photoreceptors within the organoids. RNA sequencing data showed upregulated expression of genes related to rod photoreceptors, Müller cells, horizontal cells, and amacrine cells, while genes associated with retinal pigment epithelium and retinal ganglion cells were downregulated. Variations in development and maturation were observed depending on the specific parameters of the applied electric field.

Discussion

These findings highlight the significant impact of extrinsic electrical fields on early retinal development and suggest that optimizing electrical field parameters could effectively address certain limitations in retinal organoid technology, potentially reducing the reliance on chemicals and small molecules.

Trends and challenges in organoid modeling and expansion with pluripotent stem cells and somatic tissue

The increasing demand for disease modeling, preclinical drug testing, and long waiting lists for alternative organ substitutes has posed significant challenges to current limitations in organoid technology. Consequently, organoid technology has emerged as a cutting-edge tool capable of accurately recapitulating the complexity of actual organs in physiology and functionality. To bridge the gaps between basic research and pharmaceutical as well as clinical applications, efforts have been made to develop organoids from tissue-derived stem cells or pluripotent stem cells. These developments include optimizing starting cells, refining culture systems, and introducing genetic modifications. With the rapid development of organoid technology, organoid composition has evolved from single-cell to multi-cell types, enhancing their level of biomimicry. Tissue structure has become more refined, and core challenges like vascularization are being addressed actively. These improvements are expected to pave the way for the construction of organoid atlases, automated large-scale cultivation, and universally compatible organoid biobanks. However, major obstacles remain to be overcome before urgently proof-of-concept organoids can be readily converted to practical applications. These obstacles include achieving structural and functional summarily to native tissue, remodeling the microenvironment, and scaling up production. This review aims to summarize the status of organoid development and applications, highlight recent progress, acknowledge existing limitations and challenges, and provide insights into future advancements. It is expected that this will contribute to the establishment of a reliable, scalable, and practical platform for organoid production and translation, further promoting their use in the pharmaceutical industry and regenerative medicine.

Engineering bone/cartilage organoids: strategy, progress, and application

The concept and development of bone/cartilage organoids are rapidly gaining momentum, providing opportunities for both fundamental and translational research in bone biology. Bone/cartilage organoids, essentially miniature bone/cartilage tissues grown in vitro, enable the study of complex cellular interactions, biological processes, and disease pathology in a representative and controlled environment. This review provides a comprehensive and up-to-date overview of the field, focusing on the strategies for bone/cartilage organoid construction strategies, progresses in the research, and potential applications. We delve into the significance of selecting appropriate cells, matrix gels, cytokines/inducers, and construction techniques. Moreover, we explore the role of bone/cartilage organoids in advancing our understanding of bone/cartilage reconstruction, disease modeling, drug screening, disease prevention, and treatment strategies. While acknowledging the potential of these organoids, we discuss the inherent challenges and limitations in the field and propose potential solutions, including the use of bioprinting for organoid induction, AI for improved screening processes, and the exploration of assembloids for more complex, multicellular bone/cartilage organoids models. We believe that with continuous refinement and standardization, bone/cartilage organoids can profoundly impact patient-specific therapeutic interventions and lead the way in regenerative medicine.

Advanced bioengineering strategies broaden the therapeutic landscape for corneal failure

The cornea acts as the eye foremost protective layer and is essential for its focusing power. Corneal blindness may arise from physical trauma or conditions like dystrophies, keratitis, keratoconus, or ulceration. While conventional treatments involve medical therapies and donor allografts-sometimes supplemented with keratoprostheses-these options are not suitable for all corneal defects. Consequently, the development of bioartificial corneal tissue has emerged as a critical research area, aiming to address the global shortage of human cornea donors. Bioengineered corneas hold considerable promise as substitutes, with the potential to replace either specific layers or the entire thickness of damaged corneas. This review first delves into the structural anatomy of the human cornea, identifying key attributes necessary for successful corneal tissue bioengineering. It then examines various corneal pathologies, current treatments, and their limitations. Finally, the review outlines the primary approaches in corneal tissue engineering, exploring cell-free, cell-based, and scaffold-based options as three emerging strategies to address corneal failure.

Human midbrain organoids: a powerful tool for advanced Parkinson's disease modeling and therapy exploration

Parkinson's disease (PD) is a neurodegenerative disorder marked by the loss of dopaminergic neurons in the substantia nigra. Despite progress, the pathogenesis remains unclear. Human midbrain organoids (hMLOs) have emerged as a promising model for studying PD, drug screening, and potential treatments. This review discusses the development of hMLOs, their application in PD research, and current challenges in organoid construction, highlighting possible optimization strategies.

A Comparative Analysis of Models for AAV-Mediated Gene Therapy for Inherited Retinal Diseases

Inherited retinal diseases (IRDs) represent a diverse group of genetic disorders leading to progressive degeneration of the retina due to mutations in over 280 genes. This review focuses on the various methodologies for the preclinical characterization and evaluation of adeno-associated virus (AAV)-mediated gene therapy as a potential treatment option for IRDs, particularly focusing on gene therapies targeting mutations, such as those in the RPE65 and FAM161A genes. AAV vectors, such as AAV2 and AAV5, have been utilized to deliver therapeutic genes, showing promise in preserving vision and enhancing photoreceptor function in animal models. Despite their advantages-including high production efficiency, low pathogenicity, and minimal immunogenicity-AAV-mediated therapies face limitations such as immune responses beyond the retina, vector size constraints, and challenges in large-scale manufacturing. This review systematically compares different experimental models used to investigate AAV-mediated therapies, such as mouse models, human retinal explants (HREs), and induced pluripotent stem cell (iPSC)-derived retinal organoids. Mouse models are advantageous for genetic manipulation and detailed investigations of disease mechanisms; however, anatomical differences between mice and humans may limit the translational applicability of results. HREs offer valuable insights into human retinal pathophysiology but face challenges such as tissue degradation and lack of systemic physiological effects. Retinal organoids, on the other hand, provide a robust platform that closely mimics human retinal development, thereby enabling more comprehensive studies on disease mechanisms and therapeutic strategies, including AAV-based interventions. Specific outcomes targeted in these studies include vision preservation and functional improvements of retinas damaged by genetic mutations. This review highlights the strengths and weaknesses of each experimental model and advocates for their combined use in developing targeted gene therapies for IRDs. As research advances, optimizing AAV vector design and delivery methods will be critical for enhancing therapeutic efficacy and improving clinical outcomes for patients with IRDs.

Progress and potential of brain organoids in epilepsy research

Epilepsies are disorders of the brain characterised by an imbalance in electrical activity, linked to a disruption in the excitation and inhibition of neurons. Progress in the epilepsy research field has been hindered by the lack of an appropriate model, with traditionally used 2D primary cell culture assays and animal models having a number of limitations which inhibit their ability to recapitulate the developing brain and the mechanisms behind epileptogenesis. As a result, the mechanisms behind the pathogenesis of epilepsy are largely unknown. Brain organoids are 3D aggregates of neural tissue formed in vitro and have been shown to recapitulate the gene expression patterns of the brain during development, and can successfully model a range of epilepsies and drug responses. They thus present themselves as a novel tool to advance studies into epileptogenesis. In this review, we discuss the formation of brain organoids, their recent application in studying genetic epilepsies, hyperexcitability dynamics and oxygen glucose deprivation as a hyperexcitability agent, their use as an epilepsy drug testing and development platform, as well as the limitations of their use in epilepsy research and how these can be mitigated.

iPSC-derived and Patient-Derived Organoids: Applications and challenges in scalability and reproducibility as pre-clinical models

Recent advancements in stem cell technology have led to the development of organoids - three-dimensional (3D) cell cultures that closely mimic the structural and functional characteristics of human organs. These organoids represent a significant improvement over traditional two-dimensional (2D) cell cultures by preserving native tissue architecture and cellular interactions critical for physiological relevance. This review provides a comprehensive comparison between two main types of organoids: induced Pluripotent Stem Cell (iPSC)-derived and Adult Stem Cell (ASC)-derived (also known as Patient-Derived Organoids, PDOs). iPSC-derived organoids, derived from reprogrammed cells, exhibit remarkable plasticity, and can model a wide range of tissues and developmental stages. They are particularly valuable for studying early human development, genetic disorders, and complex diseases. However, challenges such as prolonged differentiation protocols and variability in maturation levels remain significant hurdles. In contrast, ASC-derived organoids, generated directly from patient tissues, faithfully recapitulate tissue-specific characteristics and disease phenotypes. This fidelity makes them indispensable for personalized medicine applications, including drug screening, disease modeling, and understanding individualized treatment responses. The review highlights the unique advantages and limitations of each organoid type, emphasizing their roles in advancing biomedical research and drug discovery. It addresses key challenges in organoid technology, such as scalability, reproducibility, and the need for standardized culture protocols. Furthermore, it explores recent innovations in scaffold-guided organoid engineering and the integration of organoids with advanced technologies like artificial intelligence and high-throughput screening. The integration of organoids with cutting-edge technologies holds promise for enhancing their utility in modeling complex human diseases and accelerating drug discovery and development. By providing more physiologically relevant models of human organs, organoid technology is poised to revolutionize biomedical research, offering new insights into disease mechanisms and personalized therapeutic strategies.

Organoids: development and applications in disease models, drug discovery, precision medicine, and regenerative medicine

Organoids are miniature, highly accurate representations of organs that capture the structure and unique functions of specific organs. Although the field of organoids has experienced exponential growth, driven by advances in artificial intelligence, gene editing, and bioinstrumentation, a comprehensive and accurate overview of organoid applications remains necessary. This review offers a detailed exploration of the historical origins and characteristics of various organoid types, their applications-including disease modeling, drug toxicity and efficacy assessments, precision medicine, and regenerative medicine-as well as the current challenges and future directions of organoid research. Organoids have proven instrumental in elucidating genetic cell fate in hereditary diseases, infectious diseases, metabolic disorders, and malignancies, as well as in the study of processes such as embryonic development, molecular mechanisms, and host-microbe interactions. Furthermore, the integration of organoid technology with artificial intelligence and microfluidics has significantly advanced large-scale, rapid, and cost-effective drug toxicity and efficacy assessments, thereby propelling progress in precision medicine. Finally, with the advent of high-performance materials, three-dimensional printing technology, and gene editing, organoids are also gaining prominence in the field of regenerative medicine. Our insights and predictions aim to provide valuable guidance to current researchers and to support the continued advancement of this rapidly developing field.

Host-Graft Synapses Form Functional Microstructures and Shape the Host Light Responses After Stem Cell-Derived Retinal Sheet Transplantation

Purpose

Retinitis pigmentosa represents a leading cause of blindness in developed countries, yet effective treatments for the disease remain unestablished. Previous studies have demonstrated the potential of stem cell-derived retinal organoid (SC-RO) sheet transplantation to form host-graft synapses and to improve light responsiveness in animal models of retinal degeneration. However, the detailed microstructures of these de novo synapses and their functional contribution have not been well elucidated. This study aims to (1) elucidate the microstructures of the host-graft synapse, and (2) investigate the overall distribution and contribution of these synapses to host retinal light responses.

Methods

We identified host-graft synapses using a reporter system in mouse SC-RO and rd1 mice, a well-established model of end-stage retinal degeneration. Correlative array tomography was used to reveal the microstructure of host-graft synapses. Furthermore, we developed a semi-automated algorithm that robustly detects the host-graft photoreceptor synapses in the overall grafted area using the same reporter system in flat-mount retinas. We then integrated the spatial distribution of the host-graft synapses with light responses detected by multi-electrode array recording.

Results

Correlative array tomography revealed that host-graft synapses recapitulate the developmental process of photoreceptor synapse formation involving horizontal cells first and then rod bipolar cells. By integrating the spatial distribution of host-graft synapse and multi-electrode array recording, we showed that the number of light-responsive host retinal ganglion cells is positively correlated with the local density of host-graft synapses.

Conclusions

De novo host-graft synapses recapitulate the developmental microstructure of the photoreceptor synapse, and their formation contributes to the light responsiveness after SC-RO transplantation.

Cellular and circuit remodeling of the primate foveal midget pathway after acute photoreceptor loss

The retinal fovea in human and nonhuman primates is essential for high acuity and color vision. Within the fovea lies specialized circuitry in which signals from a single cone photoreceptor are largely conveyed to one ON and one OFF type midget bipolar cell (MBC), which in turn connect to a single ON or OFF midget ganglion cell (MGC), respectively. Restoring foveal vision requires not only photoreceptor replacement but also appropriate reconnection with surviving ON and OFF MBCs and MGCs. However, our current understanding of the effects of cone loss on the remaining foveal midget pathway is limited. We thus used serial block-face electron microscopy to determine the degree of plasticity and potential remodeling of this pathway in adult Macaca fascicularis several months after acute photoreceptor loss upon photocoagulation. We reconstructed MBC structure and connectivity within and adjacent to the region of cone loss. We found that MBC dendrites within the scotoma retracted and failed to reach surviving cones to form new connections. However, both surviving cones and ON and OFF MBC dendrites at the scotoma border exhibited remodeling, suggesting that these neurons can demonstrate plasticity and rewiring at maturity. At six months postlesion, disconnected OFF MBCs clearly lost output ribbon synapses with their postsynaptic partners, whereas the majority of ON MBCs maintained their axonal ribbon numbers, suggesting differential timing or extent in ON and OFF midget circuit remodeling after cone loss. Our findings raise rewiring considerations for cell replacement approaches in the restoration of foveal vision.

Brain organoids: A new tool for modelling of neurodevelopmental disorders

Neurodevelopmental disorders are mostly studied using mice as models. However, the mouse brain lacks similar cell types and structures as those of the human brain. In recent years, emergence of three-dimensional brain organoids derived from human embryonic stem cells or induced pluripotent stem cells allows for controlled monitoring and evaluation of early neurodevelopmental processes and has opened a window for studying various aspects of human brain development. However, such organoids lack original anatomical structure of the brain during maturation, and neurodevelopmental maturation processes that rely on unique cellular interactions and neural network connections are limited. Consequently, organoids are difficult to be used extensively and effectively while modelling later stages of human brain development and disease progression. To address this problem, several methods and technologies have emerged that aim to enhance the sophisticated regulation of brain organoids developmental processes through bioengineering approaches, which may alleviate some of the current limitations. This review discusses recent advances and application areas of human brain organoid culture methods, aiming to generalize optimization strategies for organoid systems, improve the ability to mimic human brain development, and enhance the application value of organoids.

Rapid and efficient generation of mature retinal organoids derived from human pluripotent stem cells via optimized pharmacological modulation of Sonic hedgehog, activin A, and retinoic acid signal transduction

Human retinal organoids have become indispensable tools for retinal disease modeling and drug screening. Despite its versatile applications, the long timeframe for their differentiation and maturation limits the throughput of such research. Here, we successfully shortened this timeframe by accelerating human retinal organoid development using unique pharmacological approaches. Our method comprised three key steps: 1) a modified self-formed ectodermal autonomous multizone (SEAM) method, including dual SMAD inhibition and bone morphogenetic protein 4 treatment, for initial neural retinal induction; 2) the concurrent use of a Sonic hedgehog agonist SAG, activin A, and all-trans retinoic acid for rapid retinal cell specification; and 3) switching to SAG treatment alone for robust retinal maturation and lamination. The generated retinal organoids preserved typical morphological features of mature retinal organoids, including hair-like surface structures and well-organized outer layers. These features were substantiated by the spatial immunostaining patterns of several retinal cell markers, including rhodopsin and L/M opsin expression in the outermost layer, which was accompanied by reduced ectopic cone photoreceptor generation. Importantly, our method required only 90 days for retinal organoid maturation, which is approximately two-thirds the time necessary for other conventional methods. These results indicate that thoroughly optimized pharmacological interventions play a pivotal role in rapid and precise photoreceptor development during human retinal organoid differentiation and maturation. Thus, our present method may expedite human retinal organoid research, eventually contributing to the development of better treatment options for various degenerative retinal diseases.

Liver organoids: updates on generation strategies and biomedical applications

The liver is the most important metabolic organ in the body. While mouse models and cell lines have further deepened our understanding of liver biology and related diseases, they are flawed in replicating key aspects of human liver tissue, particularly its complex structure and metabolic functions. The organoid model represents a major breakthrough in cell biology that revolutionized biomedical research. Organoids are in vitro three-dimensional (3D) physiological structures that recapitulate the morphological and functional characteristics of tissues in vivo, and have significant advantages over traditional cell culture methods. In this review, we discuss the generation strategies and current advances in the field focusing on their application in regenerative medicine, drug discovery and modeling diseases.

Research progress of brain organoids in the field of diabetes

Human embryonic stem cells and human induced pluripotent stem cells may be used to create 3D tissues called brain organoids. They duplicate the physiological and pathological characteristics of human brain tissue more faithfully in terms of both structure and function, and they more precisely resemble the morphology and cellular structure of the human embryonic brain. This makes them valuable models for both drug screening and in vitro studies on the development of the human brain and associated disorders. The technical breakthroughs enabled by brain organoids have a significant impact on the research of different brain regions, brain development and sickness, the connections between the brain and other tissues and organs, and brain evolution. This article discusses the development of brain organoids, their use in diabetes research, and their progress.