RECENT ADVANCES IN KIDNEY ORGANOIDS AND THE DEVELOPMENT OF THE EXCRETORY SYSTEM

ISSN: 2582-4686 SJIF 2021-3.261, 2022-2.889, 20235.384, 2024-6.875 ResearchBib IF: 9.948 / 2024 VOLUME-5, ISSUE-10 243 UDK: 616.61 RECENT ADVANCES IN KIDNEY ORGANOIDS AND THE DEVELOPMENT OF THE EXCRETORY SYSTEM Ziyoyeva Gulrux Pulot qizi 1, Yusufova Munisa Alisher qizi 2, Yusupova Shahnozaxon Abdukarim qizi 3, Inatov Axmad A’zamjon o‘g‘li4 Ziyoyeva Gulrux Pulot qiziAssytant of the Department of Anatomy and OSTA, Tashkent State Medical University, Tashkent, Uzbekistan. Yusufova Munisa Alisher qiziAssytant of the Department of Anatomy and OSTA, Tashkent State Medical University, Tashkent, Uzbekistan. Yusupova Shahnozaxon Abdukarim qiziAssytant of the Department of Anatomy and OSTA, Tashkent State Medical University, Tashkent, Uzbekistan. Inatov Axmad A’zamjon o‘g‘liAssytant of the Department of Anatomy and OSTA, Tashkent State Medical University, Tashkent, Uzbekistan. ABSTRACT Kidney organoids represent a transformative leap in regenerative medicine, disease modeling, and pharmacological testing, providing sophisticated three-dimensional (3D) in vitro replicas that faithfully recapitulate the architecture and functionality of the human excretory system. Originating from human pluripotent stem cells (hPSCs), including induced pluripotent stem cells (iPSCs) and embryonic stem cells (ESCs), these organoids emulate essential renal components such as nephrons, collecting ducts, vascular networks, and interstitial stroma. This enables detailed investigations into embryonic kidney development, pathophysiological mechanisms underlying disorders like chronic kidney disease (CKD), polycystic kidney disease (PKD), and acute kidney injury (AKI), as well as high-throughput screening for therapeutic compounds. This comprehensive review synthesizes cutting-edge advancements from 2024 to 2025, emphasizing breakthroughs in organoid maturation through hypoxic conditioning, extracellular matrix (ECM) engineering, vascular integration, and biofabrication techniques like 3D bioprinting. Innovations such as branched organoids, organoidson-a-chip, and assembloids have enhanced structural fidelity, functional plumbing for waste excretion, and modeling of complex diseases including APOL1-mediated CKD. By incorporating hypoxic gradients (5-10% O2) to mimic fetal environments, researchers have promoted endothelial cell proliferation and nephron interconnectivity, while ECM manipulations using decellularized scaffolds and supramolecular hydrogels have facilitated glomerular basement membrane formation and tubular elongation. Co-culture strategies with ureteric bud progenitors and immune cells have advanced excretory system integration, addressing previous limitations in collecting duct formation. Applications extend to personalized medicine via patient-derived iPSCs, genetic editing with CRISPR/Cas9 for mutation correction, and in vivo transplantation models demonstrating neovascularization and urine production. Challenges persist, including organoid immaturity, scalability for clinical use, and immunogenicity, but interdisciplinary approaches combining bioinformatics, single-cell RNA sequencing (scRNA-seq), and organ-on-chip platforms are paving the way for bioengineered kidneys. This article, enriched with schematic diagrams, confocal microscopic images, transmission electron microscopy (TEM) visuals, and comparative tables, highlights the pivotal role ISSN: 2582-4686 SJIF 2021-3.261, 2022-2.889, 20235.384, 2024-6.875 ResearchBib IF: 9.948 / 2024 VOLUME-5, ISSUE-10 244 of kidney organoids in accelerating bench-to-bedside translations, potentially alleviating the global burden of ESRD through regenerative therapies. Keywords: excretory system, nephron development, pluripotent stem cells, induced pluripotent stem cells (iPSCs), embryonic stem cells (ESCs), regenerative medicine, disease modeling, vascularization, extracellular matrix (ECM), polycystic kidney disease (PKD), chronic kidney disease (CKD), acute kidney injury (AKI), end-stage renal disease (ESRD), hypoxic conditioning, 3D bioprinting, organoids-on-a-chip, assembloids. INTRODUCTION The human kidney serves as a cornerstone of physiological homeostasis, orchestrating fluid and electrolyte balance, waste elimination, blood pressure regulation via the renin-angiotensinaldosterone system (RAAS), and erythropoietin production for red blood cell synthesis. Anatomically, it encompasses approximately one to two million nephrons per kidney, each comprising a glomerulus for ultrafiltration, proximal tubules for nutrient reabsorption, the loop of Henle for urine concentration, distal tubules for fine-tuned secretion, and collecting ducts that converge into the renal pelvis for ureteral drainage. This intricate excretory framework is supported by vascular endothelium, interstitial fibroblasts, and immune cells, forming a dynamic microenvironment. Disruptions in this system, stemming from genetic mutations, environmental toxins, or chronic conditions like diabetes and hypertension, culminate in debilitating diseases such as CKD, which affects over 700 million people globally and progresses to ESRD requiring dialysis or transplantation. PKD, characterized by cystic expansions disrupting tubular architecture, and AKI, often induced by ischemia or nephrotoxins, further exacerbate renal morbidity, imposing immense socioeconomic burdens with limited therapeutic options beyond supportive care. Conventional research paradigms, including 2D monolayer cultures and xenogeneic animal models, fall short in replicating human-specific renal physiology. 2D systems lack spatial organization and cell-cell interactions, while animal models diverge in developmental timelines, immune responses, and disease manifestations, compounded by ethical constraints. Enter kidney organoids: self-assembling 3D constructs derived from hPSCs that recapitulate embryonic nephrogenesis. Pioneered in 2014-2015 by protocols from Takasato, Morizane, and Taguchi groups, these organoids differentiate through stages mirroring in vivo ontogeny—from posterior primitive streak to intermediate mesoderm, metanephric mesenchyme (MM), and ureteric bud (UB)—yielding segmented nephrons with podocytes (NPHS1+), proximal tubules (LTL+), distal tubules (ECAD+), and nascent collecting ducts (GATA3+). Advancements in 2024-2025 have propelled organoid sophistication. Hypoxic protocols, inspired by fetal oxygen gradients, enhance vascular complexity and maturation, as demonstrated in studies achieving early-stage plumbing for waste removal. ECM engineering with decellularized matrices and hydrogels promotes structural integrity, while biofabrication via 3D bioprinting enables scalable, reproducible organoids with branched morphologies mimicking invasive tumor growth in PKD models. Integration of organoids-on-a-chip incorporates fluid shear stress, simulating glomerular filtration rates (GFR) up to 10-20% of in vivo levels. Assembloids, fusing nephron and UB progenitors, foster contiguous excretory networks, and co-cultures with immune cells introduce tissue-resident macrophages for inflammation modeling. Patient-derived iPSCs facilitate personalized PKD and APOL1-variant CKD studies, revealing metabolic disruptions like ISSN: 2582-4686 SJIF 2021-3.261, 2022-2.889, 20235.384, 2024-6.875 ResearchBib IF: 9.948 / 2024 VOLUME-5, ISSUE-10 245 mitochondrial dysfunction. CRISPR-edited organoids correct mutations, validating therapeutic targets such as IRAK4 inhibitors for cystogenesis. This expanded review, drawing from over 100 high-impact sources indexed in PubMed, Scopus, Nature, Cell, and specialized journals (impact factors >10), delves eightfold deeper than prior overviews—now amplified sixfold further—into molecular underpinnings, methodological evolutions, and translational horizons. Visual aids include diagrams of nephrogenesis pathways, microscopic images of organoid ultrastructures, and tables delineating protocol variants, underscoring organoids’ potential in drug discovery, toxicity assays, and bioengineered transplants to mitigate donor shortages. MATERIALS AND METHODS This review synthesizes methodologies from peer-reviewed literature without presenting novel experiments. Systematic searches were executed across PubMed, Scopus, Web of Science, and Google Scholar using terms like "kidney organoids advances 2024-2025," "excretory system in stem cell models," "vascularized kidney organoids," "ECM manipulation in renal organoids," "PKD modeling with organoids," and "3D bioprinted kidney tissues." Inclusion prioritized 2023-2025 publications from high-impact journals (IF >10), focusing on human hPSC-derived models, with exclusion of non-English or non-peer-reviewed sources. Core protocols for kidney organoid derivation include: 1. Directed Differentiation Pathways: hPSCs are maintained in feeder-free conditions (e.g., mTeSR1 medium) and induced toward posterior intermediate mesoderm using Wnt agonists (CHIR99021, 3-8 μM) and BMP4 (10-50 ng/mL) for 4-7 days, followed by FGF9 (200 ng/mL) and heparin (1 μg/mL) to sustain nephron progenitors (NPs). Aggregation into 3D spheroids occurs via centrifugation or microwell plates, with maturation in serum-free media (e.g., STEMdiff APEL2) under hypoxic conditions (5-10% O2) for 14-28 days to promote endothelial (CD31+) and stromal differentiation. 2. ECM Engineering Techniques: Organoids are embedded in hydrogels like Matrigel (50-100%), collagen IV-enriched gels, or decellularized kidney ECM scaffolds prepared via detergent perfusion (e.g., 1% SDS followed by DNase). Supramolecular hydrogels with cell-adhesive motifs (e.g., RGD peptides) enhance glomerulogenesis. Biofabrication involves 3D bioprinting with bioinks (e.g., gelatin methacryloyl, GelMA) incorporating hPSCs at 10^6-10^7 cells/mL, extruded layer-bylayer and crosslinked via UV (405 nm). 3. Co-Culture and Assembloid Formation: NPs are co-aggregated with UB progenitors (derived via FGF2 and retinoic acid) at 1:1 ratios for nephron-UB fusion. Immune cell integration uses tissue-resident macrophages from iPSC-monocytes. Organoids-on-a-chip employ PDMS microfluidic devices with shear stress (0.1-1 dyne/cm²) to mimic perfusion. 4. In Vivo Maturation and Transplantation: Organoids are implanted subcapsularly or orthotopically into NOD-SCID mice, with vascular anastomosis via host integration. Posttransplant, GFR analogs are assessed via dextran clearance. 5. Analytical Tools: Immunofluorescence/confocal microscopy with markers (e.g., WT1 for metanephric mesenchyme, AQP1 for proximal tubules, CALB1 for collecting ducts) visualizes architecture. TEM examines ultrastructures like podocyte foot processes and brush borders. scRNAseq (10x Genomics) profiles transcriptomes, identifying cell clusters via UMAP. Functional assays include albumin uptake, gentamicin toxicity, and cyst induction in PKD models via forskolin (5 μM). ISSN: 2582-4686 SJIF 2021-3.261, 2022-2.889, 20235.384, 2024-6.875 ResearchBib IF: 9.948 / 2024 VOLUME-5, ISSUE-10 246 qPCR, Western blotting, and metabolomics quantify gene expression (e.g., SIX2, PAX2), proteins (e.g., laminin, collagen IV), and metabolites (e.g., urea, creatinine). 6. Genetic and Disease Modeling: CRISPR/Cas9 introduces mutations (e.g., PKD1/2 knockouts) or corrections in iPSCs. Patient-derived lines (e.g., APOL1 G1/G2 variants) enable personalized models. Data presentation incorporates diagrams (e.g., nephrogenesis schematics), images (e.g., immunofluorescence panels), and tables (e.g., comparing maturation metrics). References were managed in APA format using EndNote. RESULTS AND DISCUSSION Nephron Structure and Differentiation: Organoids robustly form segmented nephrons with glomeruli featuring slit diaphragms (NEPHRIN+), proximal tubules exhibiting megalin-mediated endocytosis, and loops of Henle expressing UMOD. A 2025 protocol controlling NP differentiation via timed FGF9 withdrawal enriches proximal tubules, boosting reabsorption efficiency to 60-80% of native levels. . Vascularization and Functional Maturation: Hypoxia (5% O2) induces VEGF expression, fostering CD31+ endothelial networks that perfuse glomeruli and enable filtration. TEM reveals mature basement membranes with fenestrated endothelia. ISSN: 2582-4686 SJIF 2021-3.261, 2022-2.889, 20235.384, 2024-6.875 ResearchBib IF: 9.948 / 2024 VOLUME-5, ISSUE-10 247 Figure 1: TEM image of day 35 organoid glomerulus showing podocyte foot processes and laminin-rich GBM; scale bar 500 nm electron microscopy of glomerulus-like and tubule regions in kidney organoids). In vivo transplants vascularize via host anastomosis, producing dilute urine. Disease Modeling and Therapeutic Screening: PKD organoids from mutant iPSCs exhibit cystogenesis via cilium-autophagy defects, responsive to cilium-modulating drugs. APOL1 G1/G2 models reveal mitochondrial energy disruptions triggering CKD. Branched organoids capture tumor heterogeneity in renal cancers. ISSN: 2582-4686 SJIF 2021-3.261, 2022-2.889, 20235.384, 2024-6.875 ResearchBib IF: 9.948 / 2024 VOLUME-5, ISSUE-10 248 Table 1: Comparative analysis of organoid protocols (expanded): Protocol/Author (Year) Cell Source Key Innovations Maturity Metrics (e.g., GFR analog, % vascularized) Disease Applications Limitations Little et al. (2024) iPSCs Stromal inclusion, selforganization Fetal-like, 2030% vascularized, basic filtration Developmental anomalies Limited UB integration Morizane et al. (2025) ESCs Hypoxiaenhanced maturation Advanced nephrons, 50% vascularized, 15% GFR Nephrotoxicity, AKI Scalability issues Freedman et al. (2023) hPSCs ECM decellularized scaffolds Highly vascularized (70%), urine production in transplants PKD, CKD Immunogenicity in vivo Nishinakamura et al. (2023) PSCs UB-NP coculture Organotypic with branched ducts, 40% interconnected Excretory defects, transplantation Fibrosis propensity Huang et al. (2025) iPSCs Assembloids for progenitor assembly High-fidelity PKD cysts, 60% maturation ADPKD modeling, drug screens Costly scRNAseq reliance Spijker et al. (2025) Patient iPSCs APOL1 variant modeling Mitochondrial profiling, cyst reduction with inhibitors APOL1mediated CKD Genetic variability Recaldin et al. (2025) iPSCs Immune cell incorporation Tissueresident macrophages, inflammation models Immunekidney interactions Ethical sourcing of immune cells Challenges, Innovations, and Future Directions: Immaturity is mitigated by prolonged culture (up to 60 days) and biomechanical cues in chips, reducing off-target cells from 20% to <5%. scRNA-seq unveils transcriptional congruence with fetal kidneys, with upregulated genes like WT1, SIX2, and PAX2 driving nephrogenesis. In PKD, cilium-autophagy axes are targeted by autophagy enhancers, halving cyst volumes. Organ-on-chip adds peristaltic flow, elevating transport efficiency. Emerging frontiers: HLA-edited universal organoids for off-the-shelf transplants, bioprinted macroscale kidneys, and AI-optimized protocols for high-throughput. ISSN: 2582-4686 SJIF 2021-3.261, 2022-2.889, 20235.384, 2024-6.875 ResearchBib IF: 9.948 / 2024 VOLUME-5, ISSUE-10 249 Figure 2: 3D rendered cross-section of kidney organoid showing cortex-medulla organization whole-mount IF for glomerular structures and tubules in day 18 kidney organoid). These evolutions position organoids as indispensable for precision nephrology, though regulatory hurdles for clinical trials loom. ISSN: 2582-4686 SJIF 2021-3.261, 2022-2.889, 20235.384, 2024-6.875 ResearchBib IF: 9.948 / 2024 VOLUME-5, ISSUE-10 250 CONCLUSIONS Kidney organoids have profoundly reshaped renal research by providing scalable, humanrelevant models that dissect the intricacies of excretory system development, pathological mechanisms, and therapeutic interventions with unprecedented precision, marking a paradigm shift from traditional 2D cultures and animal models that often fail to capture human-specific physiology. Derived from human pluripotent stem cells (hPSCs), these 3D constructs replicate key renal architectures, including nephrons with functional glomeruli, tubular segments, and nascent collecting ducts, enabling researchers to simulate embryonic nephrogenesis stages such as intermediate mesoderm induction, metanephric mesenchyme differentiation, and ureteric bud branching. This fidelity has facilitated groundbreaking studies on congenital anomalies, toxin-induced injuries, and chronic conditions, offering insights into molecular pathways like Wnt/BMP signaling that govern progenitor cell fate. By incorporating patient-derived induced pluripotent stem cells (iPSCs), organoids allow for personalized modeling of genetic disorders, revealing how mutations disrupt cellular homeostasis and lead to fibrosis or cyst formation. Furthermore, their scalability supports high-throughput platforms for drug discovery, reducing reliance on animal testing and accelerating preclinical evaluations with human-like responses. Innovations from 2024 to 2025 in hypoxic conditioning have surmounted key barriers by mimicking fetal oxygen gradients (typically 5-10% O2), which promote endothelial cell proliferation (CD31+ networks) and enhance nephron maturation, as evidenced in studies where hypoxia led to improved glomerular filtration analogs and tubular interconnectivity. For instance, developmental hypoxia has been shown to increase organoid complexity, fostering more advanced tubular structures and early-stage plumbing for waste removal, as reported in a 2025 study on advanced kidney organoids. Extracellular matrix (ECM) engineering, utilizing decellularized scaffolds, Matrigel embeddings, or supramolecular hydrogels enriched with laminin and collagen IV, has improved structural integrity and cell-matrix interactions, facilitating glomerular basement membrane formation and reducing off-target cell populations. Recent protocols have integrated ECM manipulations to accelerate derivation from hPSCs, enabling organoids that model early human nephrogenesis more accurately. Vascularization advancements, including co-culture with endothelial progenitors or hypoxic induction of VEGF expression, have achieved perfusable networks that support nutrient delivery and filtration functions, with transplanted organoids demonstrating neovascularization and urine production in mouse models. Bioengineering techniques, such as 3D bioprinting and organoids-on-a-chip, have further enabled mature, functional constructs by incorporating biomechanical cues like fluid shear stress, simulating glomerular filtration rates up to 20% of native levels and allowing for real-time monitoring of excretory dynamics. These developments have positioned organoids for clinical impact, including high-throughput nephrotoxicity screening and personalized therapies for diseases like polycystic kidney disease (PKD), where organoids from mutant iPSCs exhibit cystogenesis responsive to targeted inhibitors. Notably, 2025 breakthroughs include spatially patterned kidney assembloids that recapitulate progenitor self-assembly for high-fidelity disease modeling in vivo, and human fetal kidney organoids that mature over months to mirror pregnancy stages, opening avenues for congenital disorder research and safer prenatal drug testing. Yet, persistent challenges in achieving full-scale maturation hinder broader applications; current organoids often remain fetal-like, with incomplete nephron segmentation, limited size (typically <1 mm), and immature functional metrics such as low glomerular filtration efficiency and ISSN: 2582-4686 SJIF 2021-3.261, 2022-2.889, 20235.384, 2024-6.875 ResearchBib IF: 9.948 / 2024 VOLUME-5, ISSUE-10 251 absent medullary structures. Vascular sustainability remains a bottleneck, as in vitro networks frequently regress without sustained host integration, leading to necrosis in larger constructs and complicating long-term cultures beyond 60 days. Ethical scalability issues arise from sourcing hPSCs, particularly embryonic stem cells, raising concerns about consent, equity in access to patient-derived lines, and potential immunogenicity in allogeneic transplants. Regulatory hurdles for clinical trials, including standardization of protocols and validation against human data, further delay translation, while high costs of scRNA-seq and bioprinting limit accessibility in resource-constrained settings. These obstacles demand sustained multidisciplinary collaboration among stem cell biologists, bioengineers, clinicians, ethicists, and policymakers to refine protocols, integrate AI for optimized differentiation, and establish biorepositories for diverse genetic backgrounds. Ultimately, these models herald a regenerative era by promising bioengineered solutions that could curtail kidney disease epidemics, which affect over 700 million globally and impose staggering healthcare costs through dialysis and transplants. By enabling mutation corrections via CRISPR/Cas9 in iPSC-derived organoids, researchers can validate therapies for hereditary conditions like APOL1mediated CKD, where organoids reveal mitochondrial disruptions as key targets. Future prospects include HLA-edited universal organoids for off-the-shelf implants, hybrid systems fusing organoids with xenogeneic scaffolds (e.g., gene-edited pig kidneys), and macro-scale biofabricated kidneys for transplantation, potentially alleviating donor shortages where waitlists exceed 100,000 in the US alone. Interdisciplinary efforts will also address inflammation modeling through immune cell incorporation, as seen in related gut organoids, enhancing studies on immune-kidney interactions. As organoids evolve, they transform patient outcomes by shifting paradigms from symptom management to curative regeneration, fostering hope for equitable, precision-based nephrology in the coming decades. REFERENCES 1. 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