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

Abstract

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.

Fig. 1

Comparation of traditional 2D models and organoids. The figure compares traditional 2D cell cultures with organoids. The 2D model shows limited physiological relevance, forced cell polarity, and restricted cell interactions, making it ideal for high-throughput screening but inadequate for complex biological processes. In contrast, organoids, derived from pluripotent stem cells or progenitor cells, offer semi-physiological structures, enabling the study of tissue morphogenesis and human development. The table highlights key differences: organoids excel in modeling complex systems but have higher experimental variability, while 2D models remain highly manipulable and suitable for basic screenings

Fig. 2

Cell microenvironment in bone/cartilage repair. The figure illustrates the distinct cellular microenvironments involved in bone and cartilage repair. On the left, the bone repair microenvironment consists of hematopoietic stem cells (HSCs), osteoprogenitor cells, and macrophages, which differentiate into osteoclasts for bone resorption, and osteoblasts for bone formation. Osteoblasts mature into osteocytes and bone-lining cells, contributing to bone homeostasis. On the right, the cartilage repair microenvironment is dominated by chondrocytes, which are responsible for cartilage matrix production. The vascularized subchondral layer supports nutrient exchange, crucial for cartilage repair and regeneration. The central image highlights the anatomical location of bone and cartilage within a joint

Fig. 3

A brief history of organoids and development of bone/cartilage organoids. The diagram outlines the key milestones in organoid research, beginning with the discovery in 1907 that dissociated sponge cells could self-organize, a foundational concept in this field. Over the decades, advances included the development of embryoid bodies (1960), the isolation of pluripotent stem cells (1981), and breakthroughs in generating organ-specific organoids such as intestinal (2009) and liver organoids (2013). The timeline also highlights the role of the extracellular matrix (1980s) in organoid development. Recent advances are marked by the generation of bone and cartilage organoids (2021), and the creation of osteochondral tissue using microcarriers and 3D bioprinting (2023). By 2024, Jiacan et al. employed a GelMA/AlgMA/HAP composite bioink to 3D bioprint large-scale biomineralized bone organoids, marking a significant leap in bone tissue engineering. The future of organoid technology points toward assembloids, which integrate multiple organoid types to mimic complex tissue interactions, offering potential for more sophisticated biological models and therapeutic applications

Fig. 4

The construction strategy of bone and cartilage organoids. The pyramid outlines the stepwise strategy for constructing bone and cartilage organoids, starting from the base and progressing to the top. Cell Selection: The foundation of the strategy includes choosing appropriate cell types, such as BMSCs, osteoblasts, osteoclasts, chondrocytes, iPSCs, and endothelial cells. Matrix Gel Selection: The next tier involves selecting the proper matrix materials to support cell growth and differentiation. Commonly used matrices include Matrigel, collagen, silk fibroin, PEG, and DNA hydrogels. Construction Techniques: Key techniques such as stepwise induction, 3D bioprinting, and the use of bioreactors are employed to guide the development of complex organoid structures. Cytokines and Inducers Selection: Specific cytokines and chemical inducers are critical for driving cell differentiation and tissue development within the organoids. At the apex, fully developed bone or cartilage organoids are formed, representing the culmination of coordinated cellular interactions, matrix scaffolding, construction techniques, and biochemical signals

Fig. 5

Overview of hydrogel systems used in the construction of bone/cartilage organoids. The figure illustrates four distinct hydrogel systems-Matrigel, Collagen-hydrogel, PEG-hydrogel, and DNA-hydrogel-used for supporting the development of bone and cartilage organoids. a Matrigel contains key extracellular matrix components such as laminin, collagen IV, and entactin, facilitating stem cell adhesion and proliferation through integrin and growth factor receptor interactions. b Collagen-hydrogel involves the incorporation of isolated mesenchymal stem cells (MSCs) into collagen fibers, forming a scaffold that promotes cell differentiation and tissue development. c PEG-hydrogel features a transglutaminase (TG) cross-linking mechanism using PEG and hyaluronic acid (HA) to form a functionalized hydrogel environment that supports the differentiation of bone marrow stromal cells (BMSCs) and hematopoietic progenitor cells (HPSCs). d DNA-hydrogel combines RGD-silk fibroin and a DNA system to create a microsphere hydrogel structure, which can be integrated into a microfluidic chip for precise tissue engineering applications

Fig. 6

Potential applications of bone/cartilage organoids. The diagram showcases the wide-ranging applications of bone and cartilage organoids in biomedical research and therapeutic development. Construction of Disease Models: Organoids are used to model diseases such as osteoarthritis, bone fractures and healing, rheumatoid arthritis, genetic bone disorders, and cartilage regeneration and repair. These models offer insight into disease mechanisms and allow for testing therapeutic interventions. Drug Discovery and Personalized Medicine: Organoids enable high-throughput drug screening, drug prediction, and intricate studies of disease mechanisms. They are also pivotal in advancing personalized medicine by tailoring treatments based on individual responses. Biomaterial Evaluation and Safety Testing: Bone and cartilage organoids allow for the evaluation of biomaterials and safety pharmacology, providing a more physiologically relevant context for material and drug testing. Genomic and Metabolomic Analysis: These organoids facilitate the study of genomic alterations and metabolic pathways, offering deeper insights into cellular responses and disease states. Biobanks and Tissue Engineering: Bone and cartilage organoids can be stored in biobanks for future research and used in genomic engineering and metabolic analysis, paving the way for advanced tissue engineering applications

Fig. 7

Integration of machine learning in bone/cartilage organoid development and evolution. a Machine Learning-Driven Optimization of Bone/Cartilage Organoids: This panel illustrates the use of machine learning to enhance organoid development. Key variables, including cell selection, matrix gel, assembly techniques, and biological activity, are forecasted and fed into a machine-learning model. The model decodes these inputs into outputs that optimize organoid structural and biological features. Data acquisition, such as image data and omics analysis, is processed to fine-tune the organoid development process through feedback optimization and in vivo validation. The algorithm continuously learns from this feedback loop, improving the design and function of organoids for regenerative purposes. b Evolution of Bone Organoid Modeling: This panel shows the progression of bone/cartilage organoid development through four stages: 1.0-Mimicking physiological characteristics of bone. 2.0-Mimicking pathological characteristics for disease modeling. 3.0-Mimicking structural characteristics, including complex features like osteons. 4.0-Advancing towards clinical translation, where bone/cartilage organoids can be applied in therapeutic contexts, including personalized medicine and large-scale tissue engineering