TO THE EDITOR
A review by Cong et al[1] highlighted the importance of stem cell-based strategies in the regeneration of hyaline cartilages, such as articular cartilages, which have limited regenerative capacity owing to their avascularity and lack of innervation[2]. The authors deftly outlined the progression from fundamental cell biology to advanced techniques, such as 3D bioprinting and translational studies, demonstrating the interdisciplinary nature of modern regenerative medicine[3]. The inclusion of cutting-edge topics, such as artificial intelligence (AI) and gene editing, provides a future direction for regenerative orthopedics. Nevertheless, an additional discussion is warranted to address the technical and conceptual challenges pertaining to cell identification and the preservation of functional integrity.
Challenges in the precise definition of cartilage progenitor cells
This review discusses multiple stem cell sources for cartilage regeneration, including bone marrow stem cells, adipose tissue stem cells, synovial stem cells, and induced pluripotent stem cells. However, the precise definition of chondrogenic progenitor cells (CPC) remains unresolved. Current research relies on broadly characterized mesenchymal stromal cells (MSCs), which exhibit substantial heterogeneity[4]. However, for successful clinical translations, defining the transcriptomic signatures and surface marker profiles that distinguish CPCs, is essential[5,6]. Reportedly, the expression of chondrogenic markers, such as SRY-box transcription factor 9 (SOX9), collagen type II alpha 1, and aggrecan, together with the suppression of runt-related transcription factor 2, helps prevent unwanted hypertrophic differentiation of CPCs[7]. As hypertrophy remains a major limitation in clinical translation, the strategies to control hypertrophic differentiation of MSC-derived chondrocytes require adequate addressing in ongoing and future clinical trials.
Transitioning from cartilage growth to mechanical stability
Another key challenge in clinical translation - briefly mentioned in the review but deserving greater emphasis - is the distinction between defect filling and restoration of functional biomechanical integrity. Although current engineering strategies, such as scaffold-free approaches and bioprinting, effectively promote tissue formation and initiate extracellular matrix (ECM) deposition, the regenerated tissue often lacks the long-term biomechanical properties of native hyaline cartilage[8]. An insufficient type II to type I collagen ratio results in mechanically inferior fibrocartilage which results in structurally weaker fibrocartilage and lacks the intricate zonal architecture of the native tissue[9]. This structural imbalance and structural deficit predispose the repair tissue to premature degradation and mechanical instability under physiological shear and compressive forces in the joint environment, thereby limiting the in vivo durability. Moreover, processes, such as chondrocyte hypertrophy, endochondral ossification, and matrix calcification, continue to limit clinical durability and warrant critical evaluation. A more detailed discussion of scaffold-free systems as well as the potential cytotoxic effects of agents, such as dexamethasone and gelatin-methacrylate, used to enhance mechanical stability in clinical translation, would further strengthen the translational relevance of the review.
Addressing the catabolic joint microenvironment
The authors summarize key signaling pathways involved in cartilage regeneration, including the regulation of Wnt/β-catenin signaling in hypertrophy, role of the transforming growth factor-β superfamily in ECM production, and hypertrophic modulation by the SOX trio (SOX5, SOX6, and SOX9). They further elaborate on scaffold design and fabrication strategies (hydrogels, 3D bio-printing), as well as growth factor delivery systems (transforming growth factor-β1, bone morphogenetic protein 2, etc.). However, greater emphasis could be placed on the degenerative cartilage microenvironment - the “hostile microenvironment” into which the scaffolds and biomaterials are implanted[10]. In chronic joint diseases, such as osteoarthritis, the microenvironment contains elevated levels of pro-inflammatory cytokines [e.g., interleukin (IL)-1β and tumor necrosis factor-α] and matrix metalloproteinases, which accelerate the degradation of the newly formed ECM, disrupt collagen organization, and inhibit cartilage repair, thereby promoting hypertrophic shifts in the implanted cells and compromising biomaterial durability[11]. Although MSCs are generally considered immune-privileged, their ability to differentiate into CPCs or chondrocytes, as well as their incorporation into bioengineered constructs, can substantially alter their immunogenic profile[12]. In addition, the role of exosomes discussed in future perspectives could be expanded to address extracellular vesicle heterogeneity, cargo profiling, and regulatory requirements that play major roles in extracellular vesicle-based regenerative strategies. For example, Chen et al[13] identified that exosomes derived from human umbilical cord MSCs showed chondrogenic and anti-inflammatory properties and promoted cartilage repair in an osteoarthritis rat model when delivered via hyaluronic acid-encapsulated sustained delivery systems. Incorporating such discussions would markedly enhance the clinical and translational relevance of this review.
Importance of detailed clinical study analysis
Although clinical studies and their outcomes are referenced, limited discussion is provided regarding study quality, sample sizes, and potential variables that may influence result interpretation. For example, Sadri et al[14] evaluated 40 patients with knee osteoarthritis, who received intra-articular injections of allogeneic adipose-derived MSCs (AD-MSCs; n = 20) against a saline placebo (n = 20). Compared to the control group (placebo), patients treated with AD-MSCs demonstrated significant improvements in clinical outcome measures, such as pain and functional scores (Western Ontario and McMaster Universities Osteoarthritis, visual analog scale). Additionally, the AD-MSC treatment was associated with a minor but significant increase in articular cartilage thickness on magnetic resonance imaging, confirming their roles in regeneration and reduced inflammation, characterized by decreased IL-6 and increased IL-10 levels[14]. Similarly, a comparison of the success rates, effect sizes, and safety profiles across different stem cell types would further strengthen the analysis of clinical results. The inclusion of a comparative table delineating various stem cell sources, biomaterials, and clinical methodologies using standardized criteria (efficacy, safety, cost, and regulatory status) would provide researchers with a valuable decision-making framework. This review discusses several gene editing strategies; however, incorporating the generation of hypoimmunogenic cells through gene editing and optimization of adeno-associated virus vectors for in vivo gene therapy would strengthen its translational relevance. These approaches after more specific, transient, and potentially safer gene editing methods that not only enhance cartilage regeneration but also ensure better control over long-term cellular behavior and joint inflammatory responses. For instance, Goodrich et al[15] demonstrated that self-complementary adeno-associated virus-mediated delivery of the IL-1 receptor antagonist gene that acts as an anti-inflammatory agent, directly into the joint, effectively attenuated osteoarthritis progression. Inclusion of similar studies would further broaden the clinical translational scope of the review. Furthermore, the review highlights the use of AI-based transcriptomic analyses to identify the chondrogenic potency of MSC subpopulations; however, current applications are limited by the availability of sufficiently large and well-annotated datasets to train AI systems for reliable marker prediction and scaffold optimization. Although Cong et al[1] appropriately identify AI as a promising tool, a deeper exploration of these limitations and future data integration strategies would enhance the translational relevance of the review. Furthermore, a structured discussion of the translational pathway from the bench to the clinic, including typical timelines, costs, and success rates, would help researchers better understand the practical prospects of these technologies.
Conclusion
Cong et al[1] delivered a thorough and insightful overview of stem cell-driven cartilage regeneration, addressing the interdisciplinary progress achieved in biomaterials, cellular therapies, and translational research. Their review integrates recent developments in scaffold engineering, stem cell selection, and in vivo regeneration strategies, providing a valuable resource for researchers in this rapidly evolving field. However, a few key aspects require more in-depth discussion to fully analyze the complexity of the field and its clinical translation. For instance, extracellular vesicles, emerging as central mediators of regenerative signaling, warrant a more detailed explanation of their standardization and underlying mechanisms. Expanding the coverage of these areas would improve the depth of the review and strengthen its guidance on improving the reproducibility, safety, and durability of stem cell-based hyaline cartilage repair strategies. Overall, this comprehensive review by Cong et al[1] underscores the significant promise of stem cell-based cartilage regeneration, successfully bridging biological strategies and innovative engineering to offer definitive clinical solutions for challenging joint and cartilage defects.
