Chondrogenic Maturation Governs hMSC Mechanoresponsiveness to Dynamic Compression.

Dynamic compression (DC) bioreactors are widely used to mimic joint loading and study how human mesenchymal stem cells (hMSCs) respond to mechanical cues. However, it remains unclear whether DC alone is sufficient to induce chondrogenesis or how such cues interact during construct maturation. In this study, hMSCs were encapsulated in fibrin hydrogels at different cell densities and subjected to DC without, during, or after TGF-β3-mediated chondrogenic induction. DC alone modestly increased SOX9 expression but failed to upregulate key cartilage matrix genes such as ACAN and COL2A1, indicating that mechanical stimulation alone is insufficient to initiate chondrogenesis. When mechanical stimulation was coupled with TGF-β3, a more mature chondrogenic phenotype was observed for high cell seeding densities (HD). To simulate a post-implantation scenario, we applied DC following growth factor withdrawal and observed marked downregulation of SOX9, ACAN, and COL2A1 in low-density (LD) constructs. This reduction was not observed in HD constructs, which maintained a more stable chondrogenic phenotype under loading. These findings show that construct maturation critically influences mechanoresponsiveness and suggest that immature grafts may not tolerate mechanical stimulation. DC bioreactors may therefore serve not only to support cartilage engineering but also to predict in vivo graft performance.