Single-cell RNA sequencing reveals chondrocyte heterogeneity and fibrocartilage transition in osteonecrosis of the femoral head.

Cartilage degeneration in osteonecrosis of the femoral head (ONFH) is closely associated with chondrocyte heterogeneity. Single-cell RNA sequencing was performed on 50,851 chondrocytes from 10 ONFH patients and 10 controls. The findings were further validated using an in vitro chondrocyte injury model induced by dexamethasone. Nine chondrocyte subtypes were identified, among which COL1A1⁺ chondrocytes (exhibiting fibrotic features and Hippo-YAP pathway activation) and COL2A1⁺ chondrocytes (displaying a reparative hyaline cartilage phenotype) were significantly enriched in ONFH. Immunohistochemistry and in vitro functional assays confirmed the upregulation of signature proteins and key transcription factors in these subtypes. Pseudotime analysis revealed that CCL20⁺ progenitor cells differentiate toward fibrotic or reparative fates via PRG4 or SOX5, respectively. Key ligand-receptor pairs, including FGF2-FGFR1 and TNFRSF11B-TNFSF11, mediated intercellular communication. This study elucidates the cellular and molecular basis of the "fibrosis-repair" imbalance in ONFH cartilage degeneration, offering a theoretical foundation for targeting chondrocyte fate regulation.