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BACKGROUND: The aim of this study was to observe the protective effect of bone marrow mesenchymal stem cell-derived extracellular vesicles (BMSC-EVs) on chondrocytes, and to clarify the possible mechanism of them combined with cartilage regeneration scaffolds in promoting the repair of late full-thickness cartilage defects. METHODS: BMSC-EVs were isolated and extracted from BMSCs through ultracentrifugation and identified. The inflammatory environment of chondrocytes was mediated using tumor necrosis factor-α (TNF-α) in vitro, and chondrocytes were co-cultured with an equivalent amount of phosphate buffered saline (PBS) and BMSC-EVs (80 μg/mL) under inflammatory response. The effects of BMSC-EVs on TNF-α-mediated chondrocyte proliferation and apoptosis were detected via methyl thiazolyl tetrazolium (MTT) assay and Hoechst33342 staining, respectively. The protein expressions of protein kinase B (AKT)/ phosphorylated (p)-AKT, extracellular signal-regulated kinase (ERK)/p-ERK, and p38/pp38 in chondrocytes in the inflammatory environment in each group were determined using Western blotting. Besides, 12 New Zealand white rabbits were randomly divided into 4 groups. The rabbit model of full-thickness knee cartilage defects was established, and an equivalent amount of normal saline, normal saline + scaffolds, and BMSC-EVs+scaffolds were applied to the cartilage defects in the 4 groups, respectively. At 6 weeks after operation, the samples were taken, and the repair effect on knee cartilage defects of laboratory rabbits was compared in each group through gross observation, hematoxylin-eosin (HE) staining, and safranin O-fast green staining. RESULTS: BMSC-EVs were successfully extracted and identified. BMSC-EVs were in an eccentric circular shape with an obvious characteristic cup-like structure, and the average diameter was 71.00 nm. It was found using Western blotting that specific surface markers CD63 and CD9 were expressed in BMSC-EVs. In the inflammatory environment, the number of chondrocyte-derived apoptotic bodies in BMSC-EVs group was smaller than that in TNF-α group, and the proliferation rate was higher in BMSC-EVs group than that in TNF-α group. The results of Western blotting showed that the expressions of p-ERK1/2 and p-p38 in chondrocytes significantly declined, while the expression of p-AKT rose in BMSC-EVs group. In the New Zealand white rabbit model of full-thickness knee cartilage defects, the repair effect on knee cartilage defects was better in BMSC-EVs + scaffold group than that in blank group and scaffold group. CONCLUSIONS: In the inflammatory environment, BMSC-EVs may inhibit chondrocyte apoptosis and promote proliferation through PI3K/ AKT, p38 MAPK and MAPK/ERK signaling pathways. BMSC-EVs can promote the repair of cartilage defects more effectively, which provide new ideas for the clinical repair of knee cartilage defects. [ABSTRACT FROM AUTHOR]