Abstract: BACKGROUND: Because of the poor regeneration ability of chondrocytes after damage, the use of stem cells as a basic treatment for repairing cartilage damage has become a popular treatment method.  
OBJECTIVE: To observe the effect of psoralen combined with transforming growth factor β1 on the differentiation of rat bone marrow mesenchymal stem cells into chondrocytes.
METHODS:  Bone marrow mesenchymal stem cells were isolated from Sprague-Dawley rats and cultured by marrow adherent method. The effective concentration to promote bone marrow mesenchymal stem cells proliferation was selected by CCK-8 assay. Bone marrow mesenchymal stem cells were assigned to four groups. In the normal control group, cells were treated with low-sugar DMEM with 10% fetal bovine serum and 100 U/mL double antibodies. In the psoralen group, cells were treated with low-sugar DMEM with 10% fetal bovine serum, 100 U/mL double antibodies, 10-7 mol/L dexamethasone, 50 mg/L vitamin C, and 10 μmol/L psoralen. In the combination group, cells were treated with low-sugar DMEM with 10% fetal bovine serum, 100 U/mL double antibodies, 10 μg/L transforming growth factor β1, 10-7 mol/L dexamethasone, 50 mg/L vitamin C, and 10 μmol/L psoralen. In the positive control group, cells were treated with low-sugar DMEM with 10% fetal bovine serum, 100 U/mL double antibodies, 10 μg/L transforming growth factor β1, 10-7 mol/L dexamethasone, and 50 mg/L vitamin C. After 21 days, the expression of type ll collagen was detected by immunocytochemistry. RT-PCR and western blot assay were utilized to detect the expression levels of differentiation marker SOX-9 and type ll collagen mRNA and protein.  
RESULTS AND CONCLUSION: (1) Immunocytochemical staining showed that no positive cells were found in the normal control group, while type ll collagen positive cells were found in the other three groups. (2) Compared with the normal control group, the expression of SOX9 and type II collagen mRNA and protein increased significantly in the psoralen group, combination group and positive control group (P < 0.05), and the combination group had the best effect (P < 0.01), and it was significantly better than that in the positive control group and the psoralen group (P < 0.05). (3) The results show that within a certain concentration range, psoralen can promote the proliferation of bone marrow mesenchymal stem cells. Psoralen can induce bone marrow mesenchymal stem cells to differentiate into cartilage, and its combined use with transforming growth factor β1 has a synergistic promoting effect.

Key words: psoralen, bone marrow mesenchymal stem cells, chondrocytes, chondrogenic differentiation, transforming growth factor β, type ll collagen, SOX9 protein, induced differentiation