Elucidation of the mechanism of TGF-βsignaling in the repair process of full-thickness defects of rat articular cartilage.

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To create full-thickness defects in rat femoral patellar groove uniformly, we invented a twin blade device. The repair process using this device was studied histologically, and the character of cells in the reparative tissue was elucidated by observing the localization of cell proliferation, N-CAM, and type II collagen mRNA.Further, the expression of TGF-β1, TGF-βRI, and TGF-βRII during the repair processes was investigated. At four days, proliferating undifferentiated spindle-shaped cells filled the entire defect, and N-CAM expression was restricted to spindlier undifferentiated cells beneath the surface area. Subsequently, these cells changed into polygonal-shaped cells, and type II collagen mRNA was detected in these cells deep in the defects at ten days. At two weeks, the first evidence of cartilage matrix indicated by safranin-O was observed, and chondrocytes surrounded by this cartilage matrix showed a high proliferative capacity. New subchondral bone at the bottom of the defects had formed through intramembranous ossification from four days. It progressed and reached to the same level of residual osteochondral junction by four weeks. By that time, new cartilage tissue similar to residual articular cartilage regenerated, and safranin-O and type II collagen wholly stained the newly synthesized cartilage matrix. TGF-β1, TGF-βRI, and TGF-β RII were coexpressed in the spindle-shaped undifferentiated cells that infiltrated into the defects, in the polygonal-shaped cells, and in proliferating chondrocytes located in the deep and middle layers. These results suggested that TGF-β signaling was involved through the chondrogenesis pathway of repair process, which included undifferentiated cell proliferation, cellular condensation, chondrocyte proliferation, and matrix synthesis.