Healing of defects in canine articular cartilage: Distribution of nonvascular alpha-smooth muscle actin-containing cells

The objective of this study was to evaluate the types of tissue resulting f rom spontaneous healing of surgically created defects in adult canine artic ular cartilage up to 29 weeks postoperatively, with specific attention dire cted toward the presence and distribution of cells containing the contracti le actin isoform, alpha-smooth muscle actin. Two 4-mm diameter defects were made in the trochlear groove to the depth of the tidemark in 20 adult mong rel dogs. The areal percentage of specific tissue types in the reparative m aterial was determined histomorphometrically. Immunohistochemistry was empl oyed to evaluate the percentage of alpha-smooth muscle actin-containing cel ls. The results showed that approximately 50% of the chondrocytes in the su perficial zone of the uninvolved articular cartilage expressed alpha-smooth muscle actin. A significantly lower percentage of alpha-smooth muscle acti n-positive chondrocytes appeared in the uninvolved deep zone. Notably, the deep zone adjacent to the defect contained a greater percentage of such cel ls than in the uninvolved deep zone. Also of interest was that a greater pe rcentage of nonvascular cells in the hyaline cartilage and fibrocartilage o f the reparative tissue contained alpha-smooth muscle actin-positive cells, compared to the fibrous tissue in the defects. The findings of this study revealed that canine articular cartilage has some potential for spontaneous regeneration, including integration with the calcified cartilage zone. By 29 weeks, up to 40% of an areal cross section of an untreated full-thicknes s chondral defect was found to fill with hyaline cartilage, with up to 19% judged histologically similar to articular cartilage. The results warrant f urther consideration of the role of alpha-smooth muscle actin in chondrocyt es in normal articular cartilage and in reparative tissue.