Healing of defects in canine articular cartilage: Distribution of nonvascular α‐smooth muscle actin‐containing cells

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Abstract
The objective of this study was to evaluate the types of tissue resulting from spontaneous healing of surgically created defects in adult canine articular cartilage up to 29 weeks postoperatively, with specific attention directed toward the presence and distribution of cells containing the contractile actin isoform, α‐smooth muscle actin. Two 4‐mm diameter defects were made in the trochlear groove to the depth of the tidemark in 20 adult mongrel dogs. The areal percentage of specific tissue types in the reparative material was determined histomorphometrically. Immunohistochemistry was employed to evaluate the percentage of α‐smooth muscle actin‐containing cells. The results showed that approximately 50% of the chondrocytes in the superficial zone of the uninvolved articular cartilage expressed α‐smooth muscle actin. A significantly lower percentage of α‐smooth muscle actin‐positive chondrocytes appeared in the uninvolved deep zone. Notably, the deep zone adjacent to the defect contained a greater percentage of such cells than in the uninvolved deep zone. Also of interest was that a greater percentage of nonvascular cells in the hyaline cartilage and fibrocartilage of the reparative tissue contained α‐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‐thickness chondral defect was found to fill with hyaline cartilage, with up to 19% judged histologically similar to articular cartilage. The results warrant further consideration of the role of α‐smooth muscle actin in chondrocytes in normal articular cartilage and in reparative tissue.