Summary:Relatively littlc is known about the contractile behavior of the human articular chondrocyte. Other connective tissue cells are known to express a contractile actin isoform, a-smooth muscle actin, in response to injury, at selected stages of wound healing, and in certain pathological conditions. This and recent work demonstrating contractile behavior in adult canine articular chondrocytes in vitro prompted the present study of the distribution of a-smooth muscle actin-containing chondrocytes in human articular cartilage. Approximately 75% of the chondrocytes in the superficial region of cartilage expressed a-smooth muscle actin as demonstrated by immunohistochemistry. In contrast. only approximately 10% of the cells in the deep region stained for this contractile actin isoform. There was no correlation of the percentage of a-smooth muscle actin-positive cells in either region with Mankin grade or with age. This is the first report of a contractile potential for human articular chondrocytes. The roles of a-smooth muscle actin in these cells warrant further investigation. The question of whether it is necessary to refer to these cells as myochondrocytes is considered.Many phenotypic traits have been reported for the articular chondrocyte in health, disease, response to injury, and selected stages of healing (8,9,45). Chondrocyte behavior in vivo has been assessed dircctly by microscopy methods (31) and biochemical and molecular biological analysis of extracted cells (39) and indirectly by determining the biochemical makeup (44) and physical properties (21) of the matrix. Evaluation of the activities of isolated chondrocytes grown in v i m (10,20,29) has also added to our understanding of the profiles of genes expressed by this cell type under a range of environmental conditions.One of the features of the articular chondrocyte that has not yet been investigated is the specific actin composition of the cytoskeleton. Of the six actin isoforms-coded by different genes (22)-all cells contain the and one of the y iorms. In some connective tissue cells (dermal fibroblasts), alterations in chemical and physical environmental conditions can upregulate or down-regulate the expression of the gene for a contractile actin isoform, u-smooth muscle actin (13,18,42). Expression of this gene has been related to the contractile behavior of cells, which may
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