Contractile actin expression in torn human menisci

The human meniscus is subject to injury that necessitates repair or removal. Many aspects of the cellular response to injury have not been well characterized. The purpose of this study was to describe the cellular distributions within the torn human meniscus. In addition to evaluating the cell density in selected regions, we investigated the cellular expression of a contractile actin isoform that has recently been found in the intact human meniscus. Included as a contemporaneous comparative group were torn human meniscal allografts. We hypothesized that a hypercellular surface zone would be found in the torn menisci, with a higher percentage of cells in this peripheral region expressing alpha-smooth muscle actin compared with other locations in the interior of the remnant. The rationale for this hypothesis was based on prior immunohistochemical investigations of the distribution of alpha-smooth muscle actin-containing cells in the torn human anterior cruciate ligament. Eighteen torn meniscal specimens were obtained from 17 patients, 0.5 to 84 months after injury, and four torn allograft meniscal samples were retrieved from three patients, 11 to 49 months after implantation. Microtomed sections of paraffin-embedded tissue were stained with hematoxylin and eosin and a monoclonal antibody to alpha-smooth muscle actin. The cell density and percentage of cells containing alpha-smooth muscle actin were determined in the following zones: synovial, vascular, hypercellular with loose collagen, hypocellular with dense collagen, and organized collagen. A cellular layer that resembled synovium was present on the surface of all but two of the specimens. Vascular regions were often continuous with the synovium abutting the more interior loose collagen zones. The total cell density was greatest in each of the zones closest to the periphery (synovium, vascular, and loose collagen; p < 0.001), when compared to the interior of the tissue. The synovium-like layer was found to have the highest percentage of alpha-smooth muscle actin-expressing cells and the highest alpha-smooth muscle actin-containing cell density (p < 0.05). Similar results were found for the torn allograft menisci. These findings confirm the working hypothesis and suggest that the torn human meniscus is capable of mounting a reparative response, including the proliferation of cells capable of contributing to wound closure. This underscores the importance of providing a bridging scaffold into which such cells can migrate.