The relationship of actin to ligament contraction

A surgical model of a contracting ligament was established in a Z-lengthened, rat medial collateral ligament. These lengthened ligaments contracted back to a normal degree of tightness in a three-week period. Immunofluorescent actin staining of cryostat ligament sections was increased in these contracted ligaments, compared with sham-operated ligaments that had not contracted. In vitro culture of ligament fibroblasts confirmed that they (like fibroblasts from other systems) produce large, actin, stress fibers thought to be involved in cell contraction. Transmission electron-microscopic examination of normal ligament demonstrated the cytoplasm of fibroblast cell processes extending out to and surrounding collagen fibrils, indicating a closer involvement with established fibrils than was expected. On the basis of these observations, it is hypothesized that intracellular proteins, such as actin, allow the fibroblast to exert forces and actively contribute to the contraction of ligamentous tissues.