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The Noncollagenous Proteins (Glycoproteins)

These molecules consist primarily of protein, but many of them also contain a few monosaccharides and oligosaccharides. Although noncollagenous proteins such as fibronectin contribute only a few percentage points to the dry weight of ligaments, they have an important role in the complex interaction of ligament cells and their environment during growth, healing, and remodeling. However, this role is poorly understood.

Fibronectins are important in an array of cellular functions, particularly those involving a cell’s interaction with its surrounding extracellular matrix. They are high-molecular-weight extracellular glycoproteins whose functions include (modulating?) intra- and extracellular matrix morphology, cellular adhesion (both cell-to-cell and cell-to-substratum), and cell migration.

Examined by electron microscopy, fibronectins appear as fine filaments or granules coating the surface of fibrillar collagens or associated with cell membranes. Fibronectins have an adhesive domain specific to fibrin, actin, hyaluronic acid, cell surface factors, and collagen. They function to attract and couple key elements in normal healing and in growing tissue.

Quantitative studies of fibronectin concentrations in rabbit ligaments demonstrate significantly (two to three times) higher amounts of fibronectin in the cruciate ligaments as compared to the collateral ligaments. This difference may reflect the fact that the cruciate ligaments are surrounded by a synovial sheath, and therefore have a higher degree of cellularity as compared to the extraarticular ligaments.

The maintenance of ligament tissue and its ability to respond to changes in loading depend on interactions between the cells and matrix. Normally the matrix macromolecules are slowly but continually degraded and replaced. The cells must synthesize new macromolecules to balance the losses due to normal degradation or microtrauma. The matrix provides to the cells protection from mechanical injury during normal loading and transmits signals generated by loading to the cells.

Cells bind to the matrix primarily through a family of cell surface proteins called integrins. These molecules mechanically link the matrix macromolecules, including fibronectin, to the internal cell cytoskeleton. They participate in cell adhesion, migration, and proliferation and in regulation of cell synthesis of new matrix macromolecules.

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