Читать книгу Musculoskeletal Disorders - Sean Gallagher - Страница 59

The extracellular matrix of connective tissues consists of ground substance and fibers and plays a critical role as a scaffold within which tissues organize. The ground substance is the component of the matrix located between the cells and the fibers. It is typically a semifluid gel medium that serves for the passage of molecules through connective tissues and for the exchange of metabolites with the circulatory system (Young & Heath, 2000). It contains a mixture of long unbranched polysaccharide chains and water stabilized by glycosaminoglycans (GAGs), proteoglycans, and glycoproteins. The glycoproteins include structural (fibrillin and fibronectin), nonfilamentous (laminin, entactin, and tenascin), and integrin proteins (cell adhesion molecules). The typical gel‐like property of the extracellular matrix is imparted by the large volume of these globular complexes and their hydrophilic nature (their highly charged side chains attract large volumes of water and ions), which, when combined, produces the characteristic turgor of the ground substance. The mechanical properties of a tissue’s particular ground substance depend on the reinforcing fibrous proteins (collagen subtypes or elastin) or minerals (bone) to which these aggregates and other components are bound.

A number of types of fibrous proteins can be found in the ground substance of connective tissue. Collagen type I is the primary collagen in the dermis of skin (Figure 3.2), tendons, ligaments, and bone. It is formed by tropocollagen triple chains into larger fibrils (Iannarone, Cruz, Hilliard, & Barbe, 2019) that are visible under light microscopy. In contrast, collagen type II is the main collagen of hyaline cartilage and consists of fine filaments that cannot be visualized under light microscopy. Collagen type III is the major component of the fine reticular network within the bone marrow. It is also the first type of collagen deposited into developing or repairing tendons and bones. Elastin is a fibrous protein typically present as discontinuous fibrils (Figure 3.2) or sheets and confers the properties of stretch and elastic recoil (Young & Heath, 2000).