Chapter 20: Integrating Cells into Tissues

20.4 The Extracellular Matrix II: Connective Tissue

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Connective tissue, such as tendon and cartilage, differs from other solid tissues in that most of its volume is made up of extracellular matrix rather than cells. This ECM is packed with insoluble protein fibers. ECM in connective tissue has several key components, some of which are found in other types of tissues as well:

Collagens, trimeric molecules that are often bundled together into fibers (fibrillar collagens)
Glycosaminoglycans (GAGs), specialized linear polysaccharide chains of specific repeating disaccharides that can be highly hydrated and confer diverse binding and physical properties (e.g., resistance to compression)
Proteoglycans, glycoproteins containing one or more covalently bound GAG chains
Multi-adhesive proteins, large multidomain proteins often comprising many copies (“repeats”) of a few distinctive domains that bind to and cross-link a variety of adhesion receptors and ECM components
Elastin, a protein that forms the amorphous core of elastic fibers

Collagen is the most abundant fibrous protein in connective tissue. Rubber-like elastin fibers, which can be stretched and relaxed, are also present in deformable sites (e.g., skin, tendons, heart). The fibronectins, a family of multi-adhesive matrix proteins, form their own distinct fibrils in the ECM of most connective tissues. Although several types of cells are found in connective tissues, the various ECM components are produced largely by cells called fibroblasts. In this section, we explore the structure and function of the various ECM components in connective tissue, and we see how the ECM is degraded and remodeled by a variety of specialized proteases.