Chapter 47: The Extracellular Matrix and Connective Tissue

The extracellular matrix serves as a critical structural and biochemical scaffold that surrounds cells and defines tissue architecture throughout the body. This chapter examines how the ECM fills intercellular spaces, mediates cell-to-cell communication, and regulates molecular transport across tissue boundaries. The ECM comprises three primary molecular classes: fibrous proteins including collagens, elastin, and laminin; proteoglycans decorated with glycosaminoglycan chains; and adhesion molecules that establish cell-ECM connections. Collagen, the most abundant protein in mammalian tissues, achieves its characteristic mechanical strength through a triple helical structure stabilized by post-translational hydroxylation of proline and lysine residues, a vitamin C dependent process whose failure produces the connective tissue disease scurvy. More than 25 collagen isoforms exist with distinct functions, including fibril forming types that provide tensile strength, network forming type IV found in basement membranes, and specialized variants that regulate angiogenesis and tumor suppression. Genetic mutations affecting collagen structure underlie osteogenesis imperfecta and Ehlers-Danlos syndrome, demonstrating how molecular defects produce skeletal fragility and connective tissue laxity. Elastin provides elastic recoil properties in lungs and blood vessels, with fibrillin microfibrils serving as essential scaffolding; fibrillin mutations cause Marfan syndrome. Laminin anchors cells within basal laminae and maintains epithelial integrity; laminin deficiency produces epidermolysis bullosa and congenital muscular dystrophy. Proteoglycans form hydrated gel networks through their glycosaminoglycan side chains, providing compressive resistance and molecular filtration; impaired degradation of these polymers causes mucopolysaccharidoses with severe skeletal and developmental consequences. Integrins function as bidirectional signaling molecules that physically link ECM proteins to the actin cytoskeleton while transmitting regulatory signals controlling adhesion, migration, and immune function. Matrix metalloproteinases degrade ECM components during normal tissue remodeling and pathological processes; their activity requires tight inhibition by tissue inhibitors of metalloproteinases to prevent excessive degradation in cancer invasion, atherosclerosis progression, and arthritis. The chapter illustrates these principles through disease mechanisms in systemic lupus erythematosus affecting joints, diabetic complications producing basement membrane thickening with urinary protein loss, and therapeutic strategies targeting osteoclasts in brittle bone disease.