Chapter 13: Extracellular Matrix

Extracellular Matrix details the unique biochemistry of collagen, including its triple-helix structure stabilized by hydroxyproline and hydroxylysine, and traces its extensive biosynthetic pathway from intracellular procollagen formation in the RER to extracellular cleavage and cross-linking by lysyl oxidase. The summary highlights critical clinical correlations, explaining how defects in collagen synthesis or processing lead to pathologies such as scurvy (vitamin C deficiency), Osteogenesis Imperfecta (brittle bone disease), and Ehlers-Danlos syndrome. The discussion then moves to elasticity within the matrix, facilitated by elastin and fibrillin, and notes the association between fibrillin gene mutations and Marfan syndrome. The text further elucidates the ground substance, a hydrated gel meshwork of proteoglycans and glycosaminoglycans (GAGs) like hyaluronic acid and chondroitin sulfate, which functions in filtration and cell signaling. A key section focuses on how cells physically attach to this matrix via the glycoprotein fibronectin and its specific interaction with transmembrane integrin receptors through the RGD (arginine-glycine-aspartate) peptide sequence, connecting the extracellular environment to the intracellular actin cytoskeleton. The unique structure and filtration function of the basal lamina are analyzed, emphasizing non-fibrillar Type IV collagen, laminin, and entactin, alongside the implications of basement membrane thickening in diabetes. Finally, the chapter contrasts animal matrices with the plant cell wall, detailing the synthesis of cellulose microfibrils at the plasma membrane via rosette complexes guided by microtubules. It explains the integration of hemicelluloses (like xyloglucan), pectins, and extensin glycoproteins, and concludes by describing the mechanisms of plant cell growth, where turgor pressure and wall acidification trigger enzymatic loosening of the rigid wall matrix to allow expansion.