Chapter 19: Cell Junctions and the Extracellular Matrix

Tight junctions, composed of claudins and occludins, seal epithelial layers to prevent paracellular leakage and establish cell polarity. Anchoring junctions link the cytoskeleton—either actin filaments or intermediate filaments—to the cell membrane via cadherins and integrins, providing mechanical strength and tissue integrity. Desmosomes and hemidesmosomes are especially important in tissues under mechanical stress, such as skin and heart muscle. Gap junctions allow for direct cytoplasmic exchange of ions and small molecules between adjacent cells via connexon channels, enabling synchronized activities like cardiac contractions. The chapter also explores the molecular basis of cell-cell adhesion through cadherins, which mediate calcium-dependent homophilic interactions and are essential for tissue morphogenesis and maintenance. Integrins, which mediate cell-ECM adhesion, are introduced as bidirectional signaling receptors that connect the ECM to the cytoskeleton. The structure and composition of the ECM is described, including fibrous proteins like collagen and elastin, adhesive glycoproteins like fibronectin and laminin, and proteoglycans that form hydrated gels for resistance and cushioning. The basal lamina, a specialized ECM layer, is highlighted for its role in tissue scaffolding and filtration. The chapter concludes by examining dynamic remodeling of adhesion and ECM components during development, wound healing, and cancer metastasis. Overall, the chapter underscores the physical and signaling roles of cell junctions and ECM in shaping multicellular life.