Cell–Cell Junctions and Extracellular Matrix (2A) - MCAT Biological and Biochemical Foundations of Living Systems

Fibronectin. It links cells to collagen via integrin-binding domains, facilitating migration and organization.

Laminin. It self-assembles into networks that organize basement membranes and support epithelial attachment.

Laminin, type IV collagen, proteoglycans, and nidogen. These components form a thin, organized sheet separating epithelia from connective tissue.

Core protein covalently linked to glycosaminoglycan (GAG) chains. This structure allows proteoglycans to attract water and ions, contributing to ECM hydration.

Proteoglycans and GAGs (for example, aggrecan, hyaluronan). Their negatively charged GAGs bind water, creating a hydrated gel that absorbs compressive forces.

Adherens junction. This junction forms a belt-like structure for circumferential adhesion and cytoskeletal linkage.

Gap junction. It facilitates rapid signaling and synchronization in tissues like cardiac muscle.

Seal adjacent cells to block paracellular transport and maintain polarity. They form a barrier at the apical-lateral border, preventing diffusion between cells and separating membrane domains.

Actin microfilaments. Catenins link cadherins to the actin cytoskeleton, enabling force transmission and cell shape maintenance.

Cadherins (for example, E-cadherin). These proteins homophilically bind across cells in a calcium-dependent manner to facilitate adhesion.

Cell–cell adhesion via actin-linked cadherins. They provide mechanical linkage and maintain tissue integrity by connecting cytoskeletons of neighboring cells.

Actin microfilaments. ZO proteins act as scaffolds connecting transmembrane components to the cytoskeleton for structural support.

Claudins and occludin. These proteins create the tight seal by forming extracellular interactions between adjacent cell membranes.

Strong cell–cell adhesion resisting mechanical stress. They distribute mechanical forces across tissues by anchoring intermediate filaments between cells.

Keratin intermediate filaments. These filaments provide tensile strength and resilience, linking to desmosomes via plaque proteins.

Desmoglein and desmocollin. These proteins form transcellular dimers, anchoring cytoskeletal elements for robust adhesion.

Direct intercellular passage of ions and small molecules. They enable metabolic and electrical coupling by forming aqueous pores between cytoplasms.

Connexins. Six connexins form a hemichannel (connexon) that docks with an adjacent cell's to create the pore.

Approximately $< 1$ kDa. The channel pore size restricts passage to small solutes, ensuring selective communication.

Anchor epithelial cells to the basement membrane (ECM). They provide stable attachment to the underlying ECM, maintaining epithelial integrity.

Integrins. These receptors bind ECM ligands like laminin, connecting to intracellular adapters.

Intermediate filaments (keratin). They link to plaque proteins like plectin, providing mechanical stability against shear forces.

Bidirectional signaling: inside-out and outside-in. Integrins transmit signals from ECM to cytoskeleton and vice versa, regulating adhesion and migration.

Structural support, signaling, and regulation of cell behavior. ECM provides a scaffold for cells, influences differentiation, and modulates growth factor activity.