Chapter 14: Cell Interactions & Adhesion

Cell Interactions & Adhesion contrasts these with tight junctions, which form sealing strands mediated by proteins like ZO-1 to establish permeability barriers and maintain membrane polarity in epithelia, and gap junctions, which utilize connexon channels made of connexin proteins to facilitate the direct transfer of ions and small molecules (ionic and metabolic coupling) for physiological synchronization. In the context of plant biology, the text examines plasmodesmata, wide membrane-lined channels containing desmotubules that connect the cytoplasm of neighboring cells (symplast) and allow for the transport of larger metabolites and viral movement proteins. The chapter then explores the molecular basis of cell adhesion, reviewing foundational experiments on sponge reaggregation and the differential adhesion hypothesis in vertebrates, which explains tissue sorting during development. A major focus is placed on Cell Adhesion Molecules (CAMs), categorized into calcium-dependent families like the cadherins (E-cadherin, N-cadherin), which interact with cytoplasmic catenins to influence cell shape, and calcium-independent molecules of the immunoglobulin superfamily, such as NCAM, where binding strength is modulated by sialic acid content. The summary concludes by applying these concepts to pathology, illustrating how bacteria (Neisseria, Pseudomonas), parasites (Plasmodium), and viruses (Rhinovirus) exploit cell surface receptors for invasion, and how cancer cells undergo metastasis by losing gap junctions and contact inhibition, altering surface glycoproteins, and secreting proteases to penetrate the extracellular matrix.