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Interaction of Cells with Other Cells (5) • Cadherins – glycoproteins that mediate Ca2+dependent cell-cell adhesion. Interaction of Cells with Other Cells (6) • Cadherins (continued) – Also involved in transmitting signals from the ECM to the cytoplasm. – Mediate many of the changes in adhesive contacts during embryonic development by forming epithelial-mesenchymal transition (EMT). Cadherins and the EMT Cadherins and the EMT Interaction of Cells with Other Cells (8) • Adherens Junctions and Desmosomes: Anchoring Cells to Other Cells – Adherens junctions – they form “belts” near apical surface called junctional complex. – Cells of an adherens junction held together by calcium-dependent linkages. Intercellular junction complex Intercellular junction complex Molecular architecture of adherens junction Interaction of Cells with Other Cells (9) • Desmosomes – disk-shaped adhesive junctions between cells. – Found in a variety of tissues. – Contain cadherins that link the two cells across a narrow gap. – Cadherins of desmososme shave different domain structures: desmogelins and desmocollins. Structure of a desmosome Interaction of Cells with Other Cells (10) • The Role of Cell-Adhesion Receptors in Transmembrane Signaling – The transfer of information across the plasma membrane is transmembrane signaling. – Integrins and cadherins can transmit signals from the extracellular environment to the cytoplasm. Interactions involving the cell surface Interaction of Cells with Other Cells (11) • The binding of an integrin with its ligand can induce a responses such as changes in growth potential. The Human Perspective: The Role of Cell Adhesion in Inflammation and Metastasis (1) • Inflammation is a response to infection or injury but can be triggered inappropriately. • Inflammatory response: – Recruitment of leukocytes to site of injury. – Neutrophils attach to P- and E-selectins. – Neutrophils start to “roll” along wall of vessel. Movement of neutrophils during inflammation The Human Perspective: The Role of Cell Adhesion in Inflammation and Metastasis (2) • As neutrophils interact with inflamed venule endothelium, platelet activating factor (PAF) is displayed. • PAF sends a signal to increase the binding activity of some integrins. • Activated integrins cause neutrophils to stop rolling and adhere firmly to wall of vessel. The Human Perspective: The Role of Cell Adhesion in Inflammation and Metastasis (3) • Cancer is the result of abnormal cell proliferation. • The spread of a tumor to other parts of the body is called metastasis. • Metastatic cells have special cell adhesion properties: – Are less adhesive. – Are able to penetrate several barriers. – Are able to invade normal tissues. Steps leading to metastatic spread The Human Perspective: The Role of Cell Adhesion in Inflammation and Metastasis (4) • During growth and development of a tumor there is loss of E-cadherin leading to less adhesion. • Changes in the numbers and types of celladhesion molecules lead to promote metastasis. 7.4 Tight Junctions: Sealing the Extracellular Space (1) • Tight junctions (TJs) – specialized contacts between epithelial cells. – Located at the very apical end of the junctional complex between adjacent cells. – TJs serve as a barrier to the free diffusion of water and solutes from the extracellular compartment. – Some TJs are permeable to specific ions or solutes. Tight junctions Tight junctions Tight Junctions: Sealing the Extracellular Space (2) • Occludins are proteins found in TJs. • Claudins form the major structural component of TJs, and may account for selective differences in TJ permeability. • TJs form the bloodbrain barrier. 7.5 Gap Junctions and Plasmodesmata: Mediating Intercellular Communication (1) • Gap junctions – sites between animal cells for intercellular communication. – Composed entirely of membrane protein connexin. – Connexins are organized into a complex called connexon. Gap junctions Gap junctions Gap Junctions and Plasmodesmata: Mediating Intercellular Communication (2) • Gap-junction intercellular communication (GJIC) allows the passage of low-weight molecules. • Gap junctions can allow integration of activities of individual cells into a functional unit. • Compatibility differences between connexins either promote or prevent communication between different cells. Passage of low-molecular-weight solutes through gap junctions Gap Junctions and Plasmodesmata: Mediating Intercellular Communication (3) • A new type of communication has been discovered – tunneling nanotubes. – It has been observed in cells growing in culture. Gap Junctions and Plasmodesmata: Mediating Intercellular Communication (4) • Plasmodesmata are cytoplasmic channels passing through cell walls of adjacent plant cells. – Are lined by plasma membrane. – Contain a central structure, the desmotubule. – Serve as sites of cell-cell communication. Plasmodesmata 7.6 Cell Walls (1) • Cell walls provide plants protection against mechanical abrasion, pathogens, and osmotic stress. – The fibrous component is cellulose. – Cellulose is organized into microfibrils, which provide rigidity to the cell wall. The plant cell wall Cell Walls (2) • The matrix of the cell wall contains hemicelluloses, pectins, and proteins. • Cell walls arise as a cell plate that forms between the plasma membranes of newly formed daughter cells. • The walls of growing cells are primary walls and allow flexibility lacking in the thicker secondary walls of mature cells. Synthesis of a plant cell wall