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Unformatted text preview: Cell Junctions, Cell Adhesions, and the Extracellular Matrix Cells must cohere to form an organized muticellular structure. The mechanics of cohesion govern the architecture of the body – its shape, strength, and the arrangement of different cell types. 1) Junctions b/w cells create pathways for communication 2) Attachments to other cells and to extracellular matrix control orientation of each cells internal structure 3) The making and breaking of attachments governs cell movements There are two different building strategies: strengthen the extracellular matrix and strengthen the cytoskeleton and cell-cell adhesions. Two broad categories of animal tissues: 1) Connective tissues : ex. Bone, tendon. The extracellular matrix is plentiful and cells are sparse w/in it. The matrix is rich in polymers, especially collagen and it is the matrix rather than the cells that bear the mechanical stress that the tissue is subjected to. Cells are attached to the matrix, usually not to one another. 2) Epithelial tissues : ex. Gut lining, epidermis. Cells are bound together in sheets called epithelia. The extracellular matrix is scanty, consisting mainly of a thin mat called the basal lamina or basement membrane, which underlies one face of the sheet. W/in the epithelium, cells are attached to each other directly by cell-cell adhesions where cytoskeletal filaments are anchored, transmitting stresses across the interior of each cell, from adhesion site to adhesion site. Four Main Functions for Junctions : 1) Anchoring Junctions : including both cell-cell adhesions and cell-matrix adhesions; transmit stresses and are tethered to cytoskeletal filaments inside the cell. a. Adherens junctions : cell-cell/actin cytoskeletons/cadherins as transmembrane proteins b. Desmosomes: cell-cell/intermediate filaments/ cadherins as transmembrane proteins c. Focal adhesions: cell-matrix/actin filaments /integrins as transmembrane proteins d. Hemidesmosomes: cell-matrix/actin filaments /integrins as transmembrane proteins 2) Occluding Junctions : seal the gaps b/w cells in epithelia so as to make the cell sheet into an impermeable or selectively permeable barrier. 3) Channel-forming Junctions: create passageways linking the cytoplasms of adjacent cells. a. Channel juctions (animals) b. Plasmadesmata (plants) 4) Signal-relaying Junctions: allow signals to be relayed from cell to cell across the plasma membrane at sites of cell-to-cell contact. a. Chemical synapses b. Immunological synapses c. Transmembrane ligand-receptor contacts Direct Interaction between Cells:Anchoring Junctions Link Cytoskeletons: This pic shows a section of mature epithelial cells and shows how cell-cell adhesions (anchoring junctions) are distributed in relation to other types of junctions. It shows a typical simple columnar epithelium where a single layer of tall cells all stand on basal lamina with their apex exposed to the extracellular medium. On their lateral surfaces they make junctions w/one another. surfaces they make junctions w/one another....
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- Spring '08
- cells, Collagen, Epithelia