Lecture 20 Extracellular Matrix

Lecture 20 Extracellular Matrix - BME 418, Quantitative...

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BME 418, Quantitative Cell Biology Alan J. Hunt #20 - Extracellular Matrix and Cellular Adhesion What is the extracellular matrix? Generally we consider anything that is outside of the plasma membrane to be "out" of a cell. However, many important cellular components are excreted and constructed outside of a cell. We have already discussed one example of this, the cell walls that surround the plasma membrane of plants and bacteria. Other examples of structures principally formed of extracellular matrix: - In joints. Bone, ligaments, tendons, and cartilage. Serving functions of load bearing supports and lines or flexible mechanical connections, shock absorbers, slip plates, and semi-flexible structures (e.g. nose). - For attaching cells to surfaces. Adhesion fibers at focal contacts. Serve as traction devices or glue. Immunofluorecent micrograph of a fixed fibroblast showing colocalization of Integrin (green) and actin-containing stess fibers (red).
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BME 418, Quantitative Cell Biology Alan J. Hunt - Basal membranes. Surrounding many cell types (e.g. muscle, nerves), blood vessels, alveoli, or any other place where cells are organized into sheets. Serve as mechanical support, guides for cell growth and migration (e.g. nerves), or filters (e.g. kidney glomerulus). Although the functions of the extracellular matrix are diverse, in animals the ECM is generally made up of various amounts of three classes of molecules: 1) Structural molecules -collagens and elastins - provide strength and modulate elastic properties. 2) Protein-polysaccharid complexes - proteoglycans - appear to be a space- filling structures, and can serve as shock-absorbers. 3) Adhesive glycoproteins - fibronectins and lamanins - serve to attach cells to the ECM. Consider each of these in turn.
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BME 418, Quantitative Cell Biology Alan J. Hunt Collagen: - Forms strong supporting fibers that give tissues mechanical strength. - High tensile strength, ~ 10 kN/cm 2 , a 1 mm diameter collagen fiber can suspend about 10 kg without snapping. - Relatively inelastic, elastic modulus of 400-1000 Mpa (Liao and Belkoff, J. Biomechanics, 1999, 32(2)), a 1 mm diameter fiber will be stretched by ~1- 3% when suspending a 10 kg weight. - Made up of bundles of collagen fibrils, which are in turn made up of triple- helix collagen molecules. - Fibers are ~100 nm diameter, individual molecules are 270 x 1.5 nm. Note also the striations in electron micrograph. These are exhibited by many collagens and have a 67 nm repeat distance. This reflects a 1/4 molecule overlap of adjacent rows of collagen molecules, which results in gaps in the molecular lattice that fill with electron-dense EM stain (a "negative" stain). Note that some collagens, including those around nerves and blood vessels, do not exhibit striations. Structural stability
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Lecture 20 Extracellular Matrix - BME 418, Quantitative...

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