Some of the pictures presented in this lecture are taken from 12/6/2018

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1-The lateral domain 12/6/2018

How the epithelial cells are attached to each other What are these structures? How the epithelial cells are attached to each other What do you think? 12/6/2018

How the epithelial cells are attached to each other??? The lateral domain What are these structures? is characterized by the presence of unique proteins TO FORM junctional specializations How the epithelial cells are attached to each other??? Before the advent of EM, the close apposition of epithelial cells was attributed to the presence of a viscous adhesive substance referred to as intercellular cement 12/6/2018

Because of its location in the terminal or apical portion of the cell and its bar like configuration, the stainable material visible in light microscopy was called The terminal bar 12/6/2018

a junctional complex The terminal bars Electron microscopy has shown that the terminal bars are specialized site that joins epithelial cells The specific structural components that makeup the terminal bars are readily identified with the EM and are collectively referred to as a junctional complex These complexes are responsible for joining individual cells together. There are three types of junctional complexes 1-Occluding junctions also called tight junctions 2-Adherens Junctions 3-Desmosomes 12/6/2018

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below adherens junction? Notice their positions Are they above or below adherens junction? Tight Junctions Tight Junctions adherens junction? adherens junction? Desmosomes 12/6/2018

How about attachment as a function??!!! 1-Occluding junctions also called tight junctions, are impermeable and allow epithelial cells to function as a barrier., occluding junctions form the primary intercellular diffusion barrier between adjacent cells. Function: by limiting the movement of water and other molecules through the intercellular space, they maintain physico-chemical separation of tissue compartments. Because they are located at the most apical point between adjoining epithelial cells, occluding junctions prevent the migration of lipids and specialized membrane proteins between the apical and lateral surfaces, thus maintaining the integrity of these two domains How about attachment as a function??!!! 12/6/2018

Where do you expect to find them? Tight Junctions consist of web like strands of transmembrane proteins that fuse together the outer surfaces of adjacent plasma membranes to seal off passage ways between adjacent cells Where do you expect to find them? 1-the stomach 2-intestines 3-urinary bladder and why? They inhibit the passage of substances between cells and prevent the contents of these organs from leaking into the blood or surrounding tissues Tight Junctions 12/6/2018

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If this is true, then we have just created a barrier How the nutrients would pass this barrier ? 12/6/2018

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2-Adherens Junctions contain plaque, a dense layer of proteins on the inside of the plasma membrane that attaches both to membrane proteins and to microfilaments of the cytoskeleton. Transmembrane glycoproteins called cadherins join the cells. Each cadherin inserts into the plaque from the opposite side of the plasma membrane, partially crosses the intercellular space (the space between the cells), and connects to cadherins of an adjacent cell. Function: adherens junctions help epithelial surfaces resist separation during various contractile activities, as when food moves through the intestines 12/6/2018

In epithelial cells, adherens junctions often form extensive zones called adhesion belts because they encircle the cell similar to the way a belt encircles your waist. Tight Junctions Tight Junctions adherens junction? adherens junction? 12/6/2018

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3-Desmosomes Like adherens junctions, desmosomes contain plaque and have transmembrane glycoproteins (cadherins) that extend into the intercellular space between adjacent cell membranes and attach cells to one another However, unlike adherens junctions, the plaque of desmosomes does not attach to microfilaments. Instead, a desmosome plaque attaches to elements of the cytoskeleton known as intermediate filaments, which consist of the protein keratin. The intermediate filaments extend from desmosomes on one side of the cell across the cytosol to desmosomes on the opposite side of the cell 12/6/2018

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Desmosomes junctions are common among the cells that make up A-The epidermis (the outermost layer of the skin) Desmosomes prevent epidermal cells from separating under tension B-cardiac muscle cells from pulling apart during contraction 12/6/2018

At gap junctions, membrane proteins called connexins form tiny fluid-filled tunnels called connexons that connect neighboring cells Through the connexons, ions and small molecules can diffuse from the cytosol of one cell to another Gap junctions allow the cells in a tissue to communicate with one another. Gap junctions also enable nerve or muscle impulses to spread rapidly 12/6/2018

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Thank you 12/6/2018