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Lesson 2 - Types of Cellular Junctions

Student Performance Objectives
1. State 3 reasons why a tissue might need a specialized junction holding its cells together. 
2. Describe tight junctions and a part of the body where they are located.
3. Describe desmosomes and a part of the body where they are located.
4. Describe gap junctions and a part of the body where they are located.

Lesson Outline
A. In General:
    1. Cells composing a tissue must anchor to each other in a way that promotes correct 
        tissue functioning. 
    2. Some tissues form barriers to the passage of molecules. E.g., the epithelia of the 
        mouth, pharynx, esophagus and the stomach.
    3. Some tissues facilitate transport of molecules across their surface. E.g., the epithelia
        of the small intestine and the kidney tubules.
    4. Some tissues must hold their cells together under tremendous stresses that 
        might pull them apart. E.g., the myocardium. 
    5. In some tissues, the cells transfer molecules directly from cell to cell as a form of 
        communication. E.g., the myocardium.
B. Tight Junctions

      1. Anchor cells of a tissue together like a zipper. The junction passes completely around each adjacent         cell. Acts as a barrier to passage of molecules.
        a. Columnar epithelium of the stomach - tight junctions prevent acid and enzymes from passing from the stomach chamber, between adjacent stomach epithelial cells, into the deeper tissues of the stomach wall. 
        b. Columnar epithelium of the small intestine - tight junctions help prevent passage of:
            (1) Undigested or partially digested molecules through the intestinal wall into the blood.
            (2) Bacteria from the intestinal lumen into the blood.
            (3) Digestive enzymes from the intestine into the intestinal wall or into the blood.
C. Desmosomes:    

    1. Firmly anchor adjacent cells in a tissue together at discreet points.
    2. Differs from a tight junction in that the junction does not encircle the cell - it is like a single 
        "spot-welded" point.
    3. Each cell contributes half of the desmosome. 
        a. A thick protein plate is found in the adjacent portion of each cell.
        b. Glycoprotein filaments span the intercellular space and become embedded in the adjacent plates.
        c. Cytoplasmic intermediate filaments embed in the intracellular side of each plate which then holds             the plate firmly to the cell's cytoskeleton. 
    4. Desmosomes hold atrial and ventricular myocytes (heart muscle cells) together (as part of intercalated discs), and are also found holding the epithelial cells of the skin's upper layer, the epidermis, together.
5. Hemidesmosomes are found in the deepest epidermal layer of the skin. Here one epidermal cell contributes 1/2 of the desmosome, but instead of "welding" to an adjacent cell, the glycoprotein filaments anchor to protein fibers in the underlying (deeper) connective tissue layer - the basement membrane (also called the basal lamina). bot.htm

   D. Gap Junctions
     1. Permit communication of adjacent cells through passage of small molecules.
    2. The junction is a small channel formed as six transmembrane proteins span across and link the cell  membranes of the two adjacent cells.
    3. This unit composed of the 6 transmembrane proteins and the channel they surround, that interconnects the two adjacent cells, is called a connexon.
    4. Found, along with desmosomes, in the intercalated discs linking adjacent heart cells. This facilitates diffusion of ions, allowing adjacent cells to stimulate each other electrically. 
    5. Found in some adjacent smooth muscle cells allowing cell-to-cell excitation, just as in the heart.