|INTERCELLULAR JUNCTIONS - Points of contact between Cells...
Multicellular organisms have evolved so-called "cell-junctions" for linking together neighboring cells. In epithelia, the plasma membranes of adjacent cells are pressed together. Several kinds of intercellular junctions made of unique protein assemblies occur in vertebrates: gap junctions, tight junctions, adherens junctions, desmosomes and hemidesmosomes. Another junction is found In many plant tissues, where the plasma membrane of each cell is continuous with that of the adjacent cells. The membranes contact each other through openings in the cell wall called plasmodesma.
Tight Junctions, also called zona occludens, are a band of membrane-proteins encircling a cell and fusing it with an adjacent cell's membranes, closing off the intracellular space between them. Their function is to form a more or less tight seal preventing the flow of materials between apical surface and the basolateral surface of cells making them impermeable (see figure). Tight junctions seal adjacent epithelial cells in a narrow band just beneath their apical surface. Materials must actually enter the cells (by diffusion or active transport) in order to pass through the tissue. The Zona adherens is a junction which encircles a cell providing adhesion of one cell to another. Zona adherens are rich in actin, myosin, tropomyosin and vinculin microfilaments.