Cell Junctions Essay

1070 Words 4 Pages
Cell junctions in multicellular organisms are responsible for connecting cells together. The three main kinds are Occluding Junctions that seal cells together by their plasma membranes, Anchoring Junctions that connect cytoskeletal filaments of one cell to another and Communicating Junctions which aid in the coordination of cellular activities (Hardin et al., 2012). The junctions, although serve the same main purpose: to link cells together, have various similarities and differences between each of their structural and functional features.
The first type of junction, occluding junctions, are found commonly in the epithelia. In vertebrates occluding junctions are called tight junctions while those in invertebrates are named septate junctions.
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Tight junctions are formed when long rows of transmembrane adhesion proteins called claudins, occludins and junctional adhesion molecules of neighbouring cells attach to each other through interactions between their extracellular domains and close the intercellular gap between the cells. These proteins, that appear as sealing strands, also interact with intracellular peripheral membrane proteins such as ZO1 to anchor the actin cytoskeleton of the cell (Alberts et al., 2002). This binding of ZO-1 to actin is dependent on calcium ions and lower levels of intracellular calcium affects the permeability of the tight junction (Brown and Davis, 2002). The strands span the entire circumference of cells forming physical paracellular barriers or "ridges" that prevent the movement of molecules through the gaps between the cells. This ensures that ions and solutes can only go through the tissue if they …show more content…
This requirement is met by Anchoring Junctions. There are four kinds of anchoring junctions: Adherens and Desmosomes that join cells to cells and Focal Adherens and Hemidesmosomes that join cells to their extracellular matrix (Alberts, 2002). In this essay, we are concerned only with the two that are involved in cell to cell linking. The transmembrane adhesion protein in Adherens and Desmosomes is called Cadherin which, like in occluding junctions, are calcium dependant. The calcium ions bring about structural changes in the extracellular domains of adjacent cadherins and allow them to bind together given that they are the same type of cadherins ( Brooker et al. 2011). However the cadherin bond holds the linked cells 20-25nm apart, unlike in occluding junctions where there is no space between cells (Hardin et al., 2012). Adherens junctions are also similar to occluding junctions in that they connect to actin filaments but adherens junctions additionally provide "contractile strength" to cells (DBriers.com, 2012). The intracellular domain of cadherin in adherens junctions bind to the intracellular anchor proteins called catenin, vinculin and α-actinin in the cytoplasm to form a complex which in turn binds to bundles of actin filaments (Alberts, 2002). This accounts for the strength of adherens junctions. In

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