29.7: Overview of Cell-Cell Junctions
The complex three-dimensional arrangement of cells in any multicellular organism is defined and maintained by interactions of cells with each other and the extracellular matrix. Cell-cell junctions are specialized structures where the multi-protein complexes on one cell interact with the multi-protein complexes on another cell. These cell junctions are classified into three main types based on their function — occluding, anchoring, and gap junctions.
Occluding or Tight Junctions
Tight junctions primarily function to prevent the movement of molecules such as water and solutes through the intercellular spaces. They are present as a series of transmembrane proteins that directly interact with the same proteins on the adjacent membrane. Thus, they hold the two plasma membranes in close proximity to each other, preventing the passage of molecules. Tight junctions can also be selectively permeable, allowing charge-specific ions to move through, while blocking others.
Anchoring junctions hold cells together and provide mechanical strength to the tissue structure. They form contact points where the cytoskeleton of one cell is anchored to that of its neighboring cells via transmembrane adhesion molecules called cadherins. There are two types of anchoring junctions:
- • adherens junctions that link the actin cytoskeleton and
- • desmosomes that link the intermediate filaments.
Thus, they form an extensive cytoskeletal network that functions cooperatively to maintain tissue structure and function.
Gap junctions are protein channels that facilitate communication between cells. The constituent transmembrane proteins form a channel linking the adjacent cells’ cytoplasms and allow the exchange of molecules and electrical impulses. The movement of molecules is regulated by the opening and closing of the channel proteins. For example, the signal for rhythmic contractions of the heart is passed from cell to cell via the gap junctions in cardiac muscle tissue. Plant cells, however, cannot exchange molecules via gap junctions because of the cell wall. Instead, they form channels of communication called plasmodesmata. These are membrane-lined pores connecting the cells through the cell walls such that the cytoplasm of one cell is contiguous with that of another. Thus, there is free movement of solutes and ions across plant cells connected by plasmodesmata.