Animal cells in multicellular tissues are usually joined by tight junctions, desmosomes (also called anchoring junctions), and gap junctions (also called communicating junctions). Plant cells are often joined by plasmodesmata. Each type of junction has specialized functions, such as preventing fluid leakage between membranes or allowing the free passage of small molecules and ions between adjacent cells. Tight junctions, which bind cells together, are found predominantly in epithelial tissues, such as the lining of the intestines. Tight junctions form a barrier that prevents fluids from moving between the intestinal space and the space between cells. Desmosomes, or anchoring junctions, appear as thickened patches in the cell membrane region between two cells. Desmosomes contain specialized proteins, such as keratin (the same protein found in fingernails and hair), that increase the rigidity of tissues. Desmosomes-such as those found in epithelium, smooth muscle, and many other animal tissues -are buttonlike junctions that bind cells together and also function as anchors for fibers in the cytoskeleton. In gap junctions, or communicating junctions, two cells are separated by a small gap, which is bridged by specialized channels that allow the passage of water and small molecules. Gap junctions help coordinate the activities of adjacent cells. For example, a hormone that stimulates one cell will often activate adjacent cells as well by the passage of intracellular signals through gap junctions. Plant cells, unlike animal cells, are surrounded by thick walls that form rigid tissues. They do not need the junctions found in animal tissues. But some higher plant cells are interconnected by plasmodesmata, tubelike structures that penetrate through cell walls and form fluid connections between adjacent cells. Like gap junctions in animal cells, plamodesmata allow the free exchange of small molecules and help coordinate the activities of neighboring cells.