Cells are connected by various components that form junctions.
The zonula occludens toward the apical margin, acts as an "O" ring to provide a tight seal so there is no leakage, and movement of fluids, ions, and small molecules is rigorously controlled. Key proteins include occludin and claudin as well as several zonula occludens (ZO) proteins. Actin filaments extend out into the cytoplasm.
The zonula adherens provides cellular attachment similar to caulking. Key proteins include catenins just inside the cell membranes and E-cadherin with calcium between the membranes. Actin filaments extend out into the cytoplasm.
The macula adherens, or desmosome provides cellular attachment similar to a rivet, with desmoglein and desmocollin proteins with calcium between the cell membranes linking two plaque regions with plakoglobin and desmoplakin proteins just inside the cell membrane. Intermediate filaments extend into the cytoplasm for anchoring.
Connexons with connexin protein provide conduits for fluid, ions, and molecules passing between cells.