Cell junctions that form impermeable barriers are tight junctions and septate junctions. Impermeable junctions hold cells together but prevent them from exchanging content. Junctions are structures that create points of fusion for cells that are in direct contact with one another.
Cell junctions fall into one of three categories: communicating junctions, impermeable junctions and adhering junctions. Classification of these junctions depends on their function. Communicating junctions assist in the exchange of small molecules from one cell to another. These include chemical synapses and cell junctions. Adhering junctions assist in keeping cells together. These include belt desmosomes, spot desmosomes and hemidesmosomes.