How are invertebrate epithelia made tight?

ABSTRACT Many compartments in the animal body are delineated and separated by epithelial sheets of cells. Such epithelia are described as leaky or tight depending on the extent to which the compartments on either side of them appear to be kept distinct. It has been assumed that a‘tight’ epithelium has intercellular junctions that completely seal the paracellular pathway. Indeed, in vertebrates, some tight junctions have been shown to be virtually impenetrable. However, in invertebrate tight epithelia, there is controversy over whether the paracellular pathway is sealed by a vertebrate type of tight junction or whether the septate junction, a special invertebrate junction not seen in vertebrate tissues, can fulfil the same function.