Cell Junctions in Arthropod Ion-transport Systems

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Abstract
The epithelial cells involved in the movement of ions and water form a major subset of all epithelial cell types. Both the form and the functions of cell junctions present in these cells are essentially the same as those found elsewhere. Gap junctions are believed to regulate intercellular communication; desmosomes and hemidesmosomes provide mechanical anchorage to other cells and the extracellular matrix; septate junctions play roles in providing cell to cell anchorage, and perhaps in sealing the lateral surfaces of adjacent cells together to prevent paracellular fluid and solute movement; tight junctions (of limited distribution in insects) are seals between adjacent cells. They form a barrier to the paracellular movement of solutes and water. Examination of the junctions in salivary glands and midgut provide insight into the roles of these junctions in the development and function of ion transport systems. In Manduca sexta (Johannsen) the cells of the salivary gland are joined by pleated septate and gap junctions. Individual salivary cells have numerous folds and canaliculi. The walls of the canaliculi consist of extensively folded plasma membrane in intimate association with mitochondria. Gap junctions connect adjacent parts of the same cell across membrane folds, effectively shortening diffusion distances in the cells. Hemidesmosomes are present in the walls of developing canaliculi. They are attached to pore filaments that occupy the lumen of the developing canaliculi. The hemidesmosomes and pore filaments may have a morphogenetic role as they disappear after the canaliculi are formed. In Manduca sexta the midgut cells are joined by gap and septate junctions. These junctions differ in morphology from their counterparts in the salivary gland; physiological studies show the goblet cells are not coupled to neighboring tall columnar cells. We have shown the gap junctions joining them are typical of non-coupling junctions. Preliminary studies suggest that the gap junctions change form when the cells are coupled.

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