• intercellular junctions;
  • IMPs;
  • ion transport;
  • scalariform junctions

The structure of scalariform junctions has been analysed in a number of different animal groups by a variety of techniques. These junctions generally display a highly regular intercellular cleft straddled by indistinct cross-striations. They are often in intimate spatial association with mitochondria which lie in parallel to the leaflets of their junctional membranes. Both intercellular and intramembranous features of these junctions have been studied in this report and certain common characteristics are revealed in all species examined. The intercellular cross-striations seen in this junctional type are found to be actually hollow columns or pillars, of hour-glass shape, regularly spaced in rhomboidal, or hexagonal arrays which may be anchored in place by certain of the intramembranous particles (IMPs) that occur in abundance on the membrane Pface. The density of the pillars is less than that of the IMPs. The other particles in the junctional area on both P and E faces may therefore be involved in ion transport, since the particular tissues in which these junctions are found are always involved in fluid flow. Those organisms which exhibit scalariform junctions have no closed circulatory system and thus may require some specialized osmoregulatory control in their excretory tissues. Those tissues which exhibit these junctions include organs such as rectal pads, rectal papillae, gills and nephridia, all of which share a similar physiological function even though they are found in organisms which occupy very different habitats. Such organisms include arthropods of different classes, as well as tunicates, where the presence of the scalariform junctions is restricted to a very specialized gut region. A revised model of scalariform junctions is presented here, which encompasses our new observations of their features. These include intercellular columns which are hollow and ‘hour-glass’, rather than straight sided, in side view, together with their spacing and orderly hexagonal or rhomboidal pattern of distribution; their possible tethering by only a certain percentage of the IMPs in the junctional area is also incorporated in this model.