Fine structure and development of ionocyte areas in the labyrinth of the trout (Salmo trutta fario).

The distribution of ionocyte areas in the trout labyrinth (in the semicircular canal ampullae, crus communis and utricular vesicle) is very similar to that of areas of dark cells in mammals. In all 3 regions, ionocytes begin to develop after hatching, and are cuboid at fry stages and prismatic in juveniles and adults. On electron microscopy, 3 types of cell can be seen in adult ionocyte areas: ionocytes, associate cells and basal cells. Ionocytes possess many mitochondria, occupying approximately 28% of the cytoplasmic volume, and a well-developed tubulomembranous system which opens on the basal surface at some points. These ultrastructural features, very similar to those of chloride cells, strongly suggest that ionocytes are involved in ion transport. Unlike mammalian dark cells, there are no basal or lateral infoldings of the plasma membrane in trout ionocytes. Trout associate cells have a well developed vacuolar system, few mitochondria and bundles of cytoplasmic filaments. Although less specialised than ionocytes, they may be involved in endolymph secretion.