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 semi circular canal ampullae, crus communis and utricular vesicle) is very similar to that of areas of dark cells in mammals. In all 3 regions, i onocytes begin to develop after hatching, and are cuboid at fry stages and prismatic in juveniles and adults. On electron microscopy, 3 type s of cell can be seen in adult ionocyte areas: ionocytes, associate ce lls and basal cells. Ionocytes possess many mitochondria, occupying si milar to 28 % of the cytoplasmic volume, and a well developed tubulome mbranous system which opens on the basal surface at some points. These ultrastructural features, very similar to those of chloride cells, st rongly suggest that ionocytes are involved in ion transport. Unlike ma mmalian dark cells, there are no basal or lateral infoldings of the pl asma membrane in trout ionocytes. Trout associate cells have a well de veloped vacuolar system, few mitochondria and bundles of cytoplasmic f ilaments. Although less specialised than ionocytes, they may be involv ed in endolymph secretion.