ULTRASTRUCTURAL-STUDY OF THE SACCULAR EPITHELIUM OF THE INNER-EAR OF 2 TELEOSTS, ONCORHYNCHUS-MYKISS AND PSETTA-MAXIMA

The secretory cells and ionocytes of the saccular epithelium of the in ner ear of trout (Oncorhynchus mykiss) and turbot (Psetta maxima) have been studied by electron microscopy. In these species, the saccular e pithelium may be subdivided into four zones: the ''macula'', the ''mes hwork area'', the ''patches area'', and the ''intermediate area''. In addition to the sensory ''hair cells'' and their supporting cells, the macula contains, at its periphery, ''granular cells'' that have the u ltrastructural characteristics of secretory cells. The ''meshwork area '' around the macula contains large ionocytes endowed with pseudopods, many mitochondria, and three intracytoplasmic membrane systems (endop lasmic reticulum, tubular, and vesicular systems). The patches area, l ocated at some distance from the macula, consists of groups of small m itochondria-rich ionocytes characterized by infoldings of their latera l plasma membrane. In the intermediate area, the size and organelle-co ntent of cells decrease from the meshwork area to the patches area. Th ere is no significant difference in cell composition or structure of t he saccular epithelium between the trout and the turbot. The secreting cells might be involved in secretion of endolymph and formation of th e otolith, whereas the ionocytes probably regulate the ionic compositi on of the endolymph.