Ultrastructural immunolocalization of the otolith water-soluble-matrix in the inner ear of rainbow trout just-hatched fry

Abstract

In order to investigate the process of otolith-matrix production and deposition, ultrastructural immunolocalization of an EDTA-soluble fraction of otolith matrix (otolith soluble-matrix, OSM) was studied in the sacculus of rainbow trout, Oncorhynchus mykiss, just-hatched fry. In the transitional epithelial cells of a cylindrical type, OSM was packed in the vesicles whose diameter was 100-250 nm. In the transitional epithelial cells of a cuboidal type and squamous epithelial cells, OSM was packed into the vesicles whose diameter was 0.5-1.5 μm. Sensory epithelium was immunonegative. Exocytosis of small vesicles, dilations of translucent vesicles, and extrusions of cytoplasm were frequently associated with the apical surface of the transitional and squamous epithelial cells. The OSM-immunopositive fibrous substance existed in the endolymph and was deposited onto the otolith surface. Otolith matrix was an OSM-immunopositive fibrous substance arranged into the meshwork. Densely packed layers and less densely packed layers of the matrix were deposited alternatively. The present results indicate that OSM is produced in the transitional and squamous epithelial cells, is stored in the two types of secretory granules, is secreted into the endolymph, and is deposited onto the otolith surface.