Characterization of ootolith soluble-matrix producing cells in the saccular epithelium of rainbow trout (Oncorhynchus mykiss) inner ear.

Although the organic matrix may play an important role in the growth of teleost otoliths, cellular contributions to the production of the organic matrix have been studied in only a small number of teleost species with limited methods, and are still poorly understood. In order to characterize saccular epithelial cells which produce otolith matrix, antiserum was raised against an EDTA-soluble fraction of otolith matrix (otolith soluble-matrix, OSM) of the rainbow trout. The components in the OSM and in the endolymph were characterized by immunoblotting. The saccular epithelium was immunohistochemically stained with the antiserum and the ultrastructure of OSM immunoreactive cells was studied. By immunoblotting, multiple components (> 94.0 kDa [smeared] and 43.0 kDa) in the OSM reacted with the antiserum, whereas only one band (> 94.0 kDa) was detected in the endolymph. Under immunohistochemical staining, reactions to the antiserum were observed in columnar cells lined at the most peripheral region of the sensory epithelium, transitional epithelial cells, and squamous epithelial cells. Electron microscopic observations revealed that all three types of cells were equipped with extended rough endoplasmic reticulum and prominent Golgi apparatus, suggesting the active production of organic material(s). Dilations of translucent vesicles, apocrine-like extrusions of cytoplasm, and vesicles containing many minute globules were frequently associated with the apical surface of these cells. Some ruptured vesicles were observed, releasing their contents into the endolymphatic space. The present study identified columnar cells lining the most peripheral region of the sensory epithelium, transitional epithelial cells, and squamous epithelial cells as the OSM-producing cells. We suggest that the OSM components are secreted and dissolved into the endolymph and subsequently deposited onto the otolith.