Meshwork arrangement of mitochondria-rich, Na+,K+-ATPase-rich cells in the saccular epithelium of rainbow trout (Oncorhynchus mykiss) inner ear

Background: Electrolyte composition of the teleost fish inner ear endolymph is characterized by a high potassium concentration, From the ultrastructural characteristics, the mitochondria-rich cells (MRCs) in the inner ear epithelium are suggested to regulate the ionic composition of the endolymph. Methods: In the present study, the ultrastructure of MRCs in the saccular epithelium of the rainbow trout (Onchorhynchus mykiss) was studied, and the immunocytochemical detection of Na+,K+-ATPase, the key enzyme of the ion-transport, in the saccular epithelium was conducted, Electrolyte composition of the saccular endolymph was also determined. Results: Electron-microscopic observations revealed that MRCs located at the periphery of the sensory macula have numerous elongated mitochondria and a well-developed tubular system, Immunocytochemical detection of Na+,K+-ATPase on paraffin sections showed that immunoreactive (ir-) cells were distributed specifically around the sensory macula, Judging from their shape, size, and localization, the Na+,K+-ATPase ir-cells corresponded to the MRCs. The whole-mount immunocytochemistry using Na+,K+-ATPase as a marker for the MRC revealed that MRCs were connected with one another by extended cellular processes, and thus forming a dense meshwork structure around the macula, In the endolymph, potassium levels were 13 times higher than those in plasma, chloride levels were slightly higher whereas sodium, calcium, magnesium, and phosphate levels were lower. Conclusions: Thus, the saccular MRCs abundant in Na+,K+-ATPase are distributed around the sensory macula forming a dense meshwork structure, with the suggested function to regulate the electrolyte composition of the saccular endolymph. (C) 1997 Wiley-Liss, Inc.