Golgi-canalicular reticulum system in ion transporting fibrocytes and outer sulcus epithelium of gerbil cochlea

Five types of highly specialized fibrocytes have been identified in the spiral ligament of the gerbil cochlea. Type I, II, and IV fibrocytes function in cycling back to the stria vascularis K+ effluxed from outer hair cells and nerves during auditory transduction. Thus, evidence exists for a transcellular path of K+ movement from outer sulcus cells through fibrocytes to the strial interstitial space, but a mechanism for facilitating such ion flow within the cells has not been elucidated.

The spiral ligament of glutaraldehyde-osmium tetroxide-fixed and Epon-embedded gerbil cochlea was examined by transmission electron microscopy.

Ultrastructural examination disclosed an extensive membrane limited reticulum in the cytoplasm of type I, II, IV, and V fibrocytes of the lateral wall and in outer sulcus cells and their root processes. This system resembled the tubulocisternal endoplasmic reticulum present in some ion-transporting epithelia but appeared more to constitute a network of canaliculi and is referred to as the canalicular reticulum (CR). Many typical small Golgi complexes invariably accompanied the CR in the fibrocytes and sulcus cells, as we have found to be true of other epithelia known to contain CR and function in ion transport. Numerous mitochondria populated cytosol-containing CR.

The data support the concept of transcellular K+ flux in type I, II, IV, and V fibrocytes and outer sulcus cells in the cochlea and lend credence to the view of CR as functioning in the movement of ions through cells. The constant and precise association of Golgi complexes with CR in the different cell types implies a functional relationship possibly concerned with biosynthesis of CR by Golgi elements, and the abundance of mitochondria near CR indicates an energy requirement for function of the reticulum or its biosynthesis.