Ultrastructure indicative of ion transport in tectal, Deiters, and tunnel cells: Differences between gerbil and chinchilla basal and apical cochlea

Abstract

Ultrastructural examination revealed an epithelium of about five tectal cells (TCs) roofing the outer tunnel (OT) in the mid to upper, but not the basal, region of gerbil and chinchilla cochlea. Structures in TCs that are apparently specialized for retrieval of K ϩ released into tunnel fluid from outer hair cells (OHCs) include surface fimbriae in the gerbil and canalicular reticulum in the chinchilla. A tunnel roof of organelle-rich TCs appeared to be better equipped for ion resorption than a roof composed of organelle-poor Hensen cells (HCs). Fimbriae, filopodia, and the cell body of TCs descended to contact the third Deiters cell (DC3) in the gerbil, and the hypertrophied DC3 phalanx rose to contact TCs in the chinchilla, which suggests a solute exchange between TCs and DCs. Previously unrecognized structures that are speculated to provide ATP ligand for cochlear purinoreceptors occurred in the chinchilla DC and gerbil TC. The observation of a microtubule stalk in DCs indicated that they also function in cochlear mechanics. A newly delineated lateral tunnel cell (LTC) intervened between the DC3 and HC in both species. The apicomedial plasmalemma of all DCs fitted closely to the base of OHCs and enveloped afferent nerves. The morphologic specializations reported here provide further support for the proposed transcellular lateral flow route for K ϩ currents generated by sound exposure and neural activity. The previously demonstrated expansion of Boettcher cells, outer sulcus cell roots, type Il and IV fibrocytes, and apical microvilli on HCs and Claudius cells (CCs) in the base of the cochlea is postulated here to mediate a basal parallel current that could supply the increased K ϩ transport required for the basally elevated electric potential

Expression of LHX3 and SOX2 during mouse inner ear development

“…While SOX2 and MYO6 expression persist in the adult ( Figure 4G, I), LHX3 and ATOH1 expression are lost from the auditory sensory epithelium ( Figure 4H and data not shown). SOX2 expression is detected in a subset of differentiated support cells, including Deiters', pillar, inner phalangeal, tectal and lateral tunnel cells ( Figures 4G, 5C, D) (Spicer et al, 2003). Because of the three-dimensional organization of the adult organ of Corti, the full extent of SOX2 expression is best appreciated in the brightest point projection Z-series of an immunolabeled whole-mount preparation ( Figure 5C, D).…”

“…Along the medial to lateral axis of the mid-basal cochlea, SOX2-expressing nuclei are detected in both hair cell (MYO6+) and support cell layers spanning from inner hair cells and inner phalangeal cells to outer hair cells, Deiters' cells and lateral tunnel cells ( Figure 2Q, R) (Spicer et al, 2003). PROX1, a homeobox transcription factor, is expressed some support cells in the cochlea at this stage of development, therefore, we tested whether these populations coincided (Bermingham-McDonogh et al, 2006).…”

Chemical exposure and hearing loss

“…Clearly, K + recycling from the organ of Corti back to the endolymph or to the stria vascularis plays an important role, preventing an adverse local effect on the hair cells and allowing K + ions to be saved. 65 Solvents transit from the outer sulcus into the organ of Corti, where they likely disrupt the role played by the Hensen and Tectal cells, thereby affecting K + recycling.…”

Novel structures in marginal and intermediate cells presumably relate to functions of apical versus basal strial strata

“…In addition, a silent current recovers K + that diffuses extracellularly into scalae vestibuli and tympani (Zidanic and Brownell, 1994). Evidence also has been presented for a parallel third K + current that flows independent of sound exposure and moves K + from scalae media and tympani through cells to the lateral wall, mainly in the cochlear region tuned to high sound frequencies (Spicer et al, 2003). These currents largely explained the source of ions which were known to derive from perilymph rather than blood plasma (Konishi et al, 1978;Woda et al, 1979;Marcus, 1986;Salt and Konishi, 1986;Sterkers et al, 1988) for maintaining the high [K + ] in endolymph.…”

“…MCs were connected apically at tight junctions and exhibited a nucleus close under the luminal surface. The apical cell body contained numerous ribosomes and mitochondria and exhibited Golgi zones, small and large lysosomes (Spoendlin, 1967;Wright and Lee, 1989) and a network of canaliculi (Forge, 1982) resembling the canalicular reticulum present in other cochlear cells (Spicer et al, 2003).…”