Short reviewGap junction systems in the mammalian cochlea
Introduction
The mammalian cochlea has a unique arrangement of extracellular fluid-filled compartments that support the mechanosensory transduction process. The apical surfaces of the cochlear hair cells are bathed in endolymph, which has an ionic composition of high K+ and low Na+. The cochlear endolymph is also characterized by the presence of the endolymphatic potential, of approximately 100 mV. When hair cells are activated by sound, their receptor potentials are generated by the flow of K+ from endolymph into hair cells. The K+ is then released to extracellular space and from here is recirculated back to endolymph. The gap junction systems are most likely the pathway for the recirculation of cochlear K+ [5].
Recent molecular biological studies 2, 3have shown that mutation in connexin 26 gene can cause non-syndromic sensorineural hearing loss, and also strongly support our hypothesis regarding the role of gap junctions in mechanisms of the inner ear.
Section snippets
Distribution of connexin 26 in the cochlea
Immunohistochemical localization of connexin 26 at the light microscopic and ultrastructural levels in the rat inner ear was studied in detail 4, 5. We have since confirmed the immunostaining results in mouse, gerbil, guinea pig, chinchilla, cat, and human, and believe that the initial results are representative of a general organization of mammalian ears.
The mammalian cochlea is comprised of a variety of cell components, including various epithelial and connective tissue cells. Connexin
Epithelial cell gap junction system
All cochlear supporting cells are directly connected to adjacent supporting cells with gap junctions. No definitive gap junctions or connexin 26-like immunoreactivity is detected between the cochlear sensory cells and the supporting cells. In the lateral end of the cochlear supporting cells, gap junctions are observed between Claudius cells and root cells. The root cells extend their cytoplasmic processes, which are also connected by gap junctions, deep into the lower part of the spiral
Connective tissue cell gap junction system
Connective tissue cells in the spiral limbus are coupled to each other and make a large gap junctional network. Mesenchymal cells, which line the scala vestibuli, possess scattered gap junctions, and also have gap junctional connections with fibrocytes in the spiral limbus and with the fibrocytes in the suprastrial zone of the spiral ligament. Fibrocytes in the spiral ligament are classified into the four different type of cells 5, 7. They are composed of types I, II, III, and IV fibrocytes.
Functional significance of gap junction systems in the ion transport mechanism
It is well-established that the high level of K+ in the scala media that is necessary for normal functioning of hair cells is maintained by the activity of marginal cells of the stria vascularis. The marginal cells accumulate K+ using a combination of Na,K-ATPase and Na–K–Cl cotransporter found in their basolateral plasma membranes [8].
It has been established that the K+ accumulated by the stria vascularis come from the perilymphatic space, but K+ in perilymph are prevented direct to access to
Acknowledgements
This work was supported by Research Grant No. 10671581 from the Ministry of Education, Science and Culture, Japan and NIDCD/NIH Grant R01 DC00073.
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