Gap junctions in the rat cochlea: immunohistochemical and ultrastructural analysis

Abstract

Gap junctions in the rat cochlea were investigated using immunostaining for connexin26 and transmission electron microscopy. Electron microscopy of normal and pre-embedded immunostained material showed that there were gap junctions between and among all cells that light microscopy showed to have immunostained appositions. Light microscopy showed immunostaining between and among cell types that electron microscopy showed to be joined by gap junctions. Immunostaining for connexin26 was therefore taken as providing a reasonable approximation of the locations of gap junctions throughout the cochlea and was used to provide an overview of the extent of those locations. Cells interconnected via gap junctions fell into one of two groups. The first group consists of nonsensory epithelial cells and includes interdental cells of the spiral limbus, inner sulcus cells, organ of Corti supporting cells, outer sulcus cells, and cells within the root processes of the spiral ligament. The second group consists of connective tissue cells and includes various fibrocyte types of the spiral limbus and spiral ligament, basal and intermediate cells of the stria vascularis, and mesenchymal cells which line the scala vestibuli. The present work represents a first attempt towards a description of how serial gap junctions among cochlear cells reflect a level of organization of the tissue. The organization described here, together with a great deal of information from previous investigators, suggest that serially arranged gap junctions of both epithelial and connective tissue cells serve as the structural basis for recycling endolymphatic potassium ions that pass through the sensory cells during the transduction process.

Immunocytochemical Traits of Type IV Fibrocytes and Their Possible Relations to Cochlear Function and Pathology

“…Sparse staining for connexin 26 for cells in this region was also noted in the initial report on its localization in rat (Kikuchi et al 1995). The sparse staining for gap junction proteins in type IV and III fibrocytes could hardly be otherwise because electron microscopy observations show that these cell types seldom contact one another or other cell types (personal unpublished observations).…”

“…Among fibrocyte types, variable staining for transport-related enzymes left ambiguities regarding the identity and properties of type IV fibrocytes (Spicer and Schulte 1991). The report that all fibrocytes within the spiral ligament of rats are connected by gap junctions (Kikuchi et al 1995), together with a variety of evidence from numerous laboratories, led to the realization that fibrocytes play key roles in K + ion uptake from perilymph and recycling of those ions back to endolymph. The reported gap junctional connectivity of all fibrocytes, together with imprecise information regarding cytochemical specializations of various cell types, has led to a common assumption that all fibrocytes within the ligament participate in K + ion recycling (e.g., Wang et al 2002;Qu et al 2007).…”

Ageing and hearing loss

“…Several possible routes for K + recycling have been proposed, including a lateral recycling pathway, whereby K + is reabsorbed through a K/Cl co-transporter in the supporting cells (Tectal and Dieter's cells) of the organ of Corti. K + may then move down its electrochemical gradient, passing between cells through gap junctions until reaching Type I spiral ligament fibrocytes (SLFs) and back to the stria vascularis [13], and also recycling routes through the perilymph to the spiral ligament, either above or below the endolymph compartment, and hence to the stria [14]. Another route is a medial recycling pathway through which excess K + is returned to the scala media through medial supporting cells, spiral limbus fibrocytes and interdental cells, all coupled by gap junctions, with transport into endolymph by Na-K-ATPase pumps present in the interdental cells [13][14][15].…”

The Membrane Properties of Cochlear Root Cells are Consistent with Roles in Potassium Recirculation and Spatial Buffering

“…Experimental evidence from in vivo studies has suggested that K + in endolymph is sourced from perilymph, rather than from the blood supply (Wada et al 1979). This observation led to a hypothesis of a recirculation of K + through the cochlear tissues (Hibino and Kurachi 2006;Kikuchi et al 2000;Kikuchi et al 1995;Wangemann 2006;, which is summarized in Figure 1.…”