EVIDENCE FOR A MEDIAL K-CELLS( RECYCLING PATHWAY FROM INNER HAIR)

K+ effluxed from enter hair cells and their nerves is thought to flow laterally to strial marginal cells for recycling into scala media. Obs ervations reported here provide evidence that K+ effluxed from inner h air cells and inner radial nerves travels medially through border cell s, inner sulcus cells (ISCs), limbal fibrocytes and interdental cells (IDCs) for return to endolymph. Morphologic features of ISCs in the me dial route resembled those of Hensen and Claudius cells in the lateral indicating an ion transport role for ISCs like that of Hensen and Cla udius cells. Na,K-ATPase in plasmalemma of IDCs testified to their cap acity to resorb and transport K+ through their known gap junctions. ID Cs were differentiated into three subgroups. The most lateral IDCs for med short and long columns. Long columns contacted the medialmost ISC inferiorly and the undersurface of the tectorial membrane superiorly p roviding thereby a potential transcellular route for K+ transit from I SCs to endolymph. Short columns faced inner sulcus below and tectorial membrane above and accordingly possessed cells with opposite polarity at the bottom and top of the column. Short columns thus appeared situ ated to resorb electrolytes from limbal stroma for release into inner sulcus and beneath tectorial membrane at opposite ends of the column. The central IDCs were positioned for resorbing and transporting K+ eff luxing from the Na,K-ATPase-rich stellate fibrocytes which spread towa rd the IDCs from near the inner sulcus. The most medial IDCs lined cup like invaginations near the attachment of Reissner's membrane and lay apposed to light fibrocytes located between supralimbal fibrocytes and the medial IDCs. Content of Na,K-ATPase and position in the K+ transp ort route likened the limbal stellate fibrocytes to the spiral ligamen t type II fibrocytes and supralimbal fibrocytes to suprastrial fibrocy tes in the lateral wall. From content of creatine kinase and position in the transport path, limbal light fibrocytes appeared analogous to s piral ligament type I fibrocytes. The additional finding that limbal f ibrocytes showed unchanged or upregulated Na,K-ATPase immunoreactivity in aged gerbils with strial atrophy provided further evidence for an independent medial transport route and for the survival of inner hair cells in presbyacusis. (C) 1998 Elsevier Science B.V.