Mechanisms of endolymph homeostasis in the inner ear.

Alec N. Salt

 

Abstract

Endolymph is unique both in terms of its composition and in the processes underlying its regulation. In contrast to other extracellular fluids, endolymph has higher potassium and lower sodium content, although the precise composition varies substantially between different parts of the inner ear. Endolymph is also not secreted in volume and in the normal state does not flow from one region to another, as other fluids do. Instead, its ion composition is maintained by local ion transport processes that recycle ions between endolymph and perilymph with no associated water flux yet demonstrated. Under abnormal conditions, when endolymph volume is disturbed, volume flow can play a part in the recovery process. Endolymph volume movements occur passively, depending on the mechanical properties of the membranous walls bounding the endolymphatic space. The saccular membranes appear to be the most compliant, so flow is directed there in a manner similar to that of air moving towards a “weak spot” of a balloon. Dilation of the endolymphatic sinus, a membranous structure between the saccule and the endolymphatic duct, seems to play a part in volume regulation. It is thought that when the sinus is dilated, pressures in the labyrinth will drive a greater volume of endolymph into the endolymphatic sac before the endolymphatic duct is closed by the sinus membrane. The sinus may thus act to regulate endolymph entry into the sac in manner that will correct endolymph volume changes. Endolymphatic hydrops could result from dysfunction at the levels of the endolymphatic sac, endolymphatic duct or endolymphatic sinus.
 
 
 

This study was supported by the National Institutes of Health through the National Institute on Deafness and other Communication Disorders, Grant number DC01368

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