Biophysical Modeling of Endolymph Transport During Inner Ear Development

Abstract

The goal of this work was to investigate the biophysical mechanisms underlying fluid transport across a single-cell epithelium in the context of inner ear development. The primordial inner ear grows as a fluid-filled spherical shell, called as the otic vesicle, to form complex sub-organs necessary for hearing and balance. The luminal fluid, called endolymph, is tightly regulated in terms of its composition and volume throughout development and whose dysregulation leads to hearing and balance disorders. Previous work showed that fluid transport creates hydrostatic pressure in the luminal cavity that feeds back to inhibit fluid transport and regulate vesicle growth rates. As a first step to linking how pressure affects fluid transport mechanisms, this study focused on the origin of the fluid, the path taken, and the biophysical mechanism underlying its transport into the luminal cavity. In order to distinguish whether intracellular fluid or fluid external to the inner ear (perilymph) contributes to transport, quantitative imaging techniques were used with small fluorescent tracer dyes. 3D+time visualizations of localization patterns provide direct evidence of the path of endolymph. The tracer dye highlighted a paracellular pathway across basal junctions of cells, lateral spaces between adjacent cells, and across apical junctions. To rule out the possibility of different paths taken by dye and fluid, several mathematical models of dye transport were evaluated by accounting for geometry and growth dynamics of the vesicle. Data from experimental perturbations were used to validate model predictions to prove that tracer dye movement is advectively coupled to fluid movement.