Differentiation of cyst-forming stria vascularis tissues in vitro

The marginal cells of the stria vascularis possess distinctive morphologica l characteristics associated with their role in endolymph production. Inter estingly, when stria-derived epithelial cells are grown in association with the underlying mesenchyme, the final differentiation of these cell types d oes not occur. Beyond the rudimentary polarity that is established, similar to that shown in epithelial monolayers, cells in culture bear only a sligh t resemblance to their marginal cell counterparts in vivo. The ultrastructu ral features that typify these epithelia, extensive cytoplasmic invaginatio ns, with an abundance of mitochondria, and darkly stained cytoplasm, are no t evident under standard culture conditions. In order to determine whether fluid transport, a key function of the stria vascularis, has an effect on t he ultrastructural morphology, we examined de novo stria vascularis tissues that formed a fluid-filled cyst in vitro. We found that only cells associa ted with the luminal structure demonstrated dark cytoplasmic staining and a mplification of the basolateral membrane of the marginal cells. Additionall y, other epithelial features, such as mitochondria-rich and microvilli-rich cells, were observed in cyst-forming tissues. The enhancement of the margi nal cell specializations was not as robust as that observed in vivo; howeve r, they were clearly more extensive when compared to cells in the same cult ure that were not associated with a fluid-filled lumen. Thus it appears tha t fluid transport may be necessary to maximize differentiation of stria vas cularis tissues in vitro. (C) 1999 Published by Elsevier Science B.V. All r ights reserved.