A few years ago, a research fellow in systems biology, noticed something odd while conducting a time-lapse microscopy study of the inner ear of zebrafish. A tiny structure in the inner ear was pulsing like clockwork, inflating and deflating over and over. As the researcher probed further, he discovered that the structure was the endolymphatic sac, a fluid-filled pocket connected to the rest of the inner ear by a long, thin duct. But he not could explain why it was pulsing. Over the course of the next several years, the researchers worked to better understand the function of this mysterious structure. To do so, they had to visualize it in action. Collaborating with some of the world's leading microscopy laboratories, they pieced together different views of the endolymphatic sac until a clear picture emerged. In a study published June 19, the team reported the results of their investigation: the endolymphatic sac acts as a pressure-relief valve and is formed by a thin barrier of cellular projections that opens and closes to regulate the release of fluid from inside the inner ear. Their findings reveal a unique biological mechanism for maintaining fluid pressure and composition and may inform the study and treatment of disorders involving defects in inner ear pressure such as Meniere's disease, a condition marked by vertigo, hearing loss and ringing in the ears. The results could also help researchers study pressure control in other organs such as the eyes and kidneys, which also have liquid-filled cavities.
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