Inside the inner ear, there is a structure called the endolymphatic sac. It is a tiny feature encased in dense bone, so although it has been known to science for centuries, it has been very difficult to study. While scientists had their suspicions about its true function, only now have they finally confirmed it.
The endolymphatic sac is a pressure valve for the liquid in the inner ear. It is made of a thin barrier of cells that expand and open, thus regulating the pressure. As reported in the Journal eLife, the inner ear is crucial to our sense of balance and hearing, so the role of the endolymphatic sac cannot be understated.
While studying the inner ear of zebrafish, an animal model commonly used in science, the research team witnessed a peculiar behavior. The endolymphatic sac in these small fish were inflating and deflating like clockwork, so they set out to investigate what was happening.
"Scientists have known about the existence of the endolymphatic sac for maybe 300 years, but it wasn't understood exactly what it does," co-author Professor Sean Megason, from Harvard Medical School, said in a statement. "It's even often missing in models or textbook cartoons of the inner ear. We didn't set out to study it, but we became interested once we saw its striking behavior."
Finding the explanation has not been easy. The initial analysis used mutant zebrafish with larger endolymphatic sacs than normal. The team witnessed that the larger ones were not equally good at deflating, which led them to the suspicion that the answer might be at a cellular level – and they were right.
The team used high-resolution electron micrographs of the inner ear. They discovered that the cells in the sac are organized in flap-like membrane projections that stretch out from the cells. When the flaps overlap, they form a barrier. When it expands, they are pulled apart, letting the fluid flow out of the sac and relieving pressure.
"Every once in a while, you hear about a house being destroyed by a water heater because its pressure release valve was defective," Swinburne added. "It's important to have these safety control systems in our organs as well."
"This study was definitely a case of seeing is believing," Megason said. "It was very important to have cutting-edge microscopy on many different fronts. Each of these different microscope techniques gave us a different piece of the puzzle and when put together, we get the whole picture."