Scientists and doctors have been studying the human body for thousands of years. While, at this point, we think we have a good grasp on how the human body works, the body is still, in many ways, a source of mystery and intrigue – particularly when it comes to the function of some lesser known body parts and structures.
Although some very well known body parts, such as the appendix, remain mysterious in function, many of these puzzling structures are smaller and generally unknown to the public, despite having been identified by researchers hundreds of years ago.
One of these body parts – the endolymphatic sac – is a small, fluid-filled pouch located near the inner ear that is hard to study in humans because it is encased by extremely dense bone. While the endolymphatic sac has been known to scientists for about 300 years, no one ever knew what it did. In fact, most models and textbooks neglect to include this tiny structure in diagrams of the inner ear because its function was unknown.
Unknown, that is, until Ian Swinburne, a research fellow at Harvard Medical School noticed the tiny structure pulsing during a time-lapse microscopy study of the inner ear of zebrafish. Alongside his postdoctoral advisor, Sean Megason, Swinburne investigated this small organ and have conducted a number of studies to better understand its function.
In their most recent study, done in collaboration with some world leading microscopy laboratories, Swinburne and Megason sought to visualize the endolymphatic sac in action. They pieced together a number of different views of the sac until they managed to come up with a clear model for how it functions.
The answer? The endolymphatic sac is a kind of pressure-relief valve that pulses to open and close and regulate the release of fluid from the inner ear.
In many bodily tissues, cells are so tightly connected that fluid cannot pass between them. In the endolymphatic sac, however, Swinburne and Megason found that cells have small flap-like membrane projections called lamella, which overlap with each other to form a barrier. Within the endolymphatic sac, the cells have small gaps between them through which fluid can flow but that are also covered by the lamella which act as valves and pressure regulators.
As fluid pressure builds, Swinburne and Megason found, the sac inflates and the lamellar barrier starts to separate until it reaches a point where it opens to allow fluid to flow out of the sac and ultimately relieve pressure inside. This capacity is important within the inner ear as all of the structures there are interconnected and filled with a fluid that moves in response to sound waves or head movement.
The movement of this fluid is detected by sensory cells which can convert these inputs into neural signals that the brain can understand. It is important for the inner ear to maintain the pressure and chemical composition of this fluid or a number of disorders, such as Meniere’s disease could occur. Scientists have long suspected that the endolymphatic sac is involved in the pressure regulation of the inner ear, but it wasn’t until Swinborne noticed the structure’s function in a zebrafish embryo that it all became clear.
Although the structure and function of the endolymphatic sac is pretty rare in the biological world, the research team suspects that similar mechanisms could exist in other organs such as the eyes, brain, and kidneys which also have pressurized fluid-filled cavities.
Swinburne and Megason’s work has revealed a very unique biological mechanism for the maintenance of fluid pressure and, thus, it could be incredibly important for the future study and treatment of conditions that involve inner ear pressure issues.
Some of these conditions, such as Meniere’s disease, have symptoms such as vertigo, hearing loss, and tinnitus, which could potentially be effectively treated after more research into the function of the endolymphatic sac. Plus, this information could be useful in treating conditions in other organs, such as the eyes and kidneys, which rely on liquid-filled cavities for their proper function.
Although the endolymphatic sac is small, it certainly has a mighty big presence in our hearing health. New research and findings such as those from this study are exciting news for the world of hearing healthcare which could use this information to further develop treatments for a variety of inner ear conditions.