What actually causes hearing loss in humans? And what are the best therapeutic approaches to this problem? Modern medicine hasn't yet been able to provide doctors with the right answers in many cases, because there has been no way to observe the tissue of the inner ear, without destroying it.
This situation was the starting point for a team of scientists from EPFL's School of Engineering (STI) and Harvard Medical School. They have been presented recently at the Bertarelli Symposium at EPFL, and will also appear in the online and print version of the Journal Of Biomedical Optics.
Observing without destroying
The team's new optical method is groundbreaking in that it provides extremely clear images of inner ear tissue without any need for fluorescent labeling of the cells with antibiotics, proteins and other fluorescent markers that are usually used to "color" the targeted cells. This represents an enormous advantage compared with the traditional microscopic methods considered in recent years, which require fluorochrome markers and are therefore impossible to use in clinical practice.
"The markers irreversibly damage the tissue, which skews the analyses," said Xin Yang, co-author of the article and a doctoral assistant in Demetri Psaltis's Optics Laboratory (LO) at EPFL.
As for Magnetic Resonance Imaging (MRI), its resolution is inadequate for observing deep tissue cells.
"MRIs can't get beyond 40 microns (a micron equals one thousandth of a millimeter) and the cells we're looking at are on the order of two microns," said Yang.
To overcome the various obstacles in their path, the team takes advantage of the autofluorescence of the cells in the cochlea. Autofluorescence refers to a phenomenon where certain cells tend to remit light after absorbing it. This phenomenon makes it possible to do without external fluorescent markers. The principle is simple: a laser is aimed at a specific target, causing the absorption of the photons by the cells' molecules. This excites the electrons, and a photon is emitted in return. These photons are then analyzed, providing a high quality image.
"To achieve better penetration into the tissue and a higher resolution 3D image, we use a method called two-photon microscopy," said Yang.
A new method for endoscopic exams?
To test their approach experimentally the team used two populations of mice. One of the two groups was exposed to sounds that caused permanent damage to the inner ears, while the second group experienced a normal environment. The mice in both groups were then put down and their cochleas were removed. The scientists used their laser method and compared the results they obtained from each group.
"Among subjects exposed to the sounds, we observed irregularities in the way the cells were aligned and even some missing cells," said Yang.