Lifeless Cells Ensure Sharp Vision

It was previously known that the lens of the eye largely consists of lifeless cells.  In these cells, the cell nuclei and other structures have been degraded so that all that remains is a solution of proteins.  This clear fluid enables light to pass through the lens to hit the retina and create an image of what is being seen.  This is how the eye functions in humans and other mammals as well.

Now scientists at Lund University have discovered another fascinating dimension of the eye’s lens.  They have shown that the lens in the blue acara, a common aquarium fish, activates some of its lifeless cells every morning and every evening in order to change the concentration of protein in the cell fluid.  The change is hardly measurable, but its effect is of major importance.

“With this strategy, the fish get better color vision during the day and can see better in the dark at night,” says Marcus Schartau, a doctoral candidate in Professor Ronald Kröger’s research team at the Department of Cell and Organism Biology at Lund University.

It is the amount of protein in the lifeless cells that makes the light refract in the right way.  What happens in the morning in the blue acara eye is that the lens adapts the protein concentration so that the lens can focus light of various wave lengths (colors) at one and the same point.  The fish can then see sharp color images.  This is called making the lens multifocal.

In the evening the protein concentration is restored to the same level as before the morning change.  The lens can then only focus a single wavelength on the retina.  The eye thereby loses its ability to create sharp color images, but instead utilizes the wavelengths that are most important for night vision.  This is referred to as making the lens monofocal.

This strategy, switching between the two lens types every day, is something humans lack.  Our monofocal lens is simpler in construction, but thanks to our greater depth of focus, we can still see different colors in daylight.