Sharp Vision – Understanding Early Visual Processing in the Avian Retina Using Scanning Electron Microscopy

In the animal kingdom, birds have the largest eyes relative to their size. Since good eyesight is essential for safe flight, vision is one of the most important senses for birds. In contrast to humans, birds have four types of spectral cone photoreceptors enabling vision under ultraviolet light.

The connectivity of the cells involved in the visual process is largely unknown. Researchers from the working group Animal Navigation at the Carl von Ossietzky University of Oldenburg together with the  Center of Advanced European Studies and Research (caesar)  in Bonn, Germany have published  a study  which provides an important foundation for understanding early visual processing in the avian retina.

Using the  ZEISS MultiSEM 506  scanning electron microscope, Dr. Anja Günther and her colleagues created the first three-dimensional dataset of a bird retina. This dataset identifies previously unknown connections between photoreceptors and provides a classification of chicken bipolar cells based on their anatomy and photoreceptor contacts.

We spoke with Dr. Günther about her work, the recent publication, and the benefits of using scanning electron microscopy (SEM) for her research.

In addition to four different single cones, birds have double cones which are common in most of the animal kingdom, but the function in birds is still debated. Besides achromatic function such as movement detection or fine pattern recognition, double cones in birds were recently also suggested to be involved in light-dependent radical-pair based magnetoreception which is important for orientation and navigation in birds. Since surprisingly little is known about the ultrastructure of these retinal cells and even less is known about the connectivity and thus potential signal transduction pathways, we wanted to investigate the detailed anatomy of double cones and their connection to other photoreceptor cells and bipolar cells in the chicken retina.

Since chicken are non-migratory birds, we want to study the retina of a long-distance night migratory bird, such as the European robin. There we want to analyze the double cone circuit and compare it to the identified connections in the chicken. In addition, we are interested in the foveal circuitry of the robin retina. In contrast to chickens, robins are birds with high visual acuity and we wonder whether they use a comparable ‘midget system’ (a 1:1 connection from cones to bipolar cells to ganglion cells) as primates do. For the chicken, we want to focus next on the connectivity of the horizontal cells in the outer retina and the ganglion cells in the inner retina.