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.
What is your main research question?
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.
What did you demonstrate in your recent publication?
Through our three-dimensional electron microscopic dataset, we were able to identify previously unknown connectivity between different photoreceptor cell types including both members of the double cones, which suggests numerous interactions between rods and cones already at this early stage of visual processing. Additionally, we provided a bipolar cell classification not only based on morphological criteria, such as stratification pattern in the inner plexiform layer or the soma position in the inner nuclear layer, but also based on the connectivity to photoreceptor cell types. Surprisingly, most of the identified bipolar cells got at least parts of their input from double cones which indicates that double cones might play many different roles in visual processing.
How did scanning electron microscopy (SEM) enable your research?
We used the 91-parallel-beam MultiSEM 506 microscope to record our serial sections from the chicken retina. This microscope enabled us to record large amounts of data in a reasonably short amount of time through which we could investigate a larger retinal area than it would have been possible with conventional scanning electron microscopy in the same time period.
What do you plan to work on next?
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.