TEM Analysis for the Investigation of Phototropic Changes of Photoreceptor Outer Segments

The Stiles-Crawford effect (SCE) refers to the directional sensitivity of the retina which means that light rays impinging at the center of the eye evoke a greater response of the neural cells composing the retina respect to rays that enter the pupil more peripherally [1]. The lack of this effect in rods photoreceptors can be compensated by the presence of transient retinal phototropism (TRP), which instead it is been proved to be dominant in rods. But what is phototropism, when and where it occurs? TRP occurs mainly in the photoreceptors outer segment (OS) and it is correlated with the oblique light stimulation. Receptors, when stimulated, move towards the direction of the oblique light and this phenomenon perhaps can compensate for some light efficiency loss caused by oblique light stimulation. Although we do not totally understand the mechanism underlying this behavior, early studies have reported disc-shape change of rod photoreceptor, but we still do not have direct evidence regarding the mechanism of discs’ arrangement, that's why this project is focused on the observation of these structures using a Transmission Electron Microscopy (TEM). To discover more about these rod outer segment (ROS) changes electroretinogram (ERG) studies have been done to compare ROS movement with the hyperpolarization of retinal photoreceptors (reflected by the onset of the ERG a-wave). It has been found that ROS shrinkage happens before the onset of a- wave and so before the hyperpolarization [3], that may indicates that disc based stages of phototransduction cascade contribute to the TRP phenomenon. This behavior can also explain the lack of TRP in cones for the simple fact that in these cells discs are linked with the membrane of the OS, while in rods, discs are stacked one over the other without any linkage with the OS membrane. So, because rod’s discs are relatively free to float inside the OS, a light stimulation may entail and provoke a modification of these structures. Perhaps, shifting of the rods towards the central region can be due to the partial stimulation of discs, that can have only one side of the pigment molecules bleached, while the unstimulated area won’t undergo modification in its structure, causing a misalignment between them that may result in the ROS bending [45]. The aim of this study is the investigation of the rod's membranous disc utilizing TEM hoping to find a quantitative explanation of this process and to discover the biophysical mechanism of rod OS dynamics that can not only have a physiological importance, from the moment that reflects a functional response of the retina to a light stimulus, but can also provide a valuable tool for advanced diagnosis of retinal diseases.