Outer Segments of Retinal Photoreceptors – A Review in the Light of Novel Findings

Abstract

Aims: Recent studies have demonstrated that all molecules of the respiratory chain are present in the photoreceptor outer segment (see Calzia et al., Biol Cell 2013). Furthermore, our group (Roehlecke et al., PLoS One 2013) could show that, after blue light stress of the retina, the outer segments are significant sources of reactive oxygen species (ROS) – even more so than the mitochondria in the inner segment. These two new findings have also important implications for degenerative diseases of the retina. Methodology: In this respect we revisited the literature regarding the photoreceptor reactions after blue light and radical stress. Furthermore, we refer to the common features of mitochondria and outer segments. Results: In the light of the recent findings many unique features of the photoreceptors get understandable: they are characterized by excessive oxygen consumption - even higher than that of other neuronal cells. Photoreceptors possess, in addition to their mitochondria – rich neuronal component an outer segment packed with stacks of membrane discs harbouring the photo pigments, respiratory enzymes and enzymes of the visual cycle. Therapeutic use of red and near infrared light is often explained by ameliorating the mitochondrial function; therefore, we also discuss this topic with respect to photoreceptor outer segments. Conclusion: Revisiting the functional anatomy with the above mentioned new findings is very important also for the interpretation of the neurodegenerative diseases like age related macular disease (AMD).