ABSTRACT: Photoreceptors are one of the most energy-consuming cell types within the human body. To meet their high energy demand, photoreceptors possess a mitochondrial cluster in the inner segment of the cell. Interestingly, in several species, the inner segment of cone photoreceptors contains extremely large mitochondria that exceed 2?µm in diameter, called mega-mitochondria. We previously reported that pig retinas also contain mega-mitochondria, however, there are few reports whether mega-mitochondria are present in mammalian photoreceptors. In the present experiment, we analyzed pig, rabbit, and mouse photoreceptors under a scanning electron microscope (SEM), and compared the mitochondrial morphology. Our data showed that all three species present numerous mitochondrial clusters in the ellipsoid zone of photoreceptors, adjacent to the outer segment. In the pig retina, the inner segments of cone and rod photoreceptors were localized in different layers; consequently, we were able to distinguish them easily. Mega-mitochondria were identified only in the inner segment of cone photoreceptors. Also, mitochondria of cone photoreceptors, including mega-mitochondria, were dense cristae and high electron-densities compared to those of rod photoreceptors. In the rabbit retina, cone photoreceptors were existed within the layer of rod photoreceptor outer segment. The rod photoreceptors had a characteristic long outer segment. Cone photoreceptors had a short outer segment, and also had a thick inner segment compared to rod photoreceptors. Most of the mitochondria present in the rod photoreceptor inner segment were long and narrow, whereas mitochondria of cone photoreceptors were fragmented and short. Mega-mitochondria was not detected in rabbit retina. In the mouse retina, most of the photoreceptor cells were rod photoreceptors. Since the shape of the inner segments were very similar, we distinguished cone and rod photoreceptors based on the shape of the outer segments. Some mitochondria of both rod and cone photoreceptors were long and narrow, and there was no significant difference in mitochondrial morphology. Our data showed that mitochondrial morphology in the inner segment of photoreceptors vary among mammalian species. Although mega-mitochondria were present in pig photoreceptors, we could not observe their presence in rabbit nor mouse retinas. To our knowledge, this is a first experiment that perform the wide field observation of rabbit and mouse retina using electron microscopy, and that compare the mitochondrial morphology of photoreceptor cells in pig, rabbit and mouse.
