The Presence of Megamitochondria in the Ellipsoid of Photoreceptor Inner Segment of the Zebrafish Retina

Abstract

Although the megamitochondria (MM) were localized in various pathological conditions, normal retina of some mammalian species was reported to include MM for various physiological roles. However, it was not clearly confirmed whether the MM is present in the retina of lower vertebrate as well. In this study, we tried to show the presence of the MM in the zebrafish retina using electron microscopic technique. In all the photoreceptors including rods, cones and double cones of the zebrafish retina, MM were observed in the ellipsoid of inner segment. In the photoreceptor epllipsoid of the zebrafish retina, the mitochondria located in the central portion of the ellipsoid had a highly electron-dense matrix, which were accompanied by the mitochondria with electron-lucent matrix in the apical portion of the ellipsoid. The presence of MM was more clearly discernable in the rods, which were localized under the double cones. This finding is somewhat different from those observed in the previous studies because MM were localized in the inner segment of cones, but were not in those of rods in the case of mammalian retina. Although the exact physiological meaning for the presence of MM in some vertebrate species should be further studied, the present study could show that the MM in the ellipsoid of the retinal photoreceptors was not only restricted in some mammalian species.

Characterisation of maturation of photoreceptor cell subtypes during zebrafish retinal development

“…As early as 72 hpf, rods are the first PRCs that can be distinguished from the other PRCs by the transient accumulation of rough ER in the ellipsoid, a region that becomes distinct from the myoid area around the same time when the OSs appear ( Kim et al, 2005 ). The transient accumulation of ER in rods is not only a suitable tool to identify this particular PRC subtype at this stage, but its transient nature can also be used for determining the maturation stage.…”

ES1 is a mitochondrial enlarging factor contributing to form mega-mitochondria in zebrafish cones

“…Accordingly, cones possess higher total mitochondrial mass than rods in many vertebrate species 5 6 . In addition, extremely large mitochondria, mega-mitochondria, exceeding 2 μm in diameter are observed in cones of certain species such as shrew and zebrafish 7 8 9 10 . In a developing retina, a single mega-mitochondrion has been suggested to be formed by enlargement of a single mitochondrion 10 , not by the fusion of small mitochondria.…”

Daily mitochondrial dynamics in cone photoreceptors

“…There are hundreds of mitochondria in zebrafish cones packed tightly between the nucleus and outer segment. Previous studies revealed that these mitochondria are heterogeneous (Kim et al 2005;Tarboush et al 2014;Masuda et al 2016). In this report we quantified the differences in mitochondrial number, size, complexity and 3-D distribution in zebrafish cone subtypes.…”

“…Cones of several species (Knabe et al 1997;Utsumi et al 2020;Tyrrell et al 2019) including zebrafish (Kim et al 2005) contain extremely large megamitochondria. To examine distribution of megamitochondria in cone clusters, we 3-D rendered the three largest mitochondria in each cone from Figure 2D in their respective clusters.…”