Purpose: Comparative studies of structure related to function offer a promising means of understanding the significance of differences in cytoarchitecture. Mitochondrial crista structure is linked tightly to mitochondrial function. Non-foveal cone photoreceptors of primates contain considerably more inner segment mitochondria and have higher oxidative enzyme activity than do rods. In addition, it is suggested that light-adapted cones have a higher aerobic ATP demand than light-adapted rods. Therefore, we investigated the oxidative metabolism and three-dimensional membrane architecture of mouse rod and cone inner segment mitochondria.
Methods: We determined the number, size, cytochrome c oxidase (CO) reactivity, and membrane architecture of rod and middle wavelength-sensitive (M) cone inner segment mitochondria from 21 day old light-adapted C57BL/6 mice using conventional electron microscopy and the three-dimensional approach of single- and double-tilt electron microscope tomography. Fourteen different measurements of mitochondrial substructures were analyzed. Photoreceptor oxygen consumption was determined in dark- and light-adapted retinas.
Results: Rod and cone mitochondria displayed an orthodox conformation. Cone inner segments, compared to rods, contained 2-fold more mitochondria and were more CO reactive. Rod and cone mitochondria had similar outer-inner membrane width, contact site width, diameter and density, crista width, number of cristae/volume, number of cristae segments/volume, and fraction of cristae with multiple segments. In contrast, cone mitochondria had narrower crista junctions, greater cristae connectivity, and approximately 3-fold more cristae membrane surface area compared to rods. The increased cristae membrane surface area in cones was accomplished by connecting more cristae segments together, rather than by creating more cristae.
Conclusions: These results demonstrate that middle wavelength (M) cones have a different bioenergetic signature than do rods and suggest that the aerobic ATP demand and production is greater in light-adapted cones than in light-adapted rods. Cones utilize two complimentary strategies to increase their aerobic ATP production: increase the number of mitochondria and increase the cristae surface membrane area. The greater ATP generation by cones may also provide increased protection against metabolic insults and apoptosis compared to rods.