As previous time-lapse light microscopic and histological studies have confirmed the light-induced length decreases in rod OSs, the following aim of this study was to disclose the anatomic source of the rod OS shrinkage on a subcellular level. Our previous studies suggested the conformational change of rod OSs was correlated with the phototransduction cascades on or adjacent to the discs. Therefore, we hypothesized the overall rod OS shrinkage was related to the perturbation of the lamellar structure of the rod discs. To verify the hypothesis, comparative TEM studies were conducted on rod OSs under light- and dark-adapted conditions.
Figures 3A1 to
3A3 show representative overall views of the photoreceptors under TEM in which individual rods can be clearly identified.
Figure 3B shows the well-preserved lamellar structure of the discs in the white rectangle in
Figure 3A3, obtained with high TEM magnification.
Figures 3C1 and
3C2 are representative TEM images of disc stacks in the dark- and light-adapted rod OSs, respectively. To provide a direct impression of how light illumination changed the lamellar structure of the discs, two blue bars, with each bar covering 15 discs, were placed in the images with their bottoms horizontally aligned. The blue bar in the light-adapted rod OS is significantly shorter in length, compared with that in the dark-adapted rod OS, indicating the light adaptation reduced the length of the disc stacks. As shown in
Figure 3D, to demonstrate whether the reduction in length came from the distance between the discs (interdisc distance), the disc itself (intradisc distance) or from both, the inter- and intradisc distances were measured in both dark- and light-adapted samples and statistically compared to illustrate the difference. Further, the inter- and intradisc distances were compared based on their relative locations, that is, tip, middle, or base region (
Fig. 3A1), in the rod OSs. The means and standard deviations of the measurements are summarized in the
Table. As shown in
Figures 3E1 to
3E3, the statistical analysis demonstrated that the light illumination resulted in a significant decrease in the interdisc distances from all three regions and the overall interdisc distance, but barely affected the intradisc distances.