The death of photoreceptors is a fundamental characteristic of RP. In the present study, we evaluated the retinal morphology by OCT imaging and histology. OCT is a noninvasive imaging approach that is commonly used to monitor the progress of retinal degeneration, and it has been proven to provide accurate and reliable measurements of retinal layers.
4 Our results revealed a significantly reduced ONL at PW6, consistent with a recent study.
6 The ONL thickness further shrank at each time point examined up to PW12. Although the ONL thickness is biased toward the surviving rod photoreceptors, our data suggest that it has a strong correlation to cone survival (
Fig. 4E). There is a report showing a correlation of ONL thickness and cone density in normal eyes and eyes with RP.
53 In the normal human eye, this is in line with the idea that cone survival requires the support of trophic factors produced by rods
54; thus, the degeneration of rods compromised the survival of cone photoreceptors in
Rho knockout mice. Our data suggest the potential of utilizing OCT images in the clinic as indirect measurements/estimations for cone survival. Conversely, the ONL thickness in the WT mice was unchanged throughout the period examined from PW6 to 14. In
Rho−/− mice, cone photoreceptor degeneration is known to follow the loss of dysfunctional rods. Therefore, the gradual drop in quantification of ONL thickness illustrates the loss of both nonfunctional rods and fully functional cone photoreceptors. Thus, to evaluate cone degeneration specifically, we also quantified the number of opsin-expressing cone photoreceptors. Our results on the WT mice are comparable to those of others.
55 In contrast, the opsin+ cone photoreceptors in the
Rho−/− mice were significantly reduced at each time point and indicated a developmental degeneration of cones from 6 to 12 weeks. The data in
Figure 4E represent the number of opsin+ cones per retinal section while we could not detect the thickness of ONL on the same retina by OCT. One possibility is that opsin signal is still present in the inner/outer segment but the cone nuclei were degenerated. Measuring the thickness of inner segment/outer segment (IS/OS) may reflect the thickness of a surviving cone but still not be necessary for the surviving cone. Technically, the OCT images of IS/OS and retinal pigment epithelium could sometimes fuse together, making it difficult to measure the thickness of IS/OS. Our data in
Figure 4E suggest that the detection limit of ONL thickness of
Rho knockout mice by OCT in our protocol is approximately 8 μm.