Here, we present the first in vivo
images of the photoreceptor mosaic of a rod monochromat. At four different retinal locations along the temporal meridian, images from the monochromat revealed a severely disrupted photoreceptor mosaic. Moreover, the size and density of the visible cells were typical for rod, not cone, photoreceptors. These results suggest that, at least in this achromat, the retina is largely devoid of healthy cone photoreceptors. The intermittent gaps in the mosaic could be the result of either absence of cone development or cone degeneration. It seems likely, given the available histological data (and the small patches of retina imaged here), that there could be residual cone structure elsewhere in this patient’s retina. In the normal retina, cones are easily imaged with adaptive optics as a result of their strong waveguide nature (Roorda & Williams, 2002
). Thus, if cones were
present at the locations we imaged, they are damaged to the point that they are not functioning (given the ERG data in this patient) and not visible in the retinal images (likely due to impaired waveguiding—i.e., lack of an outer segment).
While unlikely, an alternate interpretation of our data is of course that the photoreceptors in these images are morphologically and topographically abnormal cones. Such cones could not be functioning, given the patient’s phenotype as a complete achromat (non-detectable cone ERG, no color discrimination, photophobia, nystagmus, and his homozygous CNGB3 null mutation). As mentioned above, an interpretation of the current data might suggest that achromats with different causative mutations can have varying levels of residual cone function. We suspect that imaging such individuals with AO ophthalmoscopy will reveal significant differences in their photoreceptor mosaics that correlate with the degree of residual cone function. Furthermore, examination of genetically classified patients with high-resolution retinal imaging is likely to provide valuable insight into pathogenesis of the disease as well as identify those individuals who might be most receptive to newly developed gene therapies (Alexander et al., 2007
; Komaromy et al., 2007
If as we believe these photoreceptors are rods, one wonders why they are not as easily visualized in the normal retina under similar imaging conditions. To our knowledge, there is only one report of in vivo
images of rods from the normal human retina. In high-resolution retinal images taken 15-20° from fixation in the normal retina, Choi and colleagues observed a continuous cone mosaic with numerous rods intermingled throughout the image (Choi et al., 2004
). The difficulty in imaging them, and the relative sparseness at retinal locations where they should outnumber the cones nearly 10-fold, is consistent with previous data that rods are less effective waveguides than cones (Alpern, Ching, & Kithara, 1983
; van Loo & Enoch, 1975
). It may be that this is only a limiting factor when trying to image the normal
retina, where the light from the cones may be reducing the contrast of the interleaved rods. Another possibility is that while in the normal retina cones are thought to have more mitochondria than rods (Hoang, Linsenmeier, Chung, & Curcio, 2002
), the rods in the monochromat retina might be more active due to a change in lifestyle and as a result, the rods would have an increased metabolic demand. This might result in an increase in the relative difference in refractive index, rendering them more efficient waveguides and easier to visualize with this particular imaging technique. Alternatively, the absence of cone photoreceptors in this retina may have allowed the remaining rods to swell, as has been reported to occur when rods are lost in normal aging (Curcio, 2001
). Indeed, while the photoreceptors imaged are more consistent with rod, rather than cone, diameters, they are slightly larger than values proposed in the literature (Samy & Hirsch, 1989
). Even a slight increase in size would greatly improve the ability to resolve them in the AO images, so further work remains in determining what mechanisms are acting to increase rod contrast in this retina.
Finally, the phenotypic variability in genetically identical achromats indicates that there are other mitigating factors that influence the degree of residual cone structure and function in a given rod monochromat will have. This underscores the importance of imaging the retinae of individual subjects rather than making general assumptions about the achromat retina. Moreover, Jacobson et al. (2005)
prophetically stated that “...identifying and then targeting retinal locations with retained photoreceptors will be a prerequisite for successful gene therapy in humans...”. This is worth reiterating, with the cautionary note that the story of cone survival in the mouse and dog models of achromatopsia may not hold for all human achromats. Fortunately, both the therapeutic and diagnostic tools exist for us to proceed with prudence in the quest to restore cone function to congenital rod monochromats.