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R. S. Jonnal, J. R. Besecker, J. C. Derby, O. P. Kocaoglu, W. Gao, Q. Wang, D. T. Miller; Imaging Outer Segment Renewal in Living Human Cone Photoreceptors. Invest. Ophthalmol. Vis. Sci. 2010;51(13):2933. doi: https://doi.org/.
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In vertebrate photoreceptors, the extreme optical and metabolic demands of phototransduction necessitate continual renewal of the outer segment (OS). Renewal has been directly observed in post-mortem rods using autoradiography , and indirectly observed in post-mortem cones using electron microscopy , which allows inference of renewal dynamics from the size and distribution of phagosomes in the underlying RPE cells. Both methods require dissection of the retina. The purpose of this study is to observe and quantify outer segment renewal, noninvasively, in living human cones.
We developed a flood-illumination retina camera with adaptive optics, which permits resolution of cones in the living eye. Subjects’ retinas were illuminated with a near-infrared, temporally coherent source, which transforms the OS into a "biological interferometer" , causing changes in the cone's OS length to manifest as changes in its reflectance. We imaged the retinas of three healthy human subjects, every fifteen minutes, over courses of 5 and 24 hours. Images were registered to correct for eye motion, allowing the reflectance of individual cones to be tracked over time.
In all trials, the reflectance of nearly all cones oscillated visibly with periods between 2.5 and 3 hours. Power spectra of cone reflectance series revealed peak frequencies between 0.32 and 0.4 cyc/hr. We determined that these oscillations are due to elongation of the cone outer segment, with rates ranging from 93 to 113 nm/hour (2.2 to 2.7 µm/day). We found that renewal occurred consistently over 24 hours and we were able to monitor small but significant changes (17±3 nm/hr) in renewal rate over the course of the day.
Human cone renewal can be measured in vivo using a novel method, whose sensitivity to changes in OS length is 20 times better than the axial resolution of ultra-high resolution optical coherence tomography , the best existing method for depth imaging of the living retina.
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