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Katie M. Litts, Jeffrey D. Messinger, Kenneth R. Sloan, Richard F. Spaide, Christine A. Curcio; Sources Of Reflectivity In Optical Coherence Tomography (OCT) Of Retina: Candidates Near The Inner-Outer Segment (IS/OS) Junction. Invest. Ophthalmol. Vis. Sci. 2011;52(14):1322. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
To evaluate candidate correlates of the 2nd hyper-reflective outer retinal OCT band, currently attributed to the IS/OS junction; to provide morphometric characteristics of photoreceptor IS mitochondria (Mito) and cilia (Cil), 2 refractive organelles near this location.
Fellow eyes of a 10 yr old male monkey (M. Mulatta) were preserved by immersion in 2% glutaraldehyde and 1% paraformaldehyde and processed for thin-section transmission electron microscopy in the horizontal plane. For 6 cones at each of 2 IS ellipsoid levels (EL-Base, near myoid, and EL-Apex, near OS), we determined cross-sectional diameter (D) of IS, Mito, and Cil, Mito number, fraction of IS cross-section occupied by organelles (A-Mito, A-Cil), and a ratio of electron density (Mito/surrounding cytosol), using digital planimetry, ImageJ, and MATLAB. Locations analyzed included 2 in the rod-free (RF) foveal center and 3 in rod-containing (RC) areas at ~0.4 mm (2°) eccentricity.
Right eye results are reported, as the 2 eyes were similar. Standard deviations were <11% of the means for all measurements. From RF to RC areas, cone ISD doubled (2.18 µm to 4.41 µm), Mito number in individual cones increased 7-fold in the EL-Base (15.62 to 116.00) and 3.5-fold in the EL-Apex, and A-Mito doubled (0.20 to 0.40). Relative to EL-Base, EL-Apex cone IS had 29% fewer Mito (in RC areas only). ISD, Mito number, and A-Mito, varied little for rods (1.61 µm; 9.79 EL-Base Mito and 27.56 EL-Apex Mito; 0.40). Electron density ratio (0.64) was similar in both cones and rods. Mito D was 0.18 µm (rods and RF cones) and 0.24 µm (RC cones). Cil (0.35 µm D) occupied 0.2% and 0.7% of retinal surface area occupied by cone and rod IS Mito, respectively.
Among organelles, Mito have a high refractive index (1.4) . Within the numerically dominant cone population at this eccentricity range, Mito number varies markedly as the IS ellipsoid expands. Mito cross-sectional profile in the retinal image plane is 40-60 greater that that of Cil, which appears too small to account for the intensely reflective 2nd band. These data, combined with improved measures of IS height and Mito packing geometry, can be used to derive models of 2nd band reflectivity. Wilson. J Biomed Opt. 2007;12:014010.
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