Mitochondrial distribution in the outer plexiform layer of human retina – Does it correlate with reflectivity in OCT?

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Abstract

Mitochondria are organelles essential for cellular metabolism that also contribute reflectivity to optical coherence tomography (OCT) via Mie scattering. To find reflectivity sources, we assessed mitochondrial distribution of outer plexiform layer (OPL) using comprehensive volume electron microscopy. Parafoveal retina of a 21-year-old male donor was subject to serial block-face scanning electron microscopy in 5×5×50 nm³ voxels. A convolutional neural network (U-Net) was used to train and generate a deep learning model from manually annotated mitochondria ground truth (Dragonfly 2020.1, Object Research Systems). Using this model, mitochondrial volume and occupancy were calculated as a function of retinal depth and plotted as a longitudinal profile. Photoreceptor terminals contained clusters of ovoid mitochondria. Dendrites of bipolar neurons exhibited long and slender mitochondria. The OPL mitochondrial distribution had three layers of higher density (photoreceptor terminals, bipolar/ horizontal processes, neuropil at the border of inner nuclear layer) interleaved by two bands of lower mitochondrial occupancy (8.5-13.8%). Mitochondria occupied <1% of the Henle fiber. Automated segmentation yielded exceptional performance vs ground truth (Dice coefficient=0.96). Evidence of multilaminar distribution of mitochondria in the OPL support hypothesis of a subcellular origin of OCT reflectivity bands. Correlative studies using AO-OCT is currently under ongoing investigation.