Atlas of Human Retinal Pigment Epithelium Organelles Significant for Clinical Imaging

PURPOSE. To quantify organelles impacting imaging in the cell body and intact apical processes of human retinal pigment epithelium (RPE), including melanosomes, lipofuscin-melanolipofuscin (LM), mitochondria, and nuclei. METHODS. A normal perifovea of a 21-year-old white male was preserved after rapid organ recovery. An aligned image stack was generated using serial block-face scanning electron microscopy and was annotated by expert readers (TrakEM, ImageJ). Acquired measures included cell body and nuclear volume (n = 17); organelle count in apical processes (n = 17) and cell bodies (n = 8); distance of cell body organelles along a normalized apical-basal axis (n = 8); and dimensions of organelle-bounding boxes in apical processes in selected subsamples of cell bodies and apical processes. RESULTS. In 2661 sections through 17 cells, apical processes contained 65 +/- 24 melanosomes in mononucleate (n = 15) and 131 +/- 28 in binucleate cells (n = 2). Cell bodies contained 681 +/- 153 LM and 734 +/- 170 mitochondria. LM was excluded from the basal quartile, and mitochondria from the apical quartile. Lengths of melanosomes, LM, and mitochondria, respectively were 2305 +/- 528, 1320 +/- 574, and 1195 +/- 294 nm. The ratio of cell body to nucleus volume was 4.6 +/- 0.4. LM and mitochondria covered 75% and 63%, respectively, of the retinal imaging plane. CONCLUSIONS. Among RPE signal sources for optical coherence tomography, LM and mitochondria are the most numerous reflective cell body organelles. These and our published data show that most melanosomes are in apical processes. Overlapping LM and previously mitochondria cushions may support multiple reflective bands in cell bodies. This atlas of subcellular reflectivity sources can inform development of advanced optical coherence tomography technologies.