Abstract
Purpose :
Bruch’s Membrane (BrM) is a specialized vessel wall between the RPE and the choroidal vasculature and is a critical interface between basal RPE and choriocapillaris, referred to herein as the RPE-BM-CC transport complex (RBCTC). This interface participates in the bi-directional transfer of nutrients, fluid and waste between the RPE and choroid. As a part of normal aging, BrM accumulates lipoprotein-like particles (LLP) and other materials that contribute to drusen and other aspects of age-related macular degeneration (AMD). Our purpose was to clarify the distribution of key functionally important RBCTC components by using volume electron microscopic reconstruction.
Methods :
A rapidly preserved foveal sample from a 52-year-old Caucasian male organ donor with no known or discernible ocular pathologies was imaged using serial block face electron microscopy (EM). Manually labelled 2D images were used as ground truth to train a deep learning model that automatically segmented the full volume. Separate models were trained for BrM and CC tasks. Density of LLP was measured in multiple 20 µm3 sampling areas placed throughout the segmentation. CC lumen diameter and endothelial cell (EC) protrusions into the outer collagenous layer were measured manually.
Results :
Among sampled regions, 6% of BrM was filled with LLP, however these were also observed beyond BrM, further external to the CC. LLP are slightly more dense above capillaries than capillary pillars. We found multiple evenly spaced gaps in the apical side of the CC basal lamina through which endothelial cell membranes protrude, suggesting these as part of normal physiology, as previously suggested (PMID 15161823). In many cases these protrusions form cups, with long spacing collagens appearing inside or in close proximity to them. In our sample, gaps had an average diameter of 0.56±0.16 µm (n=20). Average CC lumen diameter was 4.22±0.93 µm (n=25).
Conclusions :
Our study presents a 3D investigation of the RBCTC, reporting an unexpected distribution of LLP and endothelial cell morphology that could functionally interact with collagen networks. Ultrastructural data from this complex may inform clinical imaging and elucidate novel pathomechanisms in potentially blinding disorders like AMD.
This abstract was presented at the 2024 ARVO Annual Meeting, held in Seattle, WA, May 5-9, 2024.