The Human Foveal Connectome

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The complex relationship of cone photoreceptor cells with retinal circuits, Müller glia, and retinal pigment epithelial (RPE) cells is essential to normal vision. Yet for the cones in the very center of the fovea that mediate peak visual acuity these relationships are poorly characterized. A longstanding barrier to a comprehensive understanding of cellular and subcellular foveal structure is the myriad interactions among a great diversity of cell types embedded and miniaturized within a complex three-dimensional architecture. The broad long-term objective of this new research program is to elucidate foveal microstructure directly by application of new methods of volume electron microscopy (connectomics). We will utilize retinal tissue acquired from an innovative organ donor program that will permit pre-recovery optical coherence tomography (OCT) imaging to assess retinal health status and foveal pit morphology and to guide connectomic reconstruction. Preliminary data from two donor eyes demonstrates feasibility of complete reconstructions of foveal cones and their associated synaptic pathways, Müller cells, and RPE cells. The first reconstructions of cone microcircuits from an adult born preterm indicate that the critical cells and synaptic pathways for foveal vision differ dramatically in structure and localization anticipated from previous work on non-human primates. Therefore in Aim 1 we propose to localize, identify and reconstruct quantitatively the synaptic visual pathways that arise from the central-most foveal cones. We will characterize all of the bipolar and ganglion cell circuits arising from these cones and test the new hypothesis that the dominant “midget” pathway subserving spatial acuity may be highly variable across individuals in both circuitry and pit localization. We will further test the hypothesis that beyond the midget circuit the foveal center gives rise to over twenty distinct but as yet uncharacterized visual pathways. The first reconstructions of Müller cells revealed the intimate wrapping of cone axons and abundance of processes in the plexiform layer and foveal floor. In Aim 2 we propose complete reconstructions of Müller cells to test the hypotheses that the foveal floor contains a novel Müller cell type restricted to inner retina and that morphology of individual Müller cells and their foveal distribution accounts for the macular pigment distribution. The first reconstructions of RPE cells provided new insights on the distribution of organelles important in clinical OCT and autofluorescence imaging. Therefore, in Aim 3 we propose to reconstruct and enumerate organelles in RPE cells in the cone-only fovea and the mixed rod-cone perifovea. We will directly test the hypothesis that RPE organelle content and distribution differs between cone-only fovea and rod-rich perifovea, accounting for the appearance of OCT bands and for topography of autofluorescence signal in clinical imaging. This proposal combines expertise and innovation in neurobiology, pathology, imaging, and connectomics. Outcomes will impact retinal neurobiology, clinical image interpretation, and pathophysiology of macular diseases, especially age-related macular degeneration.