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Dennis M. Dacey; Discovering visual pathway origins in the center of the human foveola with connectomics. Invest. Ophthalmol. Vis. Sci. 2018;59(9):14.
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© ARVO (1962-2015); The Authors (2016-present)
Presentation Description :
The center of the human foveola is one of the most specialized neural architectures in the central nervous system. Extremely thin and elongated cone photoreceptor outer and inner segments are sequestered and packed into a region, often referred to as the ‘central bouquet’, about 100 µm in diameter in the center of the foveal pit. Here cone density reaches a local peak, affording both the exquisite acuity and color perception that are hallmarks of human vision. Foveolar cones give rise to long axon-like processes that terminate as synaptic pedicles the precise location of which, due to the complex geometry of the foveal pit, remains unclear. Thus the nature of the critical visual pathways that arise from the cones of the foveola are unknown. To address this question directly we utilized volume electron microscopy (EM) and connectomic reconstruction of foveolar cells. We acquired eyes from organ donors at the time of death and maintained retinal tissue in oxygenated culture medium prior to fixation to preserve excellent synaptic ultrastructure. For volume EM we used the automated tape-collecting ultramicrotome (ATUM) to cut ~4000 serial 65 nm horizontal sections through a 2 x 2 mm retinal area encompassing the foveal center of a 21 yr old male donor. Initial analysis of this volume is beginning to reveal new insights into foveal structure. First, reconstructions of ~200 cones at the very center of the foveola (~50 µm or ~10 arcmin diam) showed unexpectedly that the pedicles linked to these cones were localized to the center of the foveal pit embedded in a dense, extended matrix of enlarged Müller cell processes; these pedicles were also unusually large and interconnected by long telodendritic processes. Thus, in one human retina the foveal depression represents a previously unrecognized and anatomically specialized locus for the relatively small number of cones that initiate highest acuity vision. Second, horizontal cells connected to these pedicles form very restricted cone connections suggesting the presence of unique foveolar circuitry used to build postreceptoral receptive field structure. Finally, up to 12 distinct bipolar types receive synaptic output in parallel from single cone pedicles suggesting that further connectomic reconstruction of the inner retina will reveal the full diversity of visual pathways that arise from the foveolar center for the first time.
This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.
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