Abstract
Purpose :
Photoreceptors are the most studied cells in the living human retina using adaptive optics (AO) imaging systems. Yet major components of the photoreceptor cell, particularly its soma and axon, remain elusive to imaging because they are highly transparent and weakly reflective. They compose the outer nuclear layer (ONL), which is the dimmest band in clinical OCT images and is more than 300× dimmer than the brightest band. Here, we show that AO-OCT 3D imaging can reveal substantial cellular-level information about the ONL.
Methods :
Two healthy subjects (24 y/o female and 31 y/o male) were imaged at 2.7° and 3.7° respectively temporal to the fovea using the Indiana AO-OCT system, which has a lateral and axial resolution of 2.4 µm and 4.7 µm in tissue. We acquired 160 5-s AO-OCT videos for each retinal location with system focus at the ONL layer. The volume acquisition rate was 10 Hz per video for a 0.8°×1° field of view and 8000 volumes were acquired in a 2-hour imaging session. Volumes were registered in three dimensions with subcellular accuracy and averaged to improve image contrast-to-noise ratio. We then calculated cross-correlations to test for associations between structures in the ONL and the underlying cone inner and outer segments. Size and reflectance of bright punctate reflections were measured with custom software.
Results :
AO-OCT B-scan images at 2.7° show comb-like strands arcing temporally and extending 44 µm into the ONL from the external limiting membrane (ELM). These strands contain punctate soma-like reflections, some forming stacks of five, resembling photoreceptor soma stacks seen at this eccentricity in histology images. AO-OCT en face images show at least 4 distinct patterns at different depths from posterior to anterior ONL: mosaic-, mesh-, striation-, and diffuseness-like. The mosaic pattern was observed immediately above the ELM and correlated with cone inner segments (r=0.51) and outer segments (r=0.42), thus further supporting a cone soma origin. Interestingly, 14 bright punctate reflections were observed at seemingly random locations in the comb-like strands. Their reflections were 3.5× brighter than those of cone somas and their diameters were 30% larger than those of cone somas (~5.2 µm). Similar findings were obtained in both subjects.
Conclusions :
AO-OCT 3D imaging reveals photoreceptor somas and other cellular structures in the ONL of living human eye.
This abstract was presented at the 2024 ARVO Annual Meeting, held in Seattle, WA, May 5-9, 2024.