Structural diversity of mitochondria and quantification of synaptic vesicles from different neuron types in the simplex retina of little skate

Abstract

Purpose : The elasmobranch fish Leucoraja erinacea (little skate) has a pure-rod retina capable of functioning across scotopic and photopic lighting conditions. We know little about the structure and diversity of organelles in skate retinal neurons. Quantification and composition of different synaptic vesicle populations in skate rods is also lacking. Here, we describe the structural diversity of mitochondria in skate photoreceptor, bipolar, and horizontal cells. Further, we perform detailed quantifications of mitochondria and synaptic vesicles in rods and determine the proportion of ribbon-docked synaptic vesicles in rod terminals in order to elucidate cell characteristics associated with functional plasticity in skate retina.

Methods : SB-3DEM imaging was performed on individual retinal samples of the little skate. SB-3DEM datasets analyzed here are from several regions of interest (ROIs) in the OPL and from several full cross-section regions of the skate retina. The ROI datasets had XYZ dimensions that varied between ~28-37μm in X, 28-37μm in Y and 12-21μm in Z. Voxel size for ROI datasets varied between 4.2-4.5nm x 4.2-4.5nm x 70-75nm in XYZ, respectively. The full cross-section datasets dimensions in XYZ varied between ~57-88μm (X) x 304-315μm (Y) x 22-127 μm (Z). with a voxel size varying between 7-10nm x 7-10nm x 65-75nm. 3D reconstructions and measurements were done with Reconstruct and Amira3D software.

Results : There are ~160 mitochondria/rod and individual mitochondrial reconstructions reveal significant variation in surface area, volume and morphology between mitochondria. Subpopulations of mitochondria in skate rods possess either rod-like or cone-like characteristics. Mitochondria in putative bipolar and horizontal cell processes also vary widely is surface area, volume and morphology within individual cells. Rods have on average ~3.5x10⁴ free synaptic vesicles in their terminals, and ~5x10² ribbon-docked synaptic vesicles per ribbon. No clear association is yet evident between ribbon surface area and number of docked vesicles.

Conclusions : In previous work we have shown that skate retinal neurons possess a number of hybrid characteristics. This also seems to extend to organelles within those cells. We propose that adaptations on a number of different cellular and sub-cellular levels allow the skate retina to function across illumination ranges.

This abstract was presented at the 2023 ARVO Annual Meeting, held in New Orleans, LA, April 23-27, 2023.