Abstract
Purpose :
In duplex retinae, the processing of visual information under scotopic and photopic light conditions is separated between the rod and cone photoreceptor systems. However, the elasmobranch little skate (L. erinacea) has a simplex retina, which contains only rods. This pure-rod retina can perform under scotopic and photopic light conditions with one complement of photoreceptors. We have little knowledge about the anatomical basis of this functional plasticity in skate rods.
Methods :
Eyes from little skate were hemisected and choroid-attached pieces of retina from the tapetal area were obtained. Retinal pieces were embedded in resin blocks and SB-3DEM was performed. The dataset analyzed here was from a region of interest in the OPL of the skate retina with width/height = 34.4μm, section thickness = 0.075μm, and depth = 15μm. Voxel size was 4.5nmX4.5nmX75nm. 3D reconstructions and measurements of rod terminal features were done with Reconstruct and Amira software.
Results :
The outer segments of skate rods display typical “stacked-disk” internal membrane morphology with the outer membrane separated in space from the disc membranes. A very short connecting cilium (CC) is present between the OS and the IS: d=~250nm, length=~400nm. Inner segments have multiple mitochondria and terminals have multiple synaptic ribbons. The majority of terminals have 2-3 synaptic ribbons. On rare occasions, terminals possessed 4 synaptic ribbons. Multiple invaginating processes can be identified (~12) and multiple filopodia of varying length extend from the periphery of each terminal.
Conclusions :
Skate rods have a typical vertebrate rod OS morphology, but display hybrid characteristics in their inner segments and terminals. CCs are much shorter than in typical rods and resemble those of cones. The presence of multiple ribbons as organizing centers for transmitter release is an unusual characteristic for rods, but can be clearly seen in skate rods, suggesting the evolution of a hybrid morphology to accommodate functional plasticity.
This is a 2021 ARVO Annual Meeting abstract.