Multiple Invagination Patterns and Synaptic Efficacy in Primate and Mouse Rod Synaptic Terminals

Area of Science:

• Neuroscience
• Comparative Anatomy
• Ophthalmology

Background:

• Retinal image scaling varies significantly between species due to eye size differences.
• Rod spherules (RS) are critical synaptic structures in the retina, forming triads with rod bipolar cells (BCs) and horizontal cells (HCs).

Purpose of the Study:

• To investigate how retinal image scaling differences correlate with the structural configuration of synaptic wiring in the rod spherule (RS).
• To compare the synaptic architecture of macaque and mouse rod spherules.

Main Methods:

• Serial section transmission electron microscopy was used to examine rod bipolar cell dendrites and horizontal cell axonal processes within the rod spherule.
• Morphological parameters of synaptic structures were quantified and compared between macaque and mouse retinas.

Main Results:

• Macaque RS exhibited a higher number and greater branching of invaginating dendrites from BCs and HCs compared to mouse RS.
• Synaptic parameters such as arciform density length and ribbon length were significantly larger in macaques than in mice (1.5–1.8 ratio).
• Macaque RS displayed 10 distinct invagination patterns, while mouse RS showed only two.

Conclusions:

• Primate RS offers a more extensive BC-RS interface due to increased neurite branching and longer arciform densities.
• The larger synaptic contact area in primate RS may enhance visual signal transfer efficiency.
• Structural adaptations in RS wiring are linked to species-specific retinal scaling.