Morphology of basal optic tract terminals in the turtle, Pseudemys scripta elegans

Area of Science:

• Neuroscience
• Cell Biology
• Visual System Research

Background:

• Retinal ganglion cells (RGCs) transmit visual information from the retina to various brain targets.
• The basal optic nucleus (BON) is a key structure in the visual pathway, particularly for processing direction-specific information.
• Understanding the axonal morphology of RGC projections to the BON is crucial for deciphering visual processing mechanisms.

Purpose of the Study:

• To investigate the detailed morphologies of axon terminals from RGCs projecting to the BON via the basal optic tract (BOT) in the red-eared turtle.
• To characterize the distribution and branching patterns of these axons within the BON.

Main Methods:

• In vitro visualization of the BOT on the brainstem surface.
• Extracellular injection of biotinylated dextran amine or intracellular injection of neurobiotin into physiologically identified RGC axons.
• Para-sagittal brain sectioning, biotin, and Nissl staining.
• Computer-assisted 3D reconstruction and analysis of axonal morphology.

Main Results:

• Axon distribution within the BON varied, with more axons near the rostral border than the caudal border.
• Thick axons (2-4 µm) were located ventrally, while thin axons (<2 µm) were found throughout the nucleus.
• Intracellularly filled axons showed thick ventral trunks with extensive dorsal and oblique branching, forming dense clusters of varicosities within small volumes (<150 µm³).
• Some thin axons (<1 µm) passed through the BON, suggesting a potential role as a relay nucleus.

Conclusions:

• RGC axons exhibit distinct morphological patterns within the BON, with differential distribution based on thickness.
• The complex branching and clustering of thin axon terminals suggest specialized synaptic connections within the BON.
• These findings provide insights into the structural basis of direction selectivity in the turtle visual system.