Where Do Core Thalamocortical Axons Terminate in Mammalian Neocortex When There Is No Cytoarchitecturally Distinct...

Area of Science:

• Neuroscience
• Comparative Anatomy
• Evolutionary Biology

Background:

• The mammalian cerebral cortex is generally described as a six-layered structure.
• However, certain cortical areas and species exhibit an indistinct or absent layer 4.
• Layer 4 typically receives thalamocortical projections from the core thalamic system.

Purpose of the Study:

• To investigate the termination sites of core thalamocortical projections in species and cortical areas lacking a distinct layer 4.
• To understand the variations in thalamocortical input patterns across different mammalian brains.
• To explore the evolutionary implications of layer 4 variations in cortical organization.

Main Methods:

• Used immunolocalization of vesicular glutamate transporter 2 (VGLUT2) as a marker for core thalamocortical axon terminals.
• Examined 31 eutherian mammal species, focusing on cortical areas with atypical layer 4.
• Analyzed termination patterns in relation to cytoarchitecture.

Main Results:

• Found variations in layer 4 presence and thalamocortical projection patterns across species.
• In shrews/microchiropterans, projections targeted layers 1, 4, and inner 3.
• In primate primary motor cortex, projections terminated throughout layer 3, with no distinct layer 4.
• In elephants, cetaceans, and hippos, projections targeted inner and outer layer 3, lacking a distinct layer 4.

Conclusions:

• Cortical layer 4 and its associated thalamocortical projections show significant variation across mammals.
• These variations correlate with differences in cortical processing and perception.
• The indistinct or absent layer 4 represents an evolutionary trajectory in cortical organization.