Area 4 has layer IV in adult primates

Area of Science:

• Neuroscience
• Comparative Anatomy

Background:

• The presence of layer IV in the primary motor cortex (area 4) is debated, with historical accounts differing on its existence in adult primates.
• Area 4 is often termed 'agranular,' a classification shared with phylogenetically older limbic cortices, causing confusion in comparative neuroanatomy.

Purpose of the Study:

• To resolve the debate regarding the existence and characteristics of layer IV in the primary motor cortex of adult rhesus monkeys.
• To clarify the laminar structure of area 4 and its implications for understanding cortical evolution and organization.

Main Methods:

• Utilized the neural marker SMI-32, which selectively labels neurons in layers III and V but not layer IV.
• Analyzed cellular morphology to differentiate layer IV interneurons from projection neurons in adjacent layers.

Main Results:

• SMI-32 staining revealed a distinct unlabeled cortical stripe in area 4, corresponding to layer IV.
• This layer IV contained small interneurons, similar to those found in granular cortical areas like area 46.
• Cellular criteria confirmed these were distinct from projection neurons in surrounding layers.

Conclusions:

• The primary motor cortex (area 4) possesses a discernible layer IV, populated by specific interneurons.
• The term 'agranular' is misleading for area 4, as its laminar structure differs significantly from ancient limbic cortices.
• Understanding layer IV in area 4 is crucial for grasping the systematic variation in cortical architecture, connections, and evolution.