Neocortical Lamination: Insights from Neuron Types and Evolutionary Precursors

Area of Science:

• Neuroscience
• Evolutionary Biology
• Developmental Neuroscience

Background:

• The mammalian neocortex exhibits a distinct six-layered structure, but its evolutionary origins and functional significance remain unclear.
• Recent research suggests that understanding simpler, ancestral cortical structures may provide insights into neocortical development and function.

Purpose of the Study:

• To explore the hypothesis that the developmental program of the three-layer olfactory cortex was co-opted during evolution to form the six-layer mammalian neocortex.
• To investigate the potential evolutionary link between ancient three-layer cortical structures and the modern six-layer neocortex.

Main Methods:

• Review and synthesis of recent studies on cortical evolution and development.
• Comparative analysis of three-layer cortical structures (olfactory, hippocampal, dorsal cortex) in early amniotes.
• Examination of the proposed co-option of developmental programs and cellular components.

Main Results:

• Three-layer cortex in early amniotes was based on a module involving intratelencephalic (IT) type pyramidal neurons, excitatory/inhibitory inputs, and interneurons.
• The hypothesis suggests that this ancient developmental program was elaborated to form the six-layer neocortex.
• Specific cell types, including IT units (layers 2-6), stellate cells (layer 4), pyramidal tract (PT) cells (layer 5B), and corticothalamic (CT) cells (layer 6), are implicated in this evolutionary transition.

Conclusions:

• The evolutionary trajectory from a three-layer to a six-layer cortex likely involved the repurposing and elaboration of existing developmental programs.
• Understanding the ancestral three-layer cortex provides a framework for deciphering the formation and functional organization of the mammalian neocortex.