Systematic, cross-cortex variation in neuron numbers in rodents and primates.

Area of Science:

• Neuroscience
• Comparative Anatomy
• Developmental Neuroscience

Background:

• Previous claims suggest uniformity in neuron numbers per cortical surface area across species and cortical areas in the isocortex.
• Discrepancies in existing studies hinder resolution due to variations in species, techniques, and cortical areas examined.

Purpose of the Study:

• To conduct a stereological assessment of neuron numbers per unit of cortical surface area across the isocortex in rodents and primates.
• To evaluate the contributions of species, cortical areas, and techniques to neuron estimates.
• To resolve conflicting claims regarding neuron number uniformity and variability in the isocortex.

Main Methods:

• Stereological assessment of neuron numbers in layers II-IV and V-VI per unit of cortical surface area.
• Inclusion of rodents (hamster, agouti, paca) and primates (owl monkey, tamarin, capuchin) selected for systematic variation in cortical size.
• Assessment of contributions from species, cortical areas, and techniques (stereology, isotropic fractionator) to neuron estimates.

Main Results:

• A rostro-caudal (RC) gradient in neurons per unit of cortical surface area was observed in primates (1.64-2.13 factor) and less so in rodents (1.15-1.54 factor).
• Neurons in layers II-IV contributed most significantly to this observed variation.
• Species variation and the RC gradient in neuron numbers help account for previously conflicting study results.

Conclusions:

• The rostro-caudal gradient in adult isocortical neuron numbers mirrors developmental gradients in neurogenesis duration.
• Understanding these gradients is crucial for reconciling inter-study variability in isocortical neuron density research.
• Neuron numbers per unit of cortical surface area are not uniform, exhibiting systematic variation across species and cortical areas.