The cellular composition of the marsupial neocortex

Area of Science:

• Neuroscience
• Comparative Anatomy
• Evolutionary Biology

Background:

• The cellular organization of the neocortex varies across mammalian species.
• Understanding neuronal and non-neuronal cell distribution is key to deciphering cortical function.
• Marsupials offer a unique perspective for comparative studies with eutherian mammals.

Purpose of the Study:

• To quantify and compare neuronal and non-neuronal cell populations in the neocortex of the short-tailed opossum (Monodelphis domestica).
• To investigate the distribution of cells within primary sensory areas (S1, A1, V1) and the remaining neocortex.
• To compare cortical organization in this marsupial with findings in other mammals, particularly non-human primates.

Main Methods:

• Dissection of primary somatosensory (S1), auditory (A1), and visual (V1) cortical areas from the short-tailed opossum.
• Application of the isotropic fractionator technique for cell counting.
• Comparative analysis of cell numbers and proportions across cortical regions.

Main Results:

• Neocortical regions (V1, S1, A1, remaining cortex) showed similar total neuron counts despite differing sizes.
• The remaining neocortex had significantly more non-neuronal cells than primary sensory areas.
• Auditory cortex (A1) displayed a higher percentage of neurons compared to the remaining cortex and the overall cortex.
• Short-tailed opossums exhibited substantially lower cellular and neuronal density than eutherian mammals.

Conclusions:

• The neocortex of the short-tailed opossum demonstrates homogeneity in neuron number but heterogeneity in non-neuronal cell distribution and neuronal percentage.
• Cortical organization in this marsupial shares similarities with non-human primates but differs significantly in cell density.
• Low neuronal density in marsupials suggests that high neuronal density is a more recent evolutionary development in eutherian mammals, potentially linked to complex behaviors.