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Gradients in cytoarchitectural landscapes of the isocortex: Diprotodont marsupials in comparison to eutherian mammals

Christine J. Charvet1 | Cheryl D. Stimpson1 | Young Do Kim1 | Mary Ann Raghanti2 | Albert H. Lewandowski3 | Patrick R. Hof4 | Aida Gomez-Robles1 | Fenna M. Krienen1 | Chet C. Sherwood1

1Department of Anthropology and Center for the Advanced Study of Human Paleobiology, The George Washington University, Washington, DC
2Department of Anthropology and School of Biomedical Sciences, Kent State University, Kent, Ohio
3Cleveland Metroparks Zoo, Cleveland, Ohio
4Fishberg Department of Neuroscience and Friedman Brain Institute, Icahn School of Medicine at Mount Sinai, Mount Sinai, New York, New York

ABSTRACT
Although it has been claimed that marsupials possess a lower density of isocortical neurons compared with other mammals, little is known about cross-cortical variation in neuron distributions in this diverse taxonomic group. We quantified upper-layer (layers IIIV) and lower-layer (layers VVI) neuron numbers per unit of cortical surface area in three diprotodont marsupial species (two macropodiformes, the red kangaroo and the parma wallaby, and a vombatiform, the koala) and compared these results to eutherian mammals (e.g., xenarthrans, rodents, primates). In contrast to the notion that the marsupial isocortex contains a low density of neurons, we found that neuron numbers per unit of cortical surface area in several marsupial species overlap with those found in eutherian mammals. Furthermore, neuron numbers vary systematically across the isocortex of the marsupial mammals examined. Neuron numbers under a unit of cortical surface area are low toward the frontal cortex and high toward the caudo-medial (occipital) pole. Upper-layer neurons (i.e., layers IIIV) account for most of the variation in neuron numbers across the isocortex. The variation in neuron numbers across the rostral to the caudal pole resembles primates. These findings suggest that diprotodont marsupials and eutherian mammals share a similar cortical architecture despite their distant evolutionary divergence.

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