Phylogenetic variation of layer II cortical immature neurons in dog and horse confirms covariance with brain size and neocortical surface

Recent research in brain structural plasticity has identified "immature" or "dormant" neurons in layer II of the cerebral cortex (cortical immature neurons; cINs), cells that remain in a prolonged state of arrested development but retain the ability to resume maturation and integrate functionally into mature cortical circuits. These immature cells are far more abundant in large-brained mammals, being restricted to paleocortex (piriform cortex) in small-brained rodents and extending in the widely expanded neocortical mantle of species with large gyrencephalic brains. In a previous systematic analysis, using a comparable method for quantification in eight mammalian species, including mice, chimpanzees, and others of diverse phylogenetic backgrounds and neuroanatomical structure, cIN density showed covariation with brain size. Notably, however, members of the order Carnivora (cats and foxes) displayed the highest cIN densities with respect to sheep and chimpanzees, endowed with larger brains. Here we used the same method to characterize and quantify the cINs in the cerebral cortex of dogs (carnivores) and horses (herbivores with a very large brain) to investigate the position of these two species in the phylogenetic variation. Our results further strengthen the finding of covariance between cIN density and increasing brain size and confirm a relationship with neocortical expansion. These results support the emerging view that immature or dormant neurons may represent a reservoir of undifferentiated (stem cell-independent) neuronal cells for the widely expanded cortices of mammals endowed with high order cognitive functions.