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Abstract
The six-layered neocortex is a unique characteristic of mammals and likely provides the neural basis of their sophisticated cognitive abilities. Although all mammalian species share the layered structure of the neocortex, the sauropsids exhibit an entirely different cytoarchitecture of the corresponding pallial region. Our previous gene expression study revealed that the chicken pallium possesses neural subtypes that express orthologs of layer-specific genes of the mammalian neocortex. To understand the evolutionary steps leading toward animal group-specific neuronal arrangements in the pallium in the course of amniote diversification, we examined expression patterns of the same orthologs and a few additional genes in the pallial development of the Chinese softshell turtle Pelodiscus sinensis, and compared these patterns to those of the chicken. Our analyses highlighted similarities in neuronal arrangements between the two species; the mammalian layer 5 marker orthologs are expressed in the medial domain and the layer 2/3 marker orthologs are expressed in the lateral domain in the pallia of both species. We hypothesize that the mediolateral arrangement of the neocortical layer-specific gene-expressing neurons originated in their common ancestor and is conserved among all sauropsid groups, whereas the neuronal arrangement within the pallium could have highly diversified independently in the mammalian lineage.
Highlights
Complex cognitive functions in mammalian species are essentially encoded in the neural circuits of the neocortex, a mammalianspecific structure characterized by tangential neuronal layers and located inside the pallium
Underneath the trilaminar domain are the nuclear domains that consist of the pallial thickening (PT) and dorsal ventricular ridge (DVR)
The former is continuously extended ventrally from the dorsal cortex (DC), and the latter protrudes into the lateral ventricle from the most ventral part of the pallium (Figure 1C). Among these subdivisions of the turtle pallium, the DC, and PT are accepted as the homolog of the mammalian neocortex because of their shared features, including the expression of Emx1, and reciprocal connections with the thalamus (Heller and Ulinski, 1987; Fernandez et al, 1998; Reiner, 2000)
Summary
Complex cognitive functions in mammalian species are essentially encoded in the neural circuits of the neocortex, a mammalianspecific structure characterized by tangential neuronal layers and located inside the pallium (the dorsal part of the telencephalon). Extracortically projecting neurons reside in the deep layers 5 and 6, whereas the majority of intracortically connecting neurons are located in the more shallow layer 2/3 (Molyneaux et al, 2007) This laminar neuronal arrangement is basically shared by all studied mammalian species, including even the monotremes, and marsupials (Butler and Hodos, 2005). The sauropsids, which is a group containing the currently living reptiles and birds, possess a totally different neuronal arrangement in the corresponding pallial region to the mammalian neocortex (Figure 1). This structural difference in neocortical regions has again raised the longstanding question of how mammals acquired the layered neocortex during evolution
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