Three-dimensional tomographic analyses of fossil endocasts suggested that the size of the mammalian cerebral cortex has increased rapidly in accordance with the dependence of olfactory and somatosensory information ( Quiroga, 1979 Rowe et al., 2011) however, histological architectures of the ancestral cerebral cortex remains unknown, preventing us from tracing how the cerebral cortex has specifically evolved in the mammalian lineage.
In recent years, numerous fossil records have been identified from Paleozoic and Mesozoic sediments, which provided significant information on the process of amniote diversification. Other lineages of amniotes have also diverged into several unique animal groups that include the descent of extant reptiles ( Ruta et al., 2003). Phylogenic and paleontological evidence indicated that the forerunners of the mammalian lineage diverged from the common ancestors of amniotes at approximately 300 million years ago ( Carroll, 1988 Ruta et al., 2003, 2013). On the contrary, the origin and evolutionary process of the mammalian cortex remain elusive. Recent advances of developmental neurobiology have illuminated the molecular mechanisms that govern these complicated cellular events during corticogenesis ( Campbell, 2005 Flames and Marin, 2005 Dehay and Kennedy, 2007 Molyneaux et al., 2007 Kumamoto and Hanashima, 2014). The basic frameworks of these unique characteristics are accomplished by the dramatic increase in the number of neural stem/progenitor cells and massive irruption of distinct types of neurons, followed by the coordinated migration of differentiated neurons during embryogenesis. The cerebral cortex is characterized by tangential expansion of its surface area, which is particularly enhanced in the primate and human neocortex, and a six-layered laminar structure composed of multiple types of excitatory and inhibitory neurons ( Nieuwenhuys, 1994 Kriegstein et al., 2006 Defelipe, 2011 Lui et al., 2011). The mammalian cerebral cortex is a remarkable brain structure that is responsible for intricate social behaviors and intelligence. Applications of these techniques illuminate the developmental mechanisms underlying reptilian corticogenesis, which provides significant insight into the evolutionary steps of different types of cortex and the origin of the mammalian neocortex. We established in ovo electroporation and an ex ovo culture system to address neural stem cell dynamics, neuronal differentiation and migration. Here, we introduce a method of embryonic manipulations for the Madagascar ground gecko and Chinese softshell turtle. However, reptilian cortical development and the underlying molecular mechanisms remain unclear, mainly due to technical difficulties with sample collection and embryonic manipulation. Because all modern reptiles have a homolog of the neocortex at the dorsal pallium, developmental analyses of the reptilian cortex are valuable to explore the origin of the neocortex. However, how the mammalian neocortex emerged during evolution remains elusive.
The mammalian neocortex is a remarkable structure that is characterized by tangential surface expansion and six-layered lamination.
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