Book of Abstracts: Albany 2009
June 16-20 2009
© Adenine Press (2008)
On the Evolutionary Origin of Mammalian Specific Features of the Neocortex
The mammalian brain is known to have some unique features, one of which is the layer structure of the neocortex. Although the functional and developmental aspects of the research has been actively done, the evolutionary origin of the neocortical layer structure have not been enough studied so far. With the aim of elucidating the evolutionary origin of layer structure of the neocortex, we studied a chick brain from the viewpoint of the comparative developmental biology. We found the following four points. (I) The chick pallium possesses the similar neuronal repertoire of the mammalian neocortex, according to the expression patterns of marker genes, but spatial distribution of a variety of neurons is divergent between the mammalian neocortex and the chick pallium. (II) BrdU labeling experiment showed that the chick shares with mammals the temporal order of the neuronal differentiation. (III) Our fate-mapping experiment demonstrated that the generation site of a particular type of neuron is distinct from that of another type of neuron in the chick pallium. However, the stem cell in all the regions of the mammalian neocortex can generate all types of cortical neurons. (IV) Several lines of evidences showed that, compared with the uniform neurogenesis in the mammalian neocortex, the chick pallial neurogenesis is biased along the medio-lateral axis, medial-low and lateral-high. We concluded that the divergent neurogenetic pattern makes the difference of pallial organization between the layer structure of the mammalian neocortex and the chick non-layered pallium. This suggests the possibility that an evolutionary novel feature, the layer structure of the mammalian neocortex, may have arisen after bird-mammal divergence caused by the changes in the differentiation process from the neural stem cell.
1Laboratory for DNA Data Analysis