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Notes:
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The neocortex is the most representative and elaborated structure of the mammalian brain and is related
to the achievement of complex cognitive capabilities, which are disturbed following malformation or
lesion. Searching for the evolutionary origin of this structure continues to be one of the most important
and challenging questions incomparative neurobiology.However, this is extremely difficult because of the
highly divergent evolution of the pallium in different vertebrates, which has obscured the comparison.
Herein, we review developmental neurobiology data for trying to understand the genetic factors that
define and underlie the parcellation of homologous pallial subdivisions in different vertebrates. According
to these data, the pallium in all tetrapods parcellates during development into four major histogenetic
subdivisions, whichare homologous as fields across species. The neocortex derives fromthe dorsal pallium
and, as such, is only comparable to the sauropsidian dorsal pallium (avian hyperpallium and lizard/turtle
dorsal cortex).We also tried to identify developmental changes in phylogeny that may be responsible of
pallial divergent evolution. In particular, we point out to evolutionary differences regarding the cortical
hem (an important signaling center for pallial patterning, that also is a source of Cajal–Retzius cells,
which are involved in cortical lamination), whichmay be behind the distinct organization of the pallium
in mammals and non-mammals. In addition, we mention recent data suggesting a correlation between
the appearance and elaboration of the subventricular zone (a newgerminative cell layer of the developing
neocortex), and the evolution of novel cell layers (the supragranular layers) and interneuron subtypes.
Finally,we comment on epigenetic factors that modulate the developmental programs, leading to changes
in the formation of functional areas in the pallium (within some constraints).
This work was supported by grants to L.M. from the Spanish Ministry of Education and Science and FEDER (DGICYT-FEDER: BFU2006-14804-C02-02/BFI). |
