Comparative development of the mammalian isocortex and the reptilian dorsal ventricular ridge. Evolutionary considerations.

There has been a long debate about a possible homology between parts of the dorsal ventricular ridge (DVR) of reptiles and birds, and parts of the mammalian isocortex. Correspondence between these structures was originally proposed on the basis of connectional similarities between the DVR of birds and the mammalian auditory and extrastriate visual isocortical areas. Furthermore, the proposal of homology includes the possible embryological similarity of cells that give rise to the DVR and cells that give rise to the isocortex. Against this concept it has been claimed that the DVR and the isocortex originate in topographically different pallial compartments, an interpretation that is supported by recent developmental and molecular data. Other studies indicate that migrating cells can cross the borders between adjacent developmental compartments: cells that originate in subcortical components contribute a number of interneurons to the developing isocortex via tangential migration. This mechanism might reconcile the proposed homology with the developmental evidence, since cells originating in one compartment (the one corresponding to DVR) may become included in structures generated in a different compartment (the one corresponding to isocortex). However, there is no evidence in mammals of a structure homologous to the embryonic DVR that can produce isocortical neurons. In order to fully clarify the problem of isocortical origins, further comparative studies are needed of the embryonic development of the lateral and dorsal aspects of the cerebral hemispheres in amphibians, reptiles and mammals.