Cheung, AFP. Kondo, S. Abdel-Mannan, O. Chodroff, RA. Sirey, TM. Bluy, LE. Webber, N. DeProto, J. Karlen, SJ. Krubitzer, L. Stolp, HB. Saunders, NR. Molnar, Z. (2010) The Subventricular Zone Is the Developmental Milestone of a 6-Layered Neocortex: Comparisons in Metatherian and Eutherian Mammals. Cerebral Cortex 20:1071-1081.
Tangential migration hypothesis only applies to inhibitory neurons. Little evidence for tangential migration of projection neurons. Radial expansion from a 3 to 6-layerd neocortex is more likely due to increased prgenitor population.
Mammals have an SVZ where intermediate progenitor cells form to amplify neuron production. 2-step pattern could facilitate the formation of 6-layer cortex.Sauropsids have 3-layered cortex and lack an organized SVZ.
Marsupials (wallaby and opposum) have fewer total neurons in a normalized cortical column than placentals (mouse). However, they have similar features of SVZ, and still generate a 6-layer cortex. Several independent approaches: 1) H3 staining for mitotic cells in SVZ, 2) Nissl and H&E staining for morphology, 3) VZ/SVZ-specific mRNA expression, and 4) vascular patterns. Results suggests that like all other mammals, marsupials have SVZ with IPCs during cortical development.
The reason that this is important is that the SVZ may be serving as a copy of the VZ, where each progenitor population effectively produces a 3-layer cortex. This has different implications about the organization of cortex from Karten's work. If 6-layer cortex is really 3-layer cortex merged with the BZR (a different unit), then these are fundamentally different circuits. The BZR circuit has tangentially migrated to make 6-layer cortex. However, the duplicate progenitor in SVZ suggests that 6-layer cortex is two 3-layer cortices stacked on top of each other.
It would be interesting to figure out whether neurons from the SVZ migrate to the outer or inner layers specifically, while neurons from VZ migrate to the other layers. I know that there is an order to the migration pattern (I forget if its outside in or inside out), so if SVZ comes on later then the neurons born there will be in the same layers.