Cell-type homologies and the origins of the neocortex.

Dugas-Ford, J. Rowell, JJ. Ragsdale, CW. (2012) Cell-type homologies and the origins of the neocortex. PNAS.

The idea of these experiments is to address Harvey Karten's hypothesis that there is a homology between different layers of neocortex and the nuclei of bird brains. To answer the question they are looking for genetic markers that distinctly label L4 (input) pyramids and L5 (output) pyramids in mammalian cortex. They then basically look for the same markers in turtles, chickens and zebra finches to claim a homology.

They conclude that Karten was basically right. They find similar markers of L4 pyramids in mammals as they find in the suggested nuclei of birds. They also see homologies in turtles. 

The most interesting part is that turtle dorsal cortex appears to be structured like archeocortex in mammals (olfactory and hippocampus). They see that the input pyramids are grouped together in the rostral part of dorsal cortex, and the output pyramids are grouped in the caudal part. They even state the homology to hippocampus: "An analogous example is provided by the mammalian hipposcampus, a three-layered cortex with separate fields, CA1 and CA3, containing pyramidal cells that differ in their connections and their molecular identities and that have between the a transitional field, CA2". 

So this gives more credence to the idea that cortical processing may require two types of pyramidal cell circuits. Which means that in a way neo-cortex is two three-layer cortices stacked on top of each other. But it is really the two spatially seperated circuits like in turtle, hippocampus, olfactory, put on top of each other. Its probably a lot more efficient to wire them in the 6-layer fashion. For birds instead of stacking, its more like they just clumped together, which made them form nuclei.

So, wtf are all those black neurons? What is L2/3 and L6 adding to neocortex. They discuss a study which do similar experiment for L2/3 and says that homologous cells exist in check pallium, but say that these claims are problematic. But the question is whether these black neurons are doing something special or what? Do they have homologies in turtle?

(Note: cortex develops in an "inside-out" manner, the deep layer neurons are born first, superficial layer neurons migrate past them later: review Rakic 2009).

They talk about the development of these regions in the different species in the discussion. There's no SVZ in turtle, but due to spatial seperation it may be that the SVZ is caudal VZ of turtle.