Evolution of brain development

Molnar, Z. (2010) Evolution of brain Development. Phenotype 7: 8-10.


Two major hypotheses exist to explain the increase in mammalian cortical neurons.
Karten: additional neurons are generated outside neocortex, then migrate and integrate into neocortex. Equivalent circuit hypothesis: DVR in avian and reptillian brains establish equivalent circuits as those responsible for sensory pathways in mammalian neocortex. (Review this paper further).

Cheung, Molnar: additional site is present within mammals which is responsible for production of extra cortical neurons. Assumes direct homology between reptilian dorsal cortex and mammalian neocortex.

Ventricular zone is proliferative layer of progenitor cells (radial glial cells). Early development these cells divide symmetrically producing more neuroprogenitors. Mice radial glial cells can divide asymmetrically creating a neuron or an intermediate progenitor cell (IPC). The IPC migrates to an adjacent zone - the subventricular zone, where they divide to make more IPCs or neurons. The SVZ is not present in reptiles, and only present in a specific region of avian brains. This gives more credence to the Molnar hypothesis.