Corticogenesis

Corticogenesis is the process in which the cortex of the brain is created during neurodevelopment. The cortex is the outer layer of the brain and is composed of up to six layers. Neurons formed in the ventricular zone migrate to their final locations in one of the six layers of the cortex The process occurs from embryonic day 10 to 17 in mice and between gestational weeks seven to 18 in humans.

The preplate is the first stage in corticogenesis prior to the development of the cortical plate. The preplate is located between the pia and the ventricular zone. According to current knowledge, the preplate contains the first-born or pioneer neurons. These neurons are mainly thought to be Cajal-Retzius cells. The preplate also contains the predecessor to the subplate, which is sometimes referred to as a layer. As the cortical plate appears, the preplate separates into two components. The Cajal-Retzius cells go into the marginal zone, above the cortical plate, while the subplate moves to below the 6 cortical layers. It is during this transition from preplate to cortical plate when many malformations may arise.

The cortical plate is the final plate formed in corticogenesis. It includes the cortex layers two through six.

The subplate is located beneath the cortical plate. It is named for both its location relative to the cortical plate and for the time frame in which it is created. While cortical plate matures, the cells located in the subplate establish connections with neurons that have not yet moved to their destination layer within the cortical plate. Pioneer cells are also present in the subplate and work to create fibers and synapses within the plate.

The intermediate zone is located between the ventricular zone and the cortical plate. The white matter in this area is where neurons, that are created in the ventricular zone, migrate through in order to reach the cortical plate. This zone is only present during corticogenesis and eventually transforms into adult white matter.

The ventricular and subventricular zones exist below the intermediate zone and communicate to other zones through cell signalling, also creating neurons destined to migrate to other areas in the cortex.

The marginal zone, along with the cortical zone, make up the 6 layers that form the cortex. This zone is the predecessor for layer 1 of the cortex. Astrocytes form an outer limiting membrane to interact with the pia. In humans it has been found that the cells here also form a subpial layer. Cajal-Retzius cells are also present in this zone and release reelin along the radial axis, a key to proper neuronal migration during corticogenesis.

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