Corticogenesis

Corticogenesis is the process in which the cerebral cortex is created. The cortex is on the outer layer of the brain and is composed up of 6 layers. The process can been seen to begin at embryonic days 10 to 11.5 (E10 to E11.5) and continues throughout the gestational period. The 6 layers are formed in the ventricular zone and then migrate to their final locations in the cortical plate with the help of various signalers, to form various cortical zones and plates.



Cortical zones and plates
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 is contains the oldest neurons, or pioneer nuerons. Theses neurons are mainly thought to be Cajal-Retzius cells. The preplate also contains the predecessor to the subplate, which is sometimes referred to as 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 suplate moves to below the 6 cortical layers. This transition from preplate to cortical plate is a time when many malformations begin.

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

The subplate is located beneath the cortical plate. It is named after both its location to the cortical plate as well as the time frame in which it is created. While the cortical plate is maturing, the cells located in the subplate establish connections with nuerons that have not yet moved to their final 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. This white matter is the area 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 transitions into adult white matter.

The ventricular and subventricular zones exist below the intermediate zone and communicate to the other zones through cell signalling, along with creating neurons prior to migration.

The marginal zone, along with the cortical zone, make up the 6 layers that from the cortex. This zone is the predecessor for layer 1 of the cortex. Astrocytes form an outer limit membrane to interact with the pia. In humans it has been found that the cells 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.

Formation of layers
The cerebral cortex is divided into six different layers. The layered structure is more detailed on cerebral cortex entry. Each layer is formed by radial glial cells located in the ventricular zone or subventricular zone, and then migrate to their final destination.

Layer 1
Layer one, also known as the Molecular layer, is the first cortical layer produced during neurogenesis at E10.5 to E12.5. Of the six layers found within the cortex, "layer 1 is the most superficial layer of the cortex...composed of Cajal-Retzius neurons and pyramidal cortex cells." This layer is unique in the aspect that these cells migrate to the outer edge of the cortex opposed to the migration experienced by the other 5 layers. Layer one is also characterized by expression of reelin, transcription factor T-box brain 1, and cortical migratory neuronal marker.

Layers 2 and 3
The second and third layers, or the External Granular layer and External Pyramidal layer respectively, are formed around mouse E13.5 to E16. Theses layers are the last to form during corticogenesis and include pyramidal neurons, astrocytes, Stellates, and radial glial cells. The pyramidal and stellate nuerons express SATB2 and CUX1. SATB2 and CUX1 are DNA binding proteins involved in dertermining the fate of cortical cells.

Layers 4, 5 and 6
The third, fourth and fifth layers, or the Internal Granular layer, Internal Pyramidal layer, and Polymorphic or Multiform layer respectively, are formed during mouse E11.5 to E14.5. Included in these layers are stellates, radial glia, and pyramidal neurons. Layer six is adjacent to the ventricular zone. During the production of these layers, transcription factors TBR1 and OTX1 are expressed along with CTIP2, or corticoneuronal zinc finger protein.

Neuron migration
Neuronal Migration plays a significant role in corticogenesis. Throughout the process of creating the six cortical layers, all the neurons and cells migrate from the ventricular zone, through the subplate, and come to rest at their appropriate layer of the cortex. Neuronal migration is generally structured by using radial migration, tangential migration, and multipolar migration.

Important signals
Multiple signalling pathways and signals themselves are very closely intertwined with the appropriate formation of the cerebral cortex. While there are many not yet understand, a few signals and pathways have been carefully understood in an attempt to fully understand how corticogenesis is controlled.

Reelin-DAB1 pathway
The Reelin-DAB1 pathway is perhaps one of the best documented pathways involved in corticogenesis. Cajal-Retzius cells located in the marginal zone excrete reelin to start the cascade. Reelin is able to interact with specific neurons in the cortical plate and direct these neurons to their proper locations. It is thought that the result downstream from this signalling could influence the cytoskeleton. Reelin is only signalled in the Cajal-Retzius cells located in the marginal zone, and its receptors are confined to the cortical plate. This segregation could be used to understand the actions of Reelin.

DAB1 is a signal downstream of the reelin receptors. This protein is located in the ventricular zone and present in highest concentrations in migrating pyramidal cells. When either reelin or DAB1 are inactivated in mice, the resulting phenotypes are the same. In this case, the neurons are unable to migrate properly through the cortial plate. It does not affect the proliferation of neurons and in the wild does not seem to have detrimental affects on memory or learning.

Sonic hedgehog
Sonic hedgehog, or Shh has been shown to severely affect knockout mice, or mice without Shh, during corticogenesis. The ventral and dorsal sides of the cerebrum are affected as Shh expresses the transcription factors to Nkx2 which is important in patterning the cortex. Shh is also important to corticogenesis as it affects the cell proliferation and differentiation, helping neuronal progenitor cells in fate determination.

Bmp-7
Bmp-7, or bone morphogenetic protein 7 is an important regulator in corticogenesis, though it is not understood whether it promotes or inhibits neurogenesis. Bmp-7 can be detected in the ventricular zone and is secreted into CSF, or cerebrospinal fluid. The CSF is an area to promote nuerogenesis and it is believed that the synergy between Bmp-7 and other regulators promote cell division along with homeostasis.

Other bone morphogenetic proteins are also known to impact corticogenesis. Bmp2, 4, 5, and 6 are expressed during the process and can compensate for one another. For example, if Bmp-4 was absent from corticogenesis, very little would change in the cortex phenotype, due to the other Bmps helping accomplish the tasks of Bmp-4. However, Bmp-7 is the only Bmp that promotes radial glia survival and therefore considered more important.

Cdk5-p35 pathway
Cdk5 has a parallel pathway to the Reelin-DAB1 pathway. This pathway affects the neuronal positioning, and results in similar malformations when absent as the Reelin or DAB1 malformations except that migration is affected at an earlier stage on the cortical plate. Cdk5/p35 pathway is also responsible for actin and microtubule dynamics involved in neuronal migration.

Cyclin-dependent kinase inhibitor 1C, or p57, also affects corticogenesis. It has been shown the p57 induces leaving the cell cycle and beginning differentiation, but is dependent on Cdks. p57 is able to induce neuronal progenitor cells to start differentiating into highly specialized neurons in the cortex. However, the mechanism in which p57 is able to control this is not yet known.

Other signals
Besides the ones listed above, there are several more signals that affect corticogenesis. Cnr1 is a g protein receptor that is widely expressed throughout the brain, and in interneurons. In knockout mice, the cortex exhibited decreased immunoreactivity. Nrp1, Robo1, and Robo2 have also been shown to be present and important in the development of interneurons. Cdh8 is known to be expressed in the intermediate and subventricular zone, though not in specific neurons in that area, and it is suggested to regulate fiber releasing.

Lissencephaly
Lissencephaly, or 'smooth brain', is a disorder in which the brain does not properly form the gyri and sulci as a result from neuronal migration and cortical folding. This disorder can also result in epilepsy and cognitive impariment. Type 1 lissencephaly is due to an error in migration. LISI, also known as PAFAH1B, is expressed in both dividing and migrating cells found in the brain. When LIS1 is deleted, lissencephaly occurs.

LIS1 is thought to have several important roles in the creation of the cortex. Since LIS1 is similar to the nuclear distribution protein F (nudF), they are thought to work similarly. The nud family is know to be a factor in nuclear translocation, or moving the nuclei of daughter cells after cell division has occurred. By relation, it is thought that LIS1 is a factor in neuronal migration. LIS1 is also considered to be a factor in controlling dynein, a motor protein that affects intercellular movement such as protein sorting and the process of cell division.

Another protein that contributes to a lissencephaly disorder is DCX, or Doublecortin. DCX is a microtubule associated protein that is responsible for double cortex malformations. DCX is found in the second layer of the cortex, and in fact is still present in immature nuerons of an adult cortex. It is thought that DCX affects neuronal migration through affecting the microtubule dynamics. Since DCX malformations results as a similar phenotype as with LIS1 malformations, it is thought they interact with one another on a cellular level. However, it is not know how this occurs yet.

Tsc1 knockout
TSC, or tuberous sclerosis, is an autosomal dominate disorder. TSC1 or TSC2 inactivation can cause TSC and the associated tumors in the brain. When inactivation of TSC1 is present during corticogenesis, malformations of cortical tubers, or abnormal benign tissue growth, along with white matter nodes would form in mice. This replicates the affect TSC is found to have in humans afflicted with TSC. In the mice there would be a lack of GFAP in astrocytes however astrogliosis would not occur like in the human TSC.

Recapitulation
Recapitulation of corticogenesis in both human and mouse embryos have been accomplished with a three dimensional culture using embryonic stem cells (ESC). By carefully using embryo body intermediates and cultured in a serum free environment cortical progenitors form in a space and time related pattern similar to in vivo corticogenesis. Using immunocytochemical analysis on mouse neural stem cells derived from ESCs, after 6 days there was evidence of neuronal differentiation. This ability only follows after the knowledge of spatial and temporal patterns have been identified along with giving the knowledge that corticogenesis can occur without input from the brain.