Our brain is made up of a large number of neurons that fit together like a huge puzzle. Thanks to the fact that they are all in their correct position, our nervous system can function at full capacity and without any problems.
However, neurons are no longer born in their final position. But they form in another region of the nervous system and have to travel a long way to reach their destination. This phase of brain formation is known as neuronal migration. Any abnormality in its development can lead to serious malformations of our nervous system and consequently a large number of neurological disorders.
What is neuronal migration?
Our brain is made up of hundreds of thousands of neurons. A lot of these nerve cells they come from places other than those they will occupy once adulthood arrives.
This process is known as neuronal migration, and most of it occurs during embryonic development, More precisely between 12 and 20 weeks of gestation. During this time, neurons are generated and travel through our brain until they settle in their final position.
This displacement is possible thanks to signals from other neurons, which are already in their final position and play a role similar to that of a traffic light that directs traffic, sending different types of signals to which they respond, neurons in progress. migration.
This migratory process occurs from the ventricular area of the neural tube, where neurons originate, to the place designated for them. At the start of neuronal migration, these cells they are located between the ventricular zone and the marginal zone, Which form the intermediate zone, a transitory localization space.
Neuronal migration takes place in different phases and is very complicated. since these nerve cells have to travel a great distance and avoid many obstacles in order for the brain to develop fully and satisfactorily. For that, they are helped by a type of cell that forms what is called a radial glia, And which fulfills the function of scaffolding through which the migrating neurons move.
When some of these phases of neuronal migration do not take place correctly, they can appear from changes in the organization of the brain, to very significant brain malformations.
As mentioned in the previous section, the process of neuronal migration occurs in different phases, specifically three, each of which is essential for successful cortical training. These stages of neuronal migration are as follows.
1. Cell proliferation phase
In this first phase, which occurs from the 32nd day of the gestational cycle, nerve cells or neurons are born.
Many of these neurons originate in the germinal areas or germline matrices, hence the name of the phase. These areas are located in the walls of the lateral ventricles.
2. Neuronal migration phase
Throughout this second phase, neuronal migration itself occurs. In other words, neurons leave their place of origin to lead to their final position.
This process occurs through the radial glial system. In this system, a cell that is no longer present in the adult brain, guides the neurons to their position.
3. Horizontal and vertical organization phase
In this last phase, the differentiation and subsequent organization of neurons takes place. Due to the complexity of this last step, it will be explained below what it consists of and what are its peculiarities.
How does differentiation occur?
When the neuron has successfully reached its final location, this is when the differentiation phase begins, Achieve all the morphological and physiological qualities of a fully developed neuron. This differentiation depends as much on the way in which this neuron is genetically preconfigured, as on the interaction with other neurons and the creation of connection pathways.
In our nervous system, as well as in other vertebrates, neural cells differentiate from each other due to different progenitor cells; which are located at specific locations in the neural tube.
Once the differentiation process is complete, neurons organize themselves by coming together, End the process of neuronal migration and completely stop the development of our brain.
Defects in this biological process
As detailed in the first point, any abnormality during neuronal migration it can have consequences on the formation of our brain; from malformations to alterations in cerebral organization.
The most serious malformations are associated with impaired intellectual development and epilepsy, while in organizational problems the brain has a correct external appearance but neural connections are severely damaged due to the fact that its correct arrangement in the brain did not take place.
Some of the causes of these errors are:
- Complete failure of the migration.
- Migration interrupted or incomplete.
- Diverted migration to another location in the brain.
- Non-stop migration.
Regarding the consequences of these migration defects. Abnormal development of the process can lead to a large number of disorders and disorders. Among these disorders, we can find:
Lysencephaly is the most serious consequence of failure of neuronal migration. In this case, the neurons begin to migrate but are unable to complete it, causing severe deformities in the brain.
Depending on the severity of the malformation, lysencephaly can be divided into three different subtypes:
- Mild lysencephaly: this type of malformation causes congenital muscular dystrophy of Fukuyama, Which is characterized by occasional hypotonia, frailty and general exhaustion of the child, intellectual developmental disorder and epilepsy.
- Moderate Lysencephaly: The direct consequence of this degree of lysencephaly is ocular cerebral muscle disease, symptoms are intellectual development disorders, myoclonic seizures and congenital muscular dystrophy.
- Lysencephaly pit: is exteriorized by Walder-Walburg syndrome, Which causes severe nervous system abnormalities, eye disease and muscular dystrophy. Patients born with this type of defect die within months.
2. Periventricular heterotopia
In this case, the problem is due to corruption at the start of the migration. It affects a small group of neurons that accumulate in places other than those that normally correspond to them.
In such cases, the person has severe seizures that start in adolescence. In addition, although they tend to have normal intelligence, some patients experience learning difficulties.
In polymicrogyria, the arrangement of the neural mass creates small, abnormal convolutions that are separated by surface grooves, creating an irregular cortical surface.
In this condition, two types of polymicrogyria with different clinical pictures can be distinguished:
- Unilateral polymicrogyria: It is manifested by visual field irregularities, focal seizures, hemiparesis and cognitive impairment.
- Bilateral polymicrogyria: This malformation occurs more frequently and is linked to a large number of symptoms and clinical conditions such as bilateral frontoparietal polymicrogyria or congenital bilateral perisilvic syndrome.
Esquisencephaly is distinguished by a normal volume of gray matter but with alterations in the convolutions of smaller in size and shallower than usual and surrounded by very shallow furrows.
This pathology has no specific clinical symptomsBut these vary depending on the extent and location of the affected areas. In some cases, they may not show visible clinical signs, while in others, people may experience epileptic episodes of varying intensity.
Other neurological disorders that arise from impaired neuronal migration are:
- Heterotropy in the subcortical band.