Neuronal migration: how nerve cells move

Neurons are born in one place and then move to another space in the brain, where they settle.

Our brain is made up of a large number of neurons that fit together like a huge jigsaw puzzle. Because they are all in the right position, our nervous system can function at full capacity and without any problems.

However, neurons are no longer born in their final position. Instead, they are formed in another region of the nervous system and must travel a long way to reach their destination. This phase of brain formation is known as neuronal migration.. Any anomaly in its development can cause serious malformations in our nervous system and as a consequence a great number of neurological disorders.

What is neuronal migration?

Our brain is made up of hundreds of thousands of neurons. A large number of these nerve cells originate in locations different from those they will occupy once adulthood has arrived..

This process is known as neuronal migration, and most of it occurs most of it occurs during embryonic development, specifically between 12 and 12 months of age.specifically between 12 and 20 weeks of gestation. During this period, neurons are generated and travel through our brain until they settle in their final position.

This movement is made possible by 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 the migrating neurons respond.

This migratory process takes place from the ventricular zone of the neural tube, where neurons originate, to the place designated for them. During the beginning of neuronal migration, these cells are located between the ventricular zone and the marginal zone of the neural tube. are located between the ventricular zone and the marginal zone, which form the intermediate zone.These cells form the intermediate zone, a space of transient location.

Neuronal migration takes place in different phases and is highly complicated, since these nerve cells must travel a long distance and avoid numerous obstacles in order for the brain to develop completely and successfully. In order to do so, They are aided by a type of cell that forms what is known as radial gliawhich acts as a scaffold along which the migrating neurons move.

When some of these phases of neuronal migration are not carried out correctly, changes in the organization of the brain can appear, as well as very important brain malformations.

Phases of migration

As mentioned in the previous section, the process of neuronal migration occurs in different phases, specifically in three, each and every one of which is essential for successful cortical formation. These stages of neuronal migration are as follows.

1. Cell proliferation phase

In this first phase, which occurs from day 32 of the gestational cycle, nerve cells or neurons originate.

A large number of these neurons are born in the germinal zones or germinal matrices, hence the name of the phase. These zones are located in the walls of the lateral ventricles.

2. Neuronal migration phase

During this second phase, the neuronal migration itself takes place. That is to say, the neurons leave their place of origin to move towards their final position.

This process takes place thanks to the glial radial 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 final stage, what it consists of and its particularities will be explained below.

How does differentiation take place?

When the neuron has reached its final location, the differentiation phase begins.The differentiation phase begins when the neuron has reached its final location, achieving all the morphological and physiological qualities of a fully developed neuron. This differentiation depends both on how the neuron is genetically preconfigured, as well as on the interaction with other neurons and the creation of the connecting pathways.

In our nervous system, as well as in the rest of vertebrates, neural cells differentiate from one another from different progenitor cells, which are located in specific locations of the neural tube.

Once the differentiation process is finished, the neurons are organized by joining one to anotherOnce the process of neuronal migration is completed, the neurons organize themselves and join together, finishing the process of neuronal migration and completing the development of our brain.

Defects in this Biological process

As detailed in the first point, any anomaly in the course of neuronal migration can have consequences in the formation of our brain. can have consequences in the formation of our brainfrom malformations to alterations in brain organization.

The most severe malformations are associated with alterations in intellectual development and epilepsies, while in organizational problems the brain has a correct external appearance but the neural connections are severely damaged. the neuronal connections are very damaged because they were not because they were not correctly arranged in the brain.

Among the causes of these failures are:

• Total migration failure.
• Interrupted or incomplete migration.
• Migration diverted to another brain location.
• Failure to stop migration.

Regarding the consequences of these migration defects. An abnormal development of the process can give rise to a large number of disorders and disorders. Among these disorders we can find:

1. Lissencephaly

Lissencephaly is the most severe consequence of a failure in neuronal migration. In this case, neurons begin their migration but are unable to complete it, resulting in severe deformities of the brain.

Depending on the severity of the malformation, lissencephaly can be divided into three different subtypes:

• Mild lissencephaly: this type of malformation causes Fukuyama congenital muscular dystrophywhich is characterized by occasional hypotonia, frailty and general exhaustion in the child, intellectual development disorder and epilepsy.
• Moderate lissencephaly: the direct consequence of this degree of lissencephaly is Cerebral Muscular Eye Disease, whose symptoms are intellectual development disorder, myoclonic seizures and congenital muscular dystrophy. and congenital muscular dystrophy.
• Severe lissencephaly: is manifested by Walder-Walburg Syndrome, which causes severe abnormalities in the brain.which causes severe anomalies in the nervous system, ocular pathologies and muscular dystrophy. Patients born with this type of malformation die within a few months of age.

2. Periventricular heterotopia

In this case, the problem is due to an alteration in the initiation of migration. This affects a small group of neurons which accumulate in locations different from those to which they are normally assigned.

In these cases the person experiences strong seizures that emerge during adolescence.. In addition, although they usually have normal intelligence, some patients experience learning problems.

3. Polymicrogyria

In polymicrogyria, the arrangement of the neural mass creates small abnormal convolutions that are separated by shallow grooves, creating an irregular cortical surface.

Two types of polymicrogyria with different clinical pictures can be distinguished in this condition:

• Unilateral polymicrogyriaUnilateral polymicrogyria: manifested by visual field irregularities, focal seizures, hemiparesis and cognitive disturbances.
• Bilateral polymicrogyriaThis malformation occurs more commonly and is associated with a large number of symptoms and clinical pictures such as bilateral frontoparietal polymicrogyria or congenital bilateral perisylvian syndrome.

4. Schizencephalias

Schizencephalias are distinguished by a normal volume of gray matter but with alterations in the circumvolutions of smaller size and more superficial than usual and surrounded by very shallow grooves.

This pathology has no specific clinical symptomsThese can vary according to the extension and localization of the affected areas. In some cases there may be no visible clinical manifestations, while in others people may suffer epileptic episodes of varying intensity.

5. Other

Other neurological alterations that have their origin in an alteration of neuronal migration are:

• Subcortical band heterotopia.
• Holoprosencephaly.
• Colpocephaly.
• Porencephaly..
• Hydranencephaly.