Neuronal migration: this is how nerve cells move

March 29, 2024

Our brain is made up of a large number of neurons that fit together like an immense puzzle. Thanks to the fact that all of them are in their correct position, our nervous system can function at full capacity and without any problem.

However, neurons are not born in their final position anymore. But they are formed in another region of the nervous system and must go 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 consequently a great number of neurological disorders.

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What is neuronal migration?

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

This process is known as neuronal migration, and most of it occurs during embryonic development , specifically between 12 and 20 weeks of gestation. During this period, the neurons are generated and travel through our brain to 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 the neurons in process respond. migration.

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

Neuronal migration is carried out in different phases and is highly complicated. since these nerve cells must travel a great distance and avoid numerous obstacles so that the brain can develop completely and satisfactorily. For it, are aided by a type of cells that form what is known as radial glia , and that exerts the function of scaffolding through which the migrating neurons move.

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

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Phases of migration

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

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 germ zones or germ matrices, hence the name of the phase. These zones are located in the walls of the lateral ventricles.

2. Phase of neuronal migration

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

This process is given thanks to the radial glial system. In this system, a cell that is not already present in the adult brain guides the neurons to their position.

3. Phase of horizontal and vertical organization

In this last phase, the differentiation and subsequent organization of the neurons takes place. Due to the complexity of this final stage, the following will explain what it consists of and what its particularities are.

How is differentiation produced?

When the neuron has managed to reach its final location is when the phase of differentiation begins , getting all the morphological and physiological qualities of a fully developed neuron. This differentiation depends as much on how this neuron is genetically preconfigured, as on the interaction with other neurons and on the creation of the connection routes.

In our nervous system, as well as in the rest of vertebrates, neural cells are differentiated from each other by 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 another , ending the process of neuronal migration and completely 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 on the formation of our brain ; from malformations to alterations in brain organization.

The most severe malformations are associated with alterations in intellectual development and epilepsies, while in the problems of organization the brain has a correct external appearance but Neural connections are badly damaged because its correct disposition in the brain did not occur.

Among the causes of these failures are:

Total migration failure.
Interrupted or incomplete migration .
Migration diverted to another brain location.
Do not stop the migration.

As for the consequences of these defects in migration. An abnormal development of the process can lead to a large number of disorders and disorders. Among these disorders we can find:

1. Lissencephaly

Lissencephaly is the most serious consequence of a failure in neuronal migration. In this case, the neurons initiate their migration but they are not able to complete it, which causes serious deformities in the brain.

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

Mild leencephaly: this type of malformation causes congenital muscular dystrophy of Fukuyama , which is characterized by occasional hypotonia, fragility and general exhaustion in the child, intellectual development disorder and epilepsy.
Moderate lissencephaly: the direct consequence of this degree of lissencephaly is the Brain Eye Muscle Disease, whose symptoms are intellectual development disorder, myoclonic seizures and congenital muscular dystrophy.
Severe lissencephaly: is externalized through the Walder-Walburg Syndrome , which causes severe anomalies in the nervous system, eye pathologies and muscular dystrophy. Patients born with this type of malformation die at a few months of age.

2. Periventricular heterotopy

In this case, the problem is due to an alteration in the beginning of the migration. This affects a small group of neurons which accumulate in different locations than those that correspond to them.

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

3. Polymicrogyria

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

In this condition, two types of polymicrogyria can be distinguished with different clinical pictures:

Unilateral polymicrogiria : manifested through irregularities in the visual field, focal crises, hemiparesis and cognitive disorders.
Bilateral polymicrogyria : This malformation occurs more commonly and is related to a large number of symptoms and clinical conditions such as bilateral frontoparietal polymicrogyria or bilateral congenital perisylvian syndrome.

4. Schizencephaly

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

This pathology has no specific clinical symptoms , but these may vary depending on the extent and location of the affected areas. In some cases, there may be no visible clinical pictures, while in others, people may suffer epileptic episodes of variable intensity.