Neuroblasts: the precursors of nerve cells

Today the term neuron is widely known by the majority of the population. We know that it is the main type of cell that forms part of our nervous system, being the basic unit of said system, and that transmit bioelectrical impulses throughout the system in order to transfer orders or information to the different parts of our organism.

But do we know how or from what they arise? At what point in our development do they appear? The answer to these questions lies in the neuroblasts, which we are going to know throughout this article.

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Neuroblasts: what are they?

The neuroblasts are a type of embryonic cell of ectodermal origin that is characterized as the precursor of nerve cells, specifically of neurons and neuroglia.

It is a type of cells that appear during pregnancy, born in the neural plate from ectoderm tissue to begin to mature and migrate to its final location and end up configuring our nervous system.

Neuroblasts are especially active and visible during gestation, greatly diminishing after birth even though they may still be active. It is the immediate precursor of the neuron, becoming it after a maturation process.

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development of the nervous system

As we have said, neuroblasts are embryonic cells, which are produced during the gestation of a future individual. Before nerve tissue can form it has been necessary for some development to occur in the fetus and start the neurulation.

This occurs approximately the third week after fertilization. At this time, the ectoderm is stimulated to end up generating the neuroectoderm, until it ends up generating the neural plate.

This plate, a layer of initially epithelial cells (to be called matrix cells), will proceed to grow and expand cephalocaudally and will generate folds, in which cells will begin to differentiate ectodermal. The plate will close on itself, generating what is known as the neural tube, which will end up closing its ends during the fourth week.

The cells of the matrix are directed towards the cavity or hollow area of ​​the tube and, at this point, they proceed to divide and replicate continuously, which will cause the neural tube to increase in size. The first neuroblasts will begin to mature and form as such, losing the ability to replicate (with small exceptions) and can henceforth only finish maturing to become a nerve cell mature.

From this moment on, the neuroblast will proceed to migrate towards its final location, the point at which it will eventually become a neuron. Generally, the older the neuron, the greater the depth at which it will be found.

An example can be seen in the spinal cord. Once formed, neuroblasts begin to migrate to the periphery of the neural tube., reaching the so-called intermediate zone that will end up being the gray matter of the medulla, where begin to mature and grow peripherally until generating the marginal zone (future substance white). Other cell types will also be generated by the matrix, such as glial cells and microglia.

neuron formation

The neuroblast does not transform into a neuron immediately, but requires a maturation process to be able to form it. Initially the cell from which the neuroblast and future nerve cell will be formed have a nucleus and a protodendrite, being inserted in the wall of the neural plate. However, at the moment of migrating towards the cavity to start replicating, it loses said dendrite, becoming a nonpolar spherical nucleus.

Once the replication process has finished and as the neuroblast begins to form as such, two opposite extensions progressively appear, forming something similar to a bipolar neuron. One of these extensions lengthens and will end up becoming an axon, while the other fragments to generate future dendrites. These elements will mature over time until they end up configuring an adult neuron.

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Do they exist in adults?

Although it was previously thought that neuroblasts could only be found in gestation and in the first years of life, with the discovery of adult neurogenesis in some brain regions It has been possible to observe how neuroblasts form in some regions throughout our lives, especially in the subventricular zone of the third ventricle and in the hippocampal gyrus.

These neuroblasts are mainly directed to the olfactory bulb or the hippocampus itself, to generate GABAergic inhibitory neurons or glutamatergic excitatory neurons, and allow a large number of functions to be maintained.

The neurogenesis that its existence supposes is fundamental to allow mental plasticity, learning, and the discrimination of stimuli. At the level of pathology, it can allow the overcoming of strokes, cerebrovascular accidents and traumatisms and the at least partial recovery of lost functions.

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Possible problems and associated diseases

Given that neuroblasts are the step prior to the existence of neurons, this is one of the most relevant types of embryonic cells for our development. However, as in all types of cells, we can encounter different problems throughout their generation and maturation.

Neuroblasts may fail to mature into complete neurons, that there is an uncontrolled, sudden and harmful growth in their number, that they do not migrate to the areas where their existence would be necessary or where for some reason there are not enough of them in the body.

The causes of these alterations can be acquired, but taking into account that much of the formation and migration of neuroblasts occurs during pregnancy it is much more likely that cases are due to genetic disorders, problems during the gestation of the fetus or the appearance of mutations.

Two examples of disorders that link to neuroblasts we can find in the presence of anencephaly or in the existence of malignant tumors attached to these cells known as neuroblastomas.

Bibliographic references:

• Snell, R.S. (2007). clinical neuroanatomy. 6th edition. Panamerican Medical Editorial. Madrid Spain.
• Lopez, N. (2012). Developmental biology. Work notebook. McGraw Hill.