Today the term neuron is widely known by the majority of the population. We know that it is the main type of cell that is part of our nervous system, being the basic unit of said system, and that it transmits bioelectric impulses throughout the system in order to transfer orders or information to the different parts of our body. .
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 neuroblasts which we are going to know throughout this article.
Neuroblasts are a type of embryonic cell of ectodermal origin that is characterized by being the precursor of nerve cells specifically neurons and neuroglia.
It is a type of cells that appear during pregnancy, being 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 pregnancy, decreasing greatly after birth although they may still be active. It is the immediate precursor of the neuron, transforming into it after a maturation process.
The development of the nervous system
As we have said, neuroblasts are embryonic cells, which are produced during the gestation of a future individual. Before nervous tissue can form it has been necessary for certain development to occur in the fetus and neurulation begins.
This occurs approximately the third week after fertilization. At this moment 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 (which will be called matrix cells), will proceed to grow and expand cephalocaudal and will generate folds, in which ectodermal cells will begin to differentiate. 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 from then on only being able to finish maturing to become a mature nerve cell.
From this moment on, the neuroblast will proceed to migrate towards its final location, the point at which it will end up transforming into a neuron. Generally, the older the neuron, the greater the depth at which it will be found.
We can see an example in the spinal cord. Once formed, neuroblasts begin to migrate toward the periphery of the neural tube, reaching the so-called intermediate zone that will end up being the gray matter of the spinal cord, where they will begin to mature and grow peripherally until generating the marginal zone (future white matter). Other cell types will also be generated by the matrix, such as glial cells and microglia.
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 They have a nucleus and a protodendrite, being inserted into the wall of the neural plate. However, at the time of migrating towards the cavity to begin replicating, it loses said dendrite, becoming a nonpolar spherical nucleus.
Once the replication process is 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 elongates 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 forming an adult neuron.
Do they exist in adults?
Although it was previously thought that neuroblasts could only be found during pregnancy and in the first years of life, with the discovery of adult neurogenesis in some brain regions It has been observed 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 They 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 implies is essential to allow mental plasticity, learning, and discrimination of stimuli. At the pathology level, it can allow the overcoming of strokes, cerebrovascular accidents and trauma and the at least partial recovery of lost functions.
Given that neuroblasts are the step prior to the existence of neurons, we are faced with one of the most relevant types of embryonic cells for our development. However, as occurs in all types of cells, we can encounter different problems throughout their generation and maturation.
Neuroblasts may not be able 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 that for some reason there are not enough in the body.
The causes of these alterations may be acquired, but taking into account that a large part of the formation and migration of neuroblasts occurs during pregnancy, it is much more likely that the cases are due to genetic disorders, problems during the gestation of the fetus or the appearance of mutations.
Two examples of disorders that are linked to neuroblasts are we can find in the presence of anencephaly or in the existence of malignant tumors linked to these cells known as neuroblastomas.
