Today the term neuron is widely known by most 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 this system, and that they transmit bioelectric impulses throughout the system with the purpose of transferring 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 can be found in the neuroblasts , which we will learn about throughout this article.
Neuroblasts: what are they?
Neuroblasts are a type of embryonic cell of ectodermal origin that is characterized by being the precursor of nerve cells , specifically neurons and neuroglial cells.
This is a type of cell that appears during gestation, 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 particularly active and visible during gestation, decreasing greatly after birth although they may still be active. It is the immediate precursor of the neuron, transforming into it after a process of maturation.
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 the nerve tissue can be formed it has been necessary for some development to take place in the foetus and for neurulation to begin.
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 plaque.
This plate, a layer of cells initially epithelial (which will be called matrix cells), will proceed to grow and expand in a cephalo-caudal manner and will generate folds, in which ectodermal cells will begin to differentiate. The plate will close on itself generating the so-called 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. They will begin to mature and form the first neuroblasts as such, losing the ability to replicate (with small exceptions) and can only finish maturing to become mature nerve cells.
From this moment on, the neuroblast will proceed to migrate towards its final location, the point where it will end up transforming into a neuron. Generally, the older the neuron, the deeper it will be.
An example can be seen in the spinal cord. Once formed, the neuroblasts begin to migrate towards the periphery of the neural tube , reaching the so-called intermediate zone that will end up being the grey substance of the cord, where they will begin to mature and grow peripherally until they generate the marginal zone (future white substance). Other cell types will also be generated by the matrix, such as glial cells and microglia.
The neuroblast does not transform itself 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 are to be formed has a nucleus and a protodendrite , being inserted in the wall of the neural plate. However, at the moment of migrating towards the cavity to begin to replicate it loses this dendrite, becoming an apolar 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 ends 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 previously it was thought that neuroblasts could only be found during gestation and the first years of life, with the discovery of adult neurogenesis in some regions of the brain it has been possible to observe how neuroblasts are formed in some regions throughout our lives, especially in the subventricular zone of the third ventricle and in the hippocampus gyrus.
These neuroblasts are mainly directed to the olfactory bulb or the hippocampus itself , to generate inhibitory neurons of the gabaergic type or glutamatergic exciters, and allow a large number of functions to be maintained.
The neurogenesis that their existence implies is fundamental to allow mental plasticity, learning, and stimulus discrimination. At the pathology level, it can allow the overcoming of ictus, cerebrovascular accidents and traumatisms and the recovery of at least partial lost functions.
Possible problems and associated diseases
Since neuroblasts are the previous step to the existence of neurons, we are facing one of the most relevant types of embryonic cells for our development. However, as with all cell types, we may encounter different problems throughout their generation and maturation.
It is possible that the neuroblasts do not manage to mature to form complete neurons , that an uncontrolled, sudden and harmful growth in their number occurs, that they do not migrate to the areas where their existence would be necessary or that for some reason there are not enough of them in the organism.
The causes of these alterations can be acquired, but taking into account that much of the formation and migration of neuroblasts occurs during gestation, 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 are linked to neuroblasts can be found in the presence of anencephaly or in the existence of malignant tumours linked to these cells known as neuroblastomas.
- Snell, R.S. (2007). Clinical neuroanatomy. 6th edition. Editorial Médica Panamericana. Madrid, Spain.
- López, N. (2012). Developmental biology. Workbook. McGraw Hill.