The complexity of our nervous system, the fundamental system that connects and governs all the processes of our organism, is something that continues to amaze the many researchers and experts who study it every day. But we must take into account one fact, and that is that while when we think of a nervous system generally comes to mind a structure already mature, it is necessary that there are a number of processes since we are little more than a cluster of cells to reach a mature nervous system.

Throughout the embryonic and fetal development, a series of events will take place that will trigger the formation of the so-called neural tube, which in turn will develop during gestation until it generates the structures of the human nervous system

What is the neural tube?

The neural tube is the structure formed during gestation and is the immediate predecessor of the nervous system , whose closure and evolution will eventually generate the different structures that are part of it. Specifically, we are talking about the brain and the spinal cord, others being those of the peripheral nervous system formed by the neural crests.

Technically, the process in which the neural tube is generated and closed would start from the third week of gestation and should finish closing around the twenty-eighth day. It should be noted that it is essential that the tube be closed so that the spine and skull can protect the nerves and brain and so that they can form. This closure usually occurs correctly in most births, although sometimes it fails to close the tube, which can lead to different neural tube defects.

Neurulation: formation and evolution of the neural tube

The neural tube is produced along a process known as neurulation , in which the notochord and the whole of the mesoderm lead the ectoderm to differentiate into a neuroectoderm. This thickens and ends up detaching itself from the cell sheet, forming the neural plate.

This plate will proceed to stretch in a caudal-face manner, in such a way that it will generate folds, which will grow with the development of the fetus. Over time, the central part of the plate collapses, generating a channel whose walls will close in on themselves until they generate a tube-shaped structure: the neural tube. This tube begins to close in on itself in the middle, moving towards the ends. In this process the neural crests are also separated and detached from the tube , which will end up generating the autonomic nervous system and different organs and tissues of the different body systems

Initially the tube will be open at its ends, forming the rostral and caudal neuropores, but from the fourth week they begin to close. This closure and the development of the tube will generate various dilations in its face-cranial part, which in the future will configure the different parts of the brain. Generally, the rostral end closes first, around the 25th, while the causal end usually closes around the 27th.

There is a second process of neurulation, the so-called secondary, in which the part of the neural tube corresponding to the spine is formed and at the same time hollowed out in such a way that the internal cavity of this tube is emptied, generating a separation between epithelium and mesenchymal cells (which will form the spinal cord). In the medulla we find that motor neurons appear in the ventral part, while sensory neurons appear in the most dorsal part of it.

Formation of the different brain regions

Throughout the formation and development of the neural tube, the structures that make up our adult nervous system will be produced. The cells of the neural tube, once closed, begin to divide and generate different layers and structures. The brain will appear in the anterior or cranial part of the tube.

During the fourth week of gestation, the prosencephalon, midbrain and rhombo-brain can be observed . During the fifth, the first and third are divided from them, forming telencephalon and diencephalon in the first and metencephalon and myelinebephalon in the second. In a relatively fast way, the structure changes in a heterogeneous way, growing the different structures (being the telencephalon, the part of the cortex that develops the most).

It should be borne in mind that it is not only the wall of the neural tube that is important, but also the holes and empty spaces present inside it: they will end up forming the ventricles and the set of structures through which the cerebrospinal fluid will circulate, without which the brain could not function properly.

Neurulation defects

The process of neurulation, in which the structure of the nervous system is formed, is something fundamental for the human being. However, it can sometimes produce alterations and malformations that can have more or less severe consequences on the development and survival of the foetus. Among them, some of the best known are the following.

1. Spina bifida

One of the most common and known neural tube defects is spina bifida. This alteration implies the existence of some kind of problem that prevents a part of the neural tube from closing completely, something that can have effects of varying severity because the nerves and the spinal cord cannot be protected properly by the spine.

Within this type of alterations we can find subjects whose alteration is not visible (hidden), although it may have holes or bumps on the back, and others who have a directly perceptible hole (cystic or open). The closer to the brain, the more serious the possible nerve damage may be.

2. Anencephaly

Another well-known neural tube disorder and defect is anencephaly. In this case, we observe that the flow part of the neural tube has not closed completely. This alteration is usually incompatible with life, and it is not uncommon for abortions to occur or to have a very short life expectancy after birth . However, in some cases survival is longer. Anencephalic subjects cannot perform complex cognitive and sensory functions, not being aware of their environment or themselves and in most cases not being able to perceive (although they may have reflexes).

3. Encephalocele

Alteration produced by problems during the closure of the rostral end of the neural tube. Equivalent to spina bifida but in the skull, it implies the existence of a protrusion of part of the brain’s contents towards the outside of the skull , generally presenting a kind of sack or lump in the head with said contents. In most cases, cognitive alterations are generated, and it is not uncommon for the child to die during fetal development.

4. Chiari malformation

It is common for the presence of alterations in the development and closure of the neural tube to generate the so-called Chiari malformations, which consist of a protrusion of part of the cerebellum or part of the brain into the spinal canal, being displaced by some type of structural malformation of the skull or brain. In other words, part of the brain’s contents invades and occupies the spinal canal. It may not generate symptoms, but it may also generate pain, balance, vision and coordination problems and paresthesia.

Bibliographic references

  • López, N. (2012) Biología del desarrollo. Workbook, McGraw-Hill Education.