Neural Tube: What It Is, How It Is Formed, And Associated Diseases
The complexity of our nervous system, a fundamental system that connects and governs all the processes of our body, is something that continues to amaze the many researchers and experts who study it every day. But we must take into account a fact, and that is that although when we think of a nervous system, an already mature structure generally comes to mind, it is necessary that a series of processes take place since we are little more than a cluster of cells. to reach a mature nervous system.
Throughout embryonic and fetal development, a series of events will occur that will trigger the formation of the so-called neural tube, which in turn will develop during pregnancy until generating the structures of the human nervous system
It is known as a neural tube. the structure that is formed during gestation and is the immediate predecessor of the nervous system, its closure and evolution being what will end up generating 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 begin from the third week of gestation and should finish closing approximately around the twenty-eighth day. It must be taken into account that it is essential that the tube closes so that the spine and skull can protect the nerves and brain and so that they can form. This closure usually occurs correctly in the majority of births, although sometimes the tube fails to close, which can give rise to different neural tube defects.
Neurulation: formation and evolution of the neural tube
The neural tube occurs through a process known as neurulation, in which the notochord and the entire mesoderm lead the ectoderm to differentiate into neuroectoderm. This thickens and ends up detaching from the cell sheet, forming the neural plate.
This plate will proceed to stretch in a rostrocaudal manner, in such a way that it will generate folds, which will grow with the development of the fetus. Over time, a sinking of the central part occurs, generating a channel whose walls will close on themselves until generating a tube-shaped structure: the neural tube. Said tube begins to close on itself in the middle part, moving towards the ends. In this process The neural crests also separate and detach 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 after the fourth week they begin to close. Said enclosure and the development of the tube will generate various dilations in its cranial aspect, which in the future configures the different parts of the brain. The rostral end generally closes first, around day 25, while the causal end usually closes around day 27.
There is a second process of neurulation, the so-called secondary, in which the part of the neural tube corresponding to the spinal column is formed and at the same time hollowed out in such a way that the internal cavity of said tube is emptied. generating a separation between epithelium and mesenchymal cells (which will form the medullary cord). In the spinal cord we find that motor neurons appear in the ventral part, while sensory neurons appear in the most dorsal part of it.
Formation of 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. It will be in the anterior or cranial part of the tube in which the brain will appear.
During the fourth week of gestation, the prosencephalon, mesencephalon and rhombencephalon can be observed During the fifth, the first and third divide, forming telencephalon and diencephalon in the first and metencephalon and myelencephalon in the second. In a relatively rapid manner, the structure changes in a heterogeneous manner, with the different structures growing (the telencephalon, the part of the cortex, being the one that develops the most).
It must be taken into account that not only the wall of the neural tube is important, but also the gaps 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 neurulation process, in which the structure of the nervous system is formed, is fundamental for human beings. However, in it Sometimes alterations and malformations may occur which can have more or less severe consequences on the development and survival of the fetus. Among them, some of the best known are the following.
1. Spina bifida
One of the most common neural tube defects and known is spina bifida. This alteration implies the existence of some type of problem that prevents a part of the neural tube from closing completely, something that can have effects of varying severity as the nerves and spinal cord cannot be properly protected by the spinal column.
Within this type of alterations we can find subjects whose alteration is not visible (occult), although they may have holes or protuberances on the back, and others who have a directly perceptible hole (cystic or open). The closer it is to the brain, the more serious the possible nerve injuries may be.
Another of the best-known neural tube disorders and defects is anencephaly. In this case, we observe that the caudal part of the neural tube has not closed completely. This alteration is usually incompatible with life, and it is not strange that abortions occur or 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 the environment or themselves and in most cases not being able to perceive (although they may have reflexes).
3. Encephalocele
Alteration caused by problems during the closure of the rostral end of the neural tube. Equivalent to spina bifida but in the skull, it means the existence of a protrusion of part of the contents of the brain towards the outside of the skull, generally presenting a kind of sac or lump on the head with said content. In most cases, cognitive alterations are generated, and the death of the child during fetal development is not uncommon.
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 towards 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 invade and occupy the spinal canal. It may not cause symptoms, but it can also cause pain, problems with balance, vision and coordination, and paresthesia.
