The complexity of our nervous system, a fundamental system that links and governs all processes in our body, is something that continues to amaze the many researchers and experts who study it every day. But one fact must be kept in mind, and that is that if we think of a nervous system usually comes to mind with a mature structure, we need a series of processes because we are hardly more than a group of cells reaching a mature nervous system.
Throughout embryonic and fetal development, they will produce a series of events that will trigger the formation of the so-called neural tube, which in turn will develop during pregnancy to the point of generating the structures specific to the human nervous system
What is the neural tube?
It is known as the neural tube a the structure that forms during pregnancy and is the immediate predecessor of the nervous system, Being its closure and its evolution which will end up generating the various structures that are part of it. Specifically, we are talking about the brain and the spinal cord, along with others like those of the peripheral nervous system formed by neural crests.
Technically, the process in which the neural tube is generated and closed would start as early as 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 the skull protect the nerves and the brain and that they form. This closure usually occurs correctly in most births, although sometimes it fails to close the tube, which can lead to various neural tube defects.
Neurulation: formation and evolution of the neural tube
The neural tube it occurs throughout a process known as neurulation, Wherein the notochord and mesoderm as a whole lead the ectoderm to differentiate into neuroextoderm. This thickens and eventually pulls away from the sheet of cells, forming the neural plate.
This plaque will continue to stretch rostrocaudally, so as to generate folds which, with the development of the fetus, developed. Over time, there is a collapse of the central part, generating a channel that the walls close on themselves to generate a tube-like structure: the neural tube. This tube begins to close on itself by the middle part, advancing towards the ends. In this process the neural crests separate and also detach from the tube, Which will eventually generate the autonomic nervous system and different organs and tissues of different bodily systems
Initially, the tube will be opened at its ends, forming the rostral and flow neuropores, but from the fourth week onwards they will begin to close. This closure and the development of the tube will cause several dilations in its face-cranial part, which in the future forms the different parts of the brain. The rostral end is usually closed first, around the 25th, while the causal one usually closes around the 27th.
There is a second process of neurulation, called secondary, in which the part of the neural tube corresponding to the spine is formed and softened at the same time so that the internal cavity of said tube empties, generate a separation between the epithelium and the mesenchymal cells (Which will form the spinal cord). In the spinal cord we find that in the ventral part the motor neurons appear, while the sensory ones appear in the most dorsal part of it.
Formation of different regions of the brain
Throughout the formation and development of the neural tube, the structures that are part of our adult nervous system will be produced. The cells of the neural tube, when closed, begin to divide and generate different layers and structures. It will be in the anterior or cranial face of the tube in which the brain will appear.
During the fourth week of gestation, they can observe the forebrain, midbrain and rhomboencephalon. During the fifth, the first and third are separated from them, forming the telencephalon and diencephalon in the first and the metencephalon and myelencephalon the second. Relatively rapidly, the structure changes heterogeneously, causing the different structures to grow (being the telencephalon, the proper part of the crust, the one that develops the most).
It should be borne in mind that not only the wall of the neural tube is important, but also the interstices and empty spaces present inside: they will eventually form the ventricles and the set of structures through which the cephalo fluid -spinal will circulate, without which the brain could not function.
The process of neurulation, in which the structure of the nervous system is formed, is fundamental for humans. However, in him sometimes they can produce alterations and malformations which can have more or less serious consequences on the development and survival of the fetus. Among them, some of the best known are as follows.
1. Spina bifida
One of the most common neural tube defects and known is spina bifida. This alteration involves the existence of some type of problem that prevents part of the neural tube from closing completely, which can have effects of varying severity by not being able to properly protect the nerves and spinal cord of the spine. .
In 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 noticeable hole. (cystic or open). The closer you get to the brain, the more serious the possible nerve damage can be.
Another of the most well-known neural tube alterations and defects is anencephaly. In this case, it is observed that the flow part of the neural tube is not completely closed. This alteration is generally incompatible with life, and it is not uncommon for abortions to occur or have a very short life expectancy after birth. However, in some cases the survival is longer. Anencephalic subjects cannot perform complex cognitive and sensory functions without being aware of the environment or themselves and in most cases not being able to perceive (although they may have reflexes).
Alteration caused by problems closing the rostral end of the neural tube. Equivalent to spina bifida but in the skull it assumes the existence of a protrusion of part of the contents of the brain outwards from the skull, Usually presenting / displaying some sort of bag or package in the head with this content. In most cases, cognitive impairment is 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, some of the brain’s content invades and occupies the spinal canal. This can not only cause symptoms, but also pain, problems with balance, vision and coordination, and paresthesia.
- López, N. (2012) Developmental biology. Workbook, McGraw-Hill Education.