Neurulation: the process of formation of the neural tube
Neurulation is the process by which the neural tube is formed during intrauterine development. The neural tube is fundamental for the differentiation of the cells of the central nervous system, while the neural crests, associated structures to which we are concerned, are for the formation of the peripheral nervous system.
In this article we will describe the two phases of neurulation or formation of the neural tube : the primary, in which the neural plate begins to retract on itself, and the secondary, which culminates this process and allows the further development of the nervous system.
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What is the neural tube?
The neural tube is an embryonic structure that forms during the first month of gestation; specifically, the tube has just closed around week 28 after fertilization. It is the precursor of the central nervous system , composed of the brain and spinal cord.
As the embryonic development progresses, the neural tube is divided into four sections: the anterior brain (forebrain), the middle (mesencephalon), the hindbrain (hindbrain), and the spinal cord. Each of these parts will progress to give rise to the different elements that make up the adult central nervous system.
While most of the nervous system develops from the walls of the neural tube , the gap between the walls is also relevant: the neurocele or neural channel. This structure will progressively transform into the ventricles and the rest of the brain cavities, through which the cerebrospinal fluid circulates.
Primary neurulation
After fertilization, the zygote is formed, the primitive cell composed of the fusion of an egg and a sperm. The zygote divides successively, becoming a set of cells called a morula. Subsequently the blastocoel appears, a cavity filled with fluid, within this structure; when this happens we speak of "blastula".
Later The blastula is divided into three layers: the endoderm, the mesoderm and the ectoderm . Each of these sections will lead to different parts of the body. The ectoderm is the most important for the subject that concerns us, since from it the nervous system develops, both central and peripheral.
The notochord, a structure that is located in the mesoderm, sends signals to the cells that are around it. Those that do not receive these signals are transformed into the neural plate or neuroectoderm, a set of cells that have already specialized in nerve functions. The word "plaque" refers to the flattened appearance of the neuroectoderm.
Primary neurulation consists of the proliferation of nerve cells in the neural plate . These make the plaque transform into the neural tube, a fundamental step in the development of the organism of human beings.
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Formation and closure of the neural tube
During the process of neurulation, the neural plate flattens, lengthens and folds over itself around the neural groove, which ends up being U-shaped as the walls rise, forming the neural crests and the neural tube . At this moment of the process the tube is open at both ends; we refer to the caudal and rostral neuroporos.
The normal thing is that these openings close after a few days; but nevertheless, Sometimes the tube does not close properly , which leads to disorders such as spina bifida (which affects the spine) and anencephaly (associated with very serious malformations in the brain).
It is important to differentiate the neural tube from the neural crest because the former is transformed in most structures of the central nervous system, while the peripheral is a progression of the neural crest.
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Secondary neurulation
Secondary neurulation is the process that culminates the formation of the neural tube . This is not due to the signals sent by certain cells, as happens with primary neurulation, but occurs as a result of the development of the neural tube.
This process is associated with the division of neural tube cells between mesenchymal and epithelial cells. The first are located in the central part of the tube, and the second in its peripheral region. As these cells differentiate, cavities form between the two sets.
The mesenchymal cells that are located in this part of the embryo condense and form what we know as cord medulla; This, in turn, hollows inside to make way for the cavity of the neural tube. This phenomenon It starts in the sacral region of the spine .
Thus, while the primary neurulation consists in the retraction of the neural plate on itself, the secondary one corresponds to the emptying of the cavity of the neural tube, very associated with the differentiation of the cells of the nervous system of the embryo.
