Neurulation is the process by which the neural tube is formed during intrauterine development. The neural tube is essential for the differentiation of cells of the central nervous system, while the neural ridges, structures associated with that in question, are intended for the formation of the peripheral nervous system.
In this article we will describe the two phases of neurulation or neural tube formation: The primary, in which the neural plate begins to fold into itself, and the secondary, which culminates this process and allows the further development of the nervous system.
What is the neural tube?
The neural tube is an embryonic structure that forms during the first month of gestation; more precisely, the tube just closed around week 28 after fertilization. It is the precursor of the central nervous system, Composed of the brain and spinal cord.
As embryonic development progresses, the neural tube is divided into four sections: forebrain (forebrain), middle (midbrain), posterior (hindbrain), and 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 tubeThe space between the walls is also relevant: the neurocele or the neural channel. This structure will be gradually transformed in the ventricles and the rest of the brain cavities, through which cerebrospinal fluid circulates.
After fertilization, the zygote is formed, the original cell made up of the fusion of an egg and a sperm. The zygote divides successively, becoming a collection of cells called a morula. Later, the blastocoel, a cavity full of fluid, appears inside this structure; when this happens, we speak of “blastula”.
later the blastula is divided into three layers: endoderm, mesoderm and ectoderm. Each of these sections will give rise to different parts of the body. The ectoderm is the most important for the question before us, because from there the nervous system develops, both central and peripheral.
The notochord, a structure located in the mesoderm, sends signals to the cells around it. Those that do not receive these signals transform into a neural plate or neuroextoderm, a collection of cells already specialized in nerve functions. The word “plaque” refers to the flattened appearance of the neuroextoderm.
Primary neurulation consists of the overgrowth of nerve cells in the neural plate. These cause the transformation of the plaque into a neural tube, a key stage in the development of the human body.
Formation and closure of the neural tube
During the process of neurulation, the neural plate flattens, elongates and folds back on itself around the neural groove, which ends up being U-shaped as the walls rise, forming neural crests and neural tube. At this point in the process, the tube is open at both ends; we refer to the flow and the rostral neuropores.
It is normal for these openings to close after a few days; But, 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 brain deformities)
It is important to differentiate the neural tube from the neural crest because the former transforms into most structures of the central nervous system, while the peripheral is a progression of the neural crest.
Secondary neurulation is the process that results in the formation of the neural tube. This is not due to signals sent by certain cells, as in the case of primary neurulation, but results from the development of the neural tube itself.
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 their peripheral region. As these cells differentiate, cavities form between the two sets.
The mesenchymal cells located in this part of the embryo condense and form what we call the spinal cord; this, in turn, softens inside until it gives way to the neural tube cavity. This phenomenon it starts in the sacred region of the spine.
Thus, while the primary neurulation consists in the folding of the neural plate on itself, the secondary 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.