The development of the nervous system: stages and characteristics

The formation of our body is a long process that never stops: new cells are constantly being born and others dying. The development of the nervous system is one of the stages. Through it, tissues and organs are formed that later allow us to generate ideas, behaviors and emotions after we are born.

The development of our nervous system occurs at the beginning of pregnancy. It is a complex and specialized process that begins with the first nerve cells. The training process occurs in various phases and goes through different critical periods. Let’s explore what each one is about and what are the characteristics and pathologies associated with this training process.

The beginning of the development of the nervous system

For the nervous system to begin its development, it is essential that the following conceptual processes occur first:

Fertilization. This is the beginning of embryogenesis or embryonic development. It occurs in the fusion of the sperm and the egg. Additionally, it can occur up to three days after intercourse. Initially, the two gametes form a single cell called a zygote.Segmentation. This is the segmentation of the zygote within the uterus. It is carried out through mitosis, which allows the division of the zygote into two identical cells formed by blastomeres. Blastomers are the cells that originate from the first division of the egg.Morulation. As a consequence of the segmentation and formation of the blastomeres, the morula is triggered. It is a structure that has approximately 12 to 16 cells. And, it owes its name to its shape, similar to a small blackberry. In addition, it is surrounded by two membranes: the innermost one, called the membrane pellucida, and the outermost one, the corona radiata.Blastulation. Here the zygote has reached a large number of cells. The blastula is formed, which appears in the shape of a ball, with a cavity inside called a blastocoel.Gastrulation. It consists of the formation of the gastrula. It occurs in the third week. Additionally, during this stage three layers are formed: Endoderm. Innermost layer of the embryonic disc. Mesoderm. Intermediate layer. Ectoderm. Outermost layer of the embryonic disc.

The nervous system develops from the ectoderm, just like our skin. Each of the other layers gives rise to different parts of our body.

The formation of the neural tube, a key process in the development of the nervous system

The ectoderm develops until it becomes the flat oval plate, which consists of a groove called the neural groove, which is what will give rise to the neural tube. Then, the plate begins to fold on itself and a groove appears surrounded by two folds. These fuse and close the sulcus and form the neural tube, which initially has two openings called neuropores (rostral and caudal), which disappear after the neural tube completely fuses.

When the neural tube closes, neural crests are formed, which will give rise to the peripheral nervous system.. It consists of the external part of each of the folds being separated from the tube and the nearby ectoderm.

The neural tube begins to curve and three vesicles appear: the forebrain, the midbrain, and the hindbrain. Later the forebrain divides into two: telencephalon and diencephalon. And, in the hindbrain, two vesicles are formed: the metencephalon and the myelencephalon.

Proliferation, migration and differentiation

Proliferation begins in the ventricular zone. It is a process in which there is a large generation of nerve cells: neurogenesis. At this stage the cells are not yet differentiated.

Now, a large number of immature neurons migrate, and are guided by the radial glia cells. In fact, they move along glial extensions. When they reach the final position, they begin to differentiate into types of neurons, depending on the genetic information they contain, the place where they are located, and the nearby neurons.

Synaptogenesis and myelination

Growth cones are the extensions of axons and dendrites that promote the growth of neurons. Neurotrophic factors participate in this process. That is, chemical substances that are responsible for repelling or attracting axons.

Axons begin to branch once they reach their destination and make connections with other cells.. This is the process of synaptogenesis, which will finish developing after birth due to the experience of the individual.

Apoptosis also occurs. It is a process through which there is programmed neuronal death. Thanks to this, the best connections are preserved. And, after neuronal death, the synapses are reorganized and new ones appear that will continue to develop during growth.

In addition to these processes, myelination occurs. It consists of the formation of myelin sheaths that cover the axons. Now, myelin is a substance that favors the transmission of impulse. In addition, it protects the axons. So, with its help, nerve conduction occurs.

Pathology of the development of the nervous system

If the development process of the nervous system does not occur properly, certain pathologies may arise. Let’s look at some of them:

Hydrocephalus. When it occurs congenitally, it consists of the inadequate development of the ventricular system of the brain. This causes it to tend to accumulate cerebrospinal fluid, manifesting itself in a larger head than usual and giving rise to motor and sensory deficits.Dandy Walker Disorder. This malformation affects the cerebellum and the cavities that surround it. Symptoms associated with movements and coordination, irritability, abnormal breathing patterns and increased intracranial pressure, among others, may occur.Cerebral cortex alterations. For example, porencephaly or the appearance of clefts in the cerebral hemispheres or lissencephaly, characterized by an alteration in the brain migration process that causes an abnormal brain appearance. All of these malformations cause motor and cognitive alterations.spina bifida. It occurs due to incomplete closure of the caudal portion of the neural tube. In the process, one or more vertebral arches do not fuse correctly and the spinal cord is left unprotected. The cause could be folic acid deficiency during pregnancy.Encephalocele. It is a disorder caused by poor closure of the neural tube. Causes cognitive and motor deficit. It consists of a sac-shaped lump formed by the exit of the brain and the membranes that surround it.

New Horizons

Various research is being carried out on the relationship between the development of the nervous system and brain plasticity, to see if it can adapt to certain abnormal conditions, and find a route to solve congenital problems. For example, Morga and collaborators, in their article published in the Journal of Neurology they emphasize the link of synaptogenesis, neural plasticity, and its implication in intellectual disability.

Another study focuses on the role of thyroid hormones in the formation process of the nervous system. Through specific nuclear receptors, they are expressed early in the embryonic phase to give rise to the cerebral cortex, hippocampus, cerebellum, retina, ear and hypothalamus, among others.

As you can see, the nervous system is one of the master systems when it comes to the functioning of the body. Although we are taught each of them separately at school, the truth is that the entire body is connected and some parts cannot be conceived without others. And you, did you know these data?

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