The development of the nervous system during pregnancy

The development of the nervous system begins early in pregnancy. Initially, neurons are undifferentiated cells from any other, but the interaction of various factors makes evolve and form an elaborate web of synaptic connections that will allow the coordination of the functions of the organism.

Let's see what this process consists of and what are the main phases of the formation of the system in the prenatal stage of the life of a human being.

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The formation of the nervous system

Fertilization consists of the penetration of a sperm into the egg after reaching the fallopian tubes. Although initially the two gametes form a single cell (the zygote)During the first days of pregnancy, it divides successively, giving rise to a set of cells that is called morula.

When the zygote is implanted in the uterus, the division of its cells begins to give rise to the embryo and the placenta; during this period we refer to the embryo as a “blastula”. This moment represents the beginning of cell differentiation.

In the initial weeks of pregnancy, the embryo is made up of three layers of cells which are called endoderm, mesoderm and ectoderm respectively. Throughout intrauterine development, the body will be formed from these cellular assemblages.

The endoderm layer progressively becomes the respiratory and digestive systems, while the mesoderm gives rise to bones, muscles, circulatory system, and the notochord, from which the spine develops vertebral. The nervous system and skin arise from the ectoderm, the outermost layer of the three.

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The development of the neural tube

During the first weeks, the ectoderm evolves into a flat oval plate. This plate has a cleft, the neural groove, which will give rise to the neural tube when the segments of the plate join.

The peripheral nervous system arises from the neural crests, portions of the oval plate that separate from it as the neural tube closes. The neural tube will later become the medullary canal. and in the cerebral ventricles; the central nervous system will emerge from its walls.

Towards the end of the first month of gestation the anterior part of the neural plate divides into three sections that soon after will form the brain: the forebrain will become the cerebral cortex, the thalamus, the hypothalamus and basal ganglia, the midbrain in the brainstem and rhombencephalon in the cerebellum, pons, and medulla oblongata.

Neuronal proliferation, migration and differentiation

The ventricular zone is located on the inside of the wall of the neural tube, where cell proliferation occurs. This phenomenon, which will continue until birth, consists of the production of large numbers of nerve cells (neurogenesis) by successive mitosis or cell divisions.

At this point the neural cells are still undifferentiated. Although many will remain in the neural tube for the time being and will transform into neurons later on, others will become glial cells and they will move to other regions.

Neuronal migration consists of the movement of neuroblasts, primordial neural cells very similar to “stem cells”, from the ventricular zone of the neural tube to their respective destinations in other parts of the brain. The radial glia allows migration as future neurons move through its processes.

Upon reaching their final position, the neuroblasts begin to transform into different types of neurons depending on the genetic information they contain, the area where they are and the neurons around them (which is known as "induction"); this process is cell differentiation.

Synaptogenesis, apoptosis, and reorganization

The dendrites and axons of neurons have extensions, growth cones, which adhere to surfaces in order to promote neuron growth. Neurotrophic factors are involved in this process, chemical substances that when released by neurons attract or repel axons.

When the axons reach their destination they begin to branch, connecting with other nearby cells; thus begins synaptogenesis or synapse formation, which will develop definitively after birth, thanks to the influences of learning.

During initial neuronal proliferation and synaptogenesis, an excessive number of neurons and synapses are formed, yet allow all the basic connections to take place. Once these processes have finished apoptosis or programmed neuronal death occurs, which causes between 20 and 80% to degrade to death.

Apoptosis mainly affects the "weaker" neurons, that is, those that have not synapt with other cells or that have not been attracted by neurotrophic factors. This ensures that only the most efficient and robust connections are maintained.

After neuronal death, the synapses are reorganized: some of the connections that had been established are canceled and new ones appear until they a complex and highly interconnected neural network is constituted that will continue to evolve and perfecting itself during growth.

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Myelination and nerve conduction

In the fourth month of gestation, glial cells begin to form myelin sheaths around axons. This substance increases the speed of transmission of nerve impulses, in addition to protecting the axons.

Myelination begins in the peripheral nervous system. Later it is produced in the upper part of the spinal cord, from where it spreads to the lower and upper sections of the future body.

The nerves associated with motor skills are myelinated earlier than those associated with sensation; this is why babies are born with basic reflexes. The myelination process will intensify during the first few months after birth and will continue later, at least until puberty.