The Development Of The Nervous System During Pregnancy
The development of the nervous system begins at the beginning of pregnancy Initially neurons are cells undifferentiated from any other, but the interaction of various factors causes them to evolve and form an elaborate tissue 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.
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 group of cells that is called morula.
When the zygote implants 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 marks the beginning of cell differentiation.
In the initial weeks of pregnancy, the embryo is made up of three layers of cells, respectively called endoderm, mesoderm and ectoderm. Throughout intrauterine development the body will be formed from these cell groups.
The endoderm layer progressively becomes the respiratory and digestive systems, while the mesoderm gives rise to bones, muscles, the circulatory system and the notochord, from which the spinal column develops. The nervous system and skin arise from the ectoderm the outermost layer of the three.
During the first weeks the ectoderm evolves until it becomes the flat oval plate. This plate has a groove, the neural groove, which will give rise to the neural tube when the segments of the plate join together.
The peripheral nervous system appears from the neural crests, portions of the oval plate that separate from it when the neural tube closes. The neural tube will later become the spinal 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 shortly after will form the brain: the forebrain will become the cerebral cortex, the thalamus, the hypothalamus and the basal ganglia, the mesencephalon will become the brain stem and hindbrain in the cerebellum, pons and medulla oblongata.
Neuronal proliferation, migration and differentiation
The ventricular zone is located on the inner side 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) through successive mitosis or cell divisions.
At this point the neural cells are still undifferentiated. Although many will remain in the neural tube for now and become neurons later, others will become glial cells and move to other regions.
Neuronal migration consists of the movement of neuroblasts , primitive 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. Radial glia allow migration since future neurons move through their 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 in which they are located 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 the growth of the neuron. Neurotrophic factors intervene 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; This is how synaptogenesis or the formation of synapses begins, 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 basic connections to take place. Once these processes have finished apoptosis or programmed neuronal death occurs which causes between 20 and 80% to degrade until they die.
Apoptosis mainly affects the “weaker” neurons, that is, those that have not synapsed 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 A complex and highly interconnected neural network is formed that will continue to evolve and perfecting during growth.
Myelination and nerve conduction
In the fourth month of gestation, glial cells begin to form myelin sheaths around the axons. This substance increases the speed of transmission of nerve impulses, in addition to protecting the axons.
Myelination begins in the peripheral nervous system It subsequently occurs in the upper part of the spinal cord, from where it spreads to the lower and upper sections of the future body.
Nerves associated with motor skills become myelinated before those associated with sensation; This is why babies are born with basic reflexes. The myelination process will intensify during the first months after birth and will continue thereafter, at least until puberty.
