Development of the nervous system begins in early pregnancy. Initially, neurons are cells undifferentiated from others, 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 body.
Let’s see what this process consists of and what are the main phases in the formation of the system in the prenatal stage of human life.
The formation of the nervous system
Fertilization involves 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 called a 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 time, we call the embryo a “blastula”. This moment marks the start of cell differentiation.
During the first weeks of pregnancy, the embryo is made up of three layers of cells called endoderm, mesoderm and ectoderm, respectively. Throughout intrauterine development, the body will be formed from these sets of cells.
The endoderm layer gradually becomes the respiratory and digestive tract, while the mesoderm gives rise to bones, muscles, circulatory system and notochord, from which the spine develops. The nervous system and skin originate from the ectoderm, The outermost layer of the three.
The development of the neural tube
During the first few weeks, the ectoderm evolves into a flat oval plaque. This plate has a fissure, the neural groove, which will cause the neural tube to join the segments of the plate.
The peripheral nervous system arises from the neural crests, parts of the oval plaque that separate from it when the neural tube closes. The neural tube will later become the spinal canal and in the cerebral ventricles; from its walls will be born the central nervous system.
Towards the end of the first month of gestation, the anterior part of the neural plate divides into three sections that will soon form the brain: the forebrain will become the cerebral cortex, the thalamus, the hypothalamus, and the basal ganglia. Mesencephalon in the brainstem and hindbrain in the cerebellum, protuberance and medulla oblongata.
Neuronal proliferation, migration and differentiation
On the inner side of the wall of the neural tube is the ventricular zone, where cell proliferation occurs. This phenomenon, which will continue until birth, consists of the production of large amounts of nerve cells (neurogenesis) by mitoses or successive cell divisions.
At this point, the neural cells are still undifferentiated. While many will stay in the neural tube for now and turn into neurons later, others will become glial cells and move to other regions.
Neuronal migration is the movement of neuroblastsVery similar to “stem cells”, primordial neural cells from the ventricular area of the neural tube to their respective destinations in other parts of the brain. Radial glia allows for migration as future neurons move through their extensions.
Upon reaching their final position, neuroblasts begin to transform into different types of neurons depending on the genetic information they contain, the area they are in, and the neurons around them (so-called “induction”); this process is cell differentiation.
Synaptogenesis, apoptosis and reorganization
The dendrites and axons of neurons have extensions, the growth cones, which adhere to surfaces in order to promote the growth of the neuron. Neurotrophic factors are involved in this process, Chemicals that, when released by neurons, attract or repel axons.
When axons reach their destination, they begin to branch out, 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 amount of neurons and synapses are formed, which nonetheless allows all basic connections to take place. Once these processes are completed programmed apoptosis or neuronal death occurs, Which causes between 20 and 80% to degrade to death.
Apoptosis mainly affects “weaker” neurons, that is, those that have not synchronized with other cells or have not been attracted to neurotrophic factors. This only keeps the most efficient and strong connections.
After neuronal death, the synapses are reorganized: some of the established connections are canceled and new ones appear until it constitutes a complex and highly interconnected neural network that will continue to evolve and refinement during growth.
Myelination and nerve conduction
By 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 axons.
Myelination begins in the peripheral nervous system. It then occurs in the upper part of the spinal cord, from where it spreads to the lower and upper parts of the future body.
Motor nerves are myelinated earlier than those associated with sensation; that’s why babies are born with basic reflexes. The process of myelination will intensify during the first months after birth and continue thereafter, at least until puberty.