The development of the nervous system begins at the beginning of gestation . Initially the 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 the main phases of the formation of the system are in the prenatal stage of a human being’s life.
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 this cell divides successively, giving rise to a group of cells which is called a 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 is the beginning of cell differentiation.
In the early weeks of pregnancy the embryo is made up of three layers of cells which are called the endoderm, mesoderm and ectoderm respectively. Throughout the intrauterine development the body will be formed from these cell groups.
The endoderm layer progressively becomes the respiratory and digestive system, while the mesoderm gives rise to the bones, muscles, circulatory system and notochord, from which the spine develops. The nervous system and skin arise from the ectoderm , the outermost layer of the three.
The development of the neural tube
During the first weeks the ectoderm evolves into the flat oval plate. This plate has an indentation, the neural groove, which will give rise to the neural tube when the segments of the plate are joined.
The peripheral nervous system appears from the neural ridges, portions of the oval plate that separate from it when the neural tube closes. The neural tube will later become the spinal canal and the cerebral ventricles; from its walls will emerge the central nervous system.
Towards the end of the first month of gestation, the anterior part of the neural plate is divided into three sections that will soon form the brain: the prosencephalon will become the cerebral cortex, the thalamus, the hypothalamus and basal ganglia, the midbrain the brainstem, and the rhombo-brain the cerebellum, the medulla and the spinal cord.
Neuronal proliferation, migration and differentiation
On the inner side of the neural tube wall is located the ventricular area, where cell proliferation occurs. This phenomenon, which will continue until birth, consists of the production of large quantities 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 the time being and will transform into neurons later, others will become glial cells and move to other regions.
Neuronal migration consists of the movement of neuroblasts , primal neural cells very similar to “stem cells”, from the ventricular area of the neural tube to their respective destinations in other parts of the brain. The radial glia allows for migration as future neurons move through their extensions.
When neuroblasts reach their final position, they 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 (known as “induction”); this process is called cell differentiation.
Synaptogenesis, apoptosis and reorganization
The dendrites and axons of the neurons have extensions, the growth cones, which adhere to surfaces in order to favour the growth of the neuron. This process involves neurotrophic factors , chemicals that are released by neurons and attract or repel axons.
When the axons reach their destination, they begin to branch out, connecting with other nearby cells; thus begins the synaptogenesis or formation of synapses, which will develop definitively after birth, thanks to the influences of learning.
During the initial neuronal proliferation and synaptogenesis an excessive amount of neurons and synapses are formed, which however allows all the basic connections to take place. Once these processes have been completed , apoptosis or programmed neuronal death occurs, causing between 20 and 80% of them to degrade to death.
Apoptosis mainly affects the “weakest” neurons, that is, those that have not synaptized with other cells or have not been attracted by neurotrophic factors. This means that only the most efficient and solid connections are maintained.
After neuronal death, the synapses are reorganized: some of the connections that had been established are annulled and new ones appear until a complex and highly interconnected neural network is formed that will continue to evolve and be perfected during growth.
Myelinization 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 and protects the axons.
Myelinization 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 sections of the future body.
The nerves related to motor skills myelinize before 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 continue thereafter, at least until puberty.