Neurodevelopment

Some say the process of underdevelopment, begins long before conception since it depends on the primary cells and their conditions so that it can develop properly the Nervous System Central

The race begins with the meeting between the sperm and the ovule. Both cells must contain a specific genetic load and determined to prevent hazards effacement, you overlap, mosaicism or lack of alleles or genetic material elsewhere. Both cells combine their material, resulting in a single cell.

Also it is known that there must be a correct proteinaceous  load  to ensure the success of the process design.

So that what generates the diversity of races and physical features is genetic recombination which undergoes each generation, but each individual is genetically different from everyone else (except if you have an identical twin), since the variety of eggs or sperm that are formed along the life is so great that for practical purposes only can say that none of them is equal to the other. Thus, mutations are the raw material of genetic diversity, but is  even greater and less controllable in species with sexual reproduction, facing all the time different genomes .

Subsequent to this process is said that the Meiosis which is a process of cell division in which a diploid cell (2n) undergoes two successive divisions, with the capacity to produce four haploid cells (n).

This process is carried out in two divisions nuclear and cytoplasmic, called first and second division meiotic or simply meiosis I and meiosis II.Both are part of  of the prophase, metaphase, anaphase and telophase.

In the interface is duplicated genetic material is shared while that homologous chromosomes are divided into two daughter cells in meiosis I , the phenomenon of cross-breeding.

Once you pass this stage, it is possible the beginning of meiosis II, like in a mitosis, each chromatid migrates to a pole. The result is 4-cell daughters haploid (n).

During meiosis are matched member of each homologous pair of chromosomes during prophase, forming bivalent. During this phase it developed a protein structure called the synaptonemal complex, allowing recombination between two homologous chromosomes that occurs during this phase. 

Subsequently a large chromosomal condensation occurs and the bivalent are situated on the equatorial plate during the first metaphase, resulting in the migration of n chromosomes to each of the Poles during the first anaphase. 

This reduction division is responsible for the maintenance of the characteristic of each species chromosome number. 

In meiosis II, the chromatids that form each chromosome separate and are distributed to the daughter cells nuclei. Between these two successive stages there is no stage S (DNA replication). The maturation of the daughter cells gives rise to the gametes.

Something important to note in this regard is that the genome of a human normal consists of 23 pairs of chromosomes, the inherited by mother and father inherited that form each pair, but in total there are 24 pairs of chromosomes that 2 correspond to the sex chromosomes X and the and, which combine in XX if you are female and XY if it’s a male.

All this takes place in a relatively short period of time and in spite of being a process necessary for the reproduction of the human species, is not a perfect process; errors in meiosis are sometimes responsible for the main chromosomal anomalies. Meiosis manages to keep constant the number of chromosomes in the cells of the species to maintain the genetic information. In general, members of a chromosome pair are not in close proximity either at rest or during mitotic division cell. The only time they enter into intimate contact is during the meiotic divisions or germ cell maturation.

This process continued during the following weeks the cells begin to migrate and give way to another process called referred to as phase of cell proliferation to one in which the cells that compose the Nervous System (neurons and glial cells) originate or are born.

Of the different stages of Morphogenesis is this which can properly be considered as the phase of neurogenesis.

Since it is known that the development of the human brain starts very early, around 3 to 4th week of gestational age and continues, although at a declining rate, until adulthood. And this development is characterized by the occurrence of 2 major organizational events. 

The first begins with the conception and includes neuroregulation events, proliferation, migration, and differentiation, the second occurs after birth. It has been proposed that these events are controlled by genetic factors and epigenetic (non-mutational phenomena but that vary the expression of a gene, such as the development of proteins or blocking of certain neurotransmitters) that originate neural structures sensitive to external influences.

 In humans this stage of development occurs in the fourth week of gestation from the neuroepithelium, which is made up of the calls of CNS stem cells. This stem cell progenitor, which also glioblasts or immature neurons produce called cells. Once born neurons, that as it has been said are still immature, they lose their reproductive ability. The glioblasts, however, retain their reproductive capacity throughout life.

This phase covers until about the fifth month of gestation; although we cannot forget that it does not occur simultaneously in all neural tube, but that each region has its own period of neurogenesis. The process does not end there, but rather so that we can properly talk of nervous system cells that compose it still must go through different times.

After this phase of cell proliferation occurs cell migration, in which nerve cells migrate to their final location; the radial glia is the support through which neurons can reach their final location. 

Cells in these phases are still undifferentiated, so go to the stage of neuronal differentiation to acquire the morphological and physiological characteristics of the mature neuron. Also, establish different connections (synapses), while the development establishes many more synapses than necessary during synaptogenesis, with which many of these connections are subsequently eliminated. In addition, during fetal development the human creates many more neurons than needs, so those that are functionally superfluous die (this neuronal death is known as neuronal apoptosis and can reach between 25% and 75% of neurons created).

It is so nervous tissue formation begins with the formation of a simple tube, the so-called neural tube and from the induction of the neuroectoderm (this is part of the ectoderm that is the outermost cell primary embryo that originates the central and peripheral, nervous systems including some glial cells), this process occurs in the human between the third and fourth gestational week. 

Once formed the neural tube occurs a differentiation in three dimensions: the first leads to the spinal cord, the second will give rise to stem and brain stem and the cerebellum, while the third portion will develop the cerebral hemispheres. This stage is called a fore brain, this  process that occurs between the fifth and tenth gestational week and during which develops an active neurogenesis (neuron development) from neural precursor cells, which have a special feature and is not mature and do not proliferate, because we will have to wait for the next moment for such differentiation.

Between the eighth and eighteenth gestational week, occurs an active neuronal proliferation, the precursor cells begin to differentiate to produce new precursor cells and neuronal cells such are different neurons as glial (cells astrocytes and oligodendrocytes).

The speed of proliferation in this period is impressive since they form around 200,000 neurons per minute. However it occurs gradually, after passing through several cycles of cell division, it stops. Even if it is unknown what starts and then stops the mechanism of proliferation in any region, it is clear that the periods are rigidly determined, what determines this differentiation, however, is still a mystery, although we know that it depends on neural factors specific to the region of the brain where it occurs and functions which will exert. 

Differentiated cells begin to emigrate from ventricular areas (Central) to the more peripheral areas of the brain (neocortex) training. I.e. which begin first occupies the deeper layers of the cortex layers, while those that start later, occupy the uppermost layers.

So between the 2nd and 4th month of intrauterine life produces an explosion of cell proliferation, known as neurogenesis, while in between the 3rd and 5th month occurs the migration of neurons, guided by processes glial based on chemical signals and neural growth factors, mediated by regulatory genes that determine the activity of other genes in a defined sequence and for precise periods and in specific regions. 

It is then when cerebral malformations that relate to the brain organization, including delayed neuronal migration disorders occur.This radial migration of neurons to the periphery used glial cells as a guide since these form a scaffolding that facilitates the movement of neurons. 

Neuronal migration occurs mainly in two regions in the thalamus and hypothalamus, where the oldest neurons are pushed by more new neurons, by which the first will be located in the periphery.On the other hand, in regions of the brain structure of laminar, as it is the case of the cortex and the cerebellum, neurons more young people migrate to break through to the oldest, whereupon the latter will sit closer of the neuroepithelium and the more young people on the periphery.

 

Neuronal migration process takes place between the 10th second and the twenty fourth gestational week. 

During neurogenesis and neuronal migration, approximately 50% of neurons undergo apoptosis, i.e. die in a programmed way, probably because they do not follow the correct course of emigration or because they do not receive adequate stimuli, the correct answer is still a mystery. 

A certain proportion of the neurons that survive (20%) Trek horizontally and one after emigration radial, to allow the formation of lamination (segmentation) cortex, it is so neurons looking his way, motivated by chemical stimuli (Neurotropic factors), extending its structure in one of its ends, resulting in the so-called axonal growth cones .

Simultaneously with the neuronal migration occurs in synaptogenesis (formation of synapses), although this is much more intense between the twelfth and the twelfth fourth gestational week, but persists in a very active way until the eighth or ninth month post natal.

It is interesting to note that pre natal synaptogenesis is mainly determined by the genetic heritage of the individual. However, in the stage post natal synaptogenesis is also affected by sensory experiences, particularly through the learning process. 

Thus, during puberty, occurs a sort of freeze on neurogenesis, which has been associated with the acquisition of the own and particular character of each individual. Myelination is a late process that starts in way more intense from the 40ava week, occurs in the white matter and peripheral neurons 

Neurogenesis and the subsequent stages associated with this process morphogenic lead to the formation of approximately 100 billion neurons in the adult brain and several trillions of synapses. 

This implies that a significant number of the 30,000 genes that we have must be involved in this complex process, expressing together in simultaneous or sequential form. However, she has still not been achieved understand this prodigious process, because a region possessing 20,000 genes, is only 302 neurons and nerve tissue that form is far from having the functionality of the human brain.

The number of cells in the fetal brain is between 30 and 70% higher than the number of neurons in the adult. Surplus cells survive for a period of days to weeks, after which, on its own, starts a cascade of degenerative changes and a physiological process of programmed cell death. 

In the picture below, it is possible to observe the differences between birth and two years of development, although it seems that increased neuronal tangle, in reality there are what are they are less neurons with larger number of neural networks, connections between neurons, i.e. interneuronal communication, which allows a more robust network that ensure more specific skills.

In this sense, found that the selective removal of the synaptic connections, is a fundamental process in the cognitive development of the child, as has been observed relationship between changes in the gray matter of the frontal lobe and the evolution in the performance of cognitive tasks.

During the acceleration phase, occurs a large increase of dendritic extensions and small branch, which has been called dendritic arborization, that form numerous synapses, so that all cells and its extensions are arranged in layers and orient themselves, at the same time causing programmed cell death and differentiation and specialization neuronal This depending on the interactions with the environment and genetic factors. So crests of the neuronal branches are, density peaks occur at different ages, but also in different brain areas. 

Thus one fast and dense development both in the visual cortex and the hearing between the 3 and 4 postnatal months and maximum density, around the year of life can be observed. On the contrary, the growth of the prefrontal area is presented at the same age, but the peak is reached until after the first year of life. The only exceptions are granulated cells of the olfactory bulb, cerebellum, and hippocampus, which continue its genesis after birth and continue throughout life.

Brainly, myelination, that is an overlay of the neural connections by a specialized membrane which allows a proper transmission of nerve impulses, is fundamentally a made post natal, occurring in cycles, with a ranked stream by default, to thus start the neural connections, the most important, which will form the basis for all subsequent development. 

Thus, myelination greatly contributes to improve the functionality of the brain because it produces an increase in the speed of nerve impulse conduction. In this sense has been found that there is an increase in white matter during childhood, which probably reflects the increase in myelination.

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