Whether nerve cells are restored or not. Is the nervous system recovering? The hard way to the brain
The process by which the formation of a nerve cell occurs (both in the prenatal period and in life) is called “neurogenesis”.
The well-known statement that nerve cells do not regenerate was once made in 1928 by Santiago Ramon I Halem, a Spanish neurohistologist. This situation existed until the end of the last century until a scientific article by E. Gould and C. Cross appeared, which presented facts proving the production of new brain cells, although back in the 60–80s. some scientists tried to convey this discovery to the scientific world.
Where are cells restored?
Currently, “adult” neurogenesis has been studied to a level that allows us to draw a conclusion about where it occurs. There are two such areas.
- Subventricular zone (located around the cerebral ventricles). The process of neuronal regeneration in this section occurs continuously and has some peculiarities. In animals, stem cells (the so-called progenitors) migrate to the olfactory bulb after they divide and transform into neuroblasts, where they continue their transformation into full-fledged neurons. In the human brain, the same process occurs with the exception of migration - which is most likely due to the fact that the smell function is not as vital for humans as it is for animals.
- Hippocampus This is a paired section of the brain, which is responsible for orientation in space, consolidation of memories and the formation of emotions. Neurogenesis in this section is especially active - about 700 nerve cells appear here per day.
Some scientists argue that in the human brain, neuronal regeneration can occur in other structures - for example, the cerebral cortex.
Modern ideas that the formation of nerve cells is present in the adult period of human life opens up enormous opportunities in the invention of methods for treating degenerative diseases of the brain - Parkinson's, Alzheimer's and the like, the consequences of traumatic brain injuries, strokes.
Scientists are currently trying to figure out what exactly promotes neuronal recovery. Thus, it has been established that astrocytes (special neuroglial cells), which are the most resistant after cellular damage, produce substances that stimulate neurogenesis. It is also suggested that one of the growth factors - activin A - in combination with other chemical compounds allows nerve cells to suppress inflammation. This, in turn, promotes their regeneration. The features of both processes have not yet been sufficiently studied.
The influence of external factors on the recovery process
Neurogenesis is a constant process that can be periodically negatively affected by various factors. Some of them are known in modern neurobiology.
- Chemotherapy and radiation therapy used in the treatment of cancer. Precursor cells are affected by these processes and stop dividing.
- Chronic stress and depression. The number of brain cells that are dividing sharply decreases during the period when a person experiences negative emotional feelings.
- Age. The intensity of the process of formation of new neurons decreases with old age, which affects the processes of attention and memory.
- Ethanol. Alcohol has been found to damage astrocytes, which are involved in the production of new cells in the hippocampus.
Positive effects on neurons
Scientists are faced with the task of studying as fully as possible the effects of external factors on neurogenesis in order to understand how certain diseases arise and what can help cure them.
A study on the formation of brain neurons, which was carried out on mice, showed that exercise had a direct effect on cell division. Animals running on wheels produced positive results compared to those sitting idle. This same factor also had a positive effect on those rodents that were “old” in age. In addition, neurogenesis was enhanced by mental stress - solving problems in mazes.
Experiments are currently being intensively carried out to search for substances or other therapeutic effects that promote the formation of neurons. So, the scientific world knows about some of them.
- Stimulation of the neurogenesis process using biodegradable hydrogels has shown positive results in stem cell cultures.
- Antidepressants not only help to cope with clinical depression, but also affect the recovery of neurons in those suffering from this disease. Due to the fact that the disappearance of symptoms of depression during drug therapy occurs in about one month, and the process of cell regeneration takes the same amount of time, scientists have hypothesized that the appearance of this disease directly depends on the fact that neurogenesis in the hippocampus slows down.
- In studies aimed at finding ways to repair tissue after ischemic stroke, it was found that peripheral brain stimulation and physical therapy increased neurogenesis.
- Regular exposure to dopamine receptor agonists stimulates cell restoration after damage (for example, in Parkinson's disease). Important for this process is a different combination of drugs.
- The introduction of tenascin-C, a protein of the intercellular matrix, affects cellular receptors and increases the regeneration of axons (neuron processes).
Application of stem cells
Separately, it is necessary to say about the stimulation of neurogenesis through the introduction of stem cells, which are the precursors of neurons. This method is potentially effective as a treatment for degenerative brain diseases. Currently, it has only been conducted on animals.
For these purposes, primary cells of the mature brain are used, preserved from the time of embryonic development and capable of division. After division and transplantation, they take root and turn into neurons in the very regions already known as the places where neurogenesis occurs - the subventricular zone and the hippocampus. In other areas they form glial cells but not neurons.
After scientists realized that nerve cells are restored from neuronal stem cells, they suggested the possibility of stimulating neurogenesis through other stem cells - blood. The truth turned out to be that they penetrate the brain, but form binucleate cells, merging with existing neurons.
The main problem with the method is the immaturity of “adult” brain stem cells, so there is a risk that after transplantation they may not differentiate or die. The challenge for researchers is to determine what exactly causes a stem cell to become a neuron. This knowledge will allow, after collection, to “give” her the necessary biochemical signal to begin transformation.
Another serious difficulty encountered in the implementation of this method as a therapy is the rapid division of stem cells after their transplantation, which in a third of cases leads to the formation of cancerous tumors.
So, in the modern scientific world, the question of whether neurons are formed does not stand: it is not only known that neurons can be restored, but, to some extent, it has been determined what factors can influence this process. Although major research discoveries in this area are still ahead.
In the modern world, full of stress, emotional and mental tension, as well as hard work, the human brain experiences incredible stress, which sometimes results in various diseases. The expression “nerve cells do not recover” is familiar to everyone from early childhood, but is this true? Question: Do nerve cells regenerate? - is very controversial and can be answered with confidence either “yes” or “no”.
Scientists have only relatively recently figured out why nerve cells do not recover. This occurs due to the division gene, which is in an inactive state in neurons and cardiac muscle cells. Any other tissues of the human body are capable of replacing dead or weakened fellow tissues through division, especially hematopoietic cells and epithelial cells, but the human brain is not.
This is quite logically justified, because skin, blood, muscle tissue, intestinal tissue, liver and many others are consumable materials of the body that are spent in bruises, wounds, during the performance of their functions and under the influence of the environment. Their ability to recover is essential for maintaining the vital functions of the body.
The human brain and heart, on the contrary, are the most protected organs, which are practically not affected by external environmental factors, and if they could be restored through cell division, they would grow to incredible sizes and shapes, which cannot lead to anything good. In addition, if one of the most important organs is seriously damaged, the rest of the body will die in the next few minutes, and until the heart or brain heals, there will be no one for them to function.
At birth, the body lays down the required number of neurons, which increases to the required number during the child’s growth.
That is why it is necessary to try to develop children as much as possible, both mentally and physically, the main thing is to do this competently, so that the intended benefit does not turn into very real harm. From this feature also emerged the theory that a person uses only 10% of his brain, and the rest is in an inactive state. However, neither the first nor the second have yet found sufficient scientific evidence.
Why do nerve cells die?
Despite the fact that the human nervous system is reliably protected, nerve cells still die. This happens for many reasons, for which the person himself is to blame.
The greatest death of nerve cells occurs naturally in the human embryo, since during embryogenesis a huge excess of them is formed, which kills approximately 70% of the total before birth. Only the number necessary for existence remains.
Secondly, cells of the peripheral nervous system most often die, which occurs due to various injuries to the skin and other tissues, and various inflammations.
Many infectious, genetic and diseases caused by the irreversible consequences of negative influences destroy the human nervous system. Such diseases include encephalitis, meningitis, traumatic brain injuries, strong thermal effects of the environment, both heat and cold, natural fluctuations in body temperature during illness, neurodegenerative irreversible disorders - Alzheimer's, Parkinson's, Huntington's and many others.
However, the percentage of natural causes of brain death is quite small compared to the suicidal influence of the person himself. Now people have surrounded themselves with such a huge amount of toxic substances that you can’t help but wonder how humanity hasn’t died out at all.
The human brain and peripheral nervous system happily destroy alcohol, smoking, drugs, medications, preservatives and food chemicals, pesticides and household chemicals, hypoxia caused by high levels of carbon dioxide in the atmosphere, stress, etc.
While everything is clear about the deadly influence of injuries and chemicals, many people do not take the stressful influence seriously. This is especially true for low-income segments of the population, who consider discussions about the dangers of stress to be the lot of a capricious social class, accustomed to the comfort of a wealthy social class.
In case of danger, the adrenal glands release cortisol and adrenaline, designed to increase the speed of the brain and the reactions of the peripheral nervous system to solve the problem and save the entire body. During short-term stress, hormones have time to do their job and are removed from the blood. Constant stress creates an excess of hormones in the blood, which causes overstrain and “burning out” of neurons. In addition, continuous electrical signals through which nerve cells transmit information can accumulate and completely disrupt the entire fine structure. Even a small but constant stress can lead to serious consequences, since its hormones, even in minimal quantities, do not allow brain cells to return to a resting state, which wears them out very quickly. Stress hormones are eliminated very slowly, and sometimes even days, much less a few hours of sleep at night, are not enough to completely cleanse the body.
Is it true that nerve cells do not regenerate?
The question of whether it is true that nerve cells do not regenerate still remains quite controversial. If the nervous system had only died off without the ability to restore its cells, then humanity would hardly have survived, dying in childhood and adolescence.
Experiments conducted on worms and insects have shown that their nerve cells are capable of dividing, although they are not capable of performing mental stress.
In mammals, brain cells do not divide, but are completely regenerated with new ones, which was noticed through experiments on rats whose brains were partially destroyed by electric current. The newly formed cells were identified using a special radioactive substance that is absorbed only by newly formed neurons.
The story of songbirds is even more interesting. Scientists have noticed that every mating season the same songbird, isolated from other birds and the sounds they make, develops new trills and the singing becomes much more beautiful. Upon detailed study, it turned out that due to increased emotional stress during the mating season, a lot of brain cells die in birds, which are perfectly replaced by new ones, periodically renewing the entire brain.
In humans, nerve cells are also restored in certain ways. The patient who has survived the operation loses sensitivity in the incision area, which is restored after a long period of time. This is explained by a disruption of neural connections between nerve cells, which are carried out using axons - special processes of incredible length for transmitting impulses. The axon of one cell can reach 120 cm in length, which is truly impressive, because the average human height is 1.5 – 2 meters. If you imagine how many nerve cells and their processes there are in the body, you will get an amazing picture of the most complex, intricate nervous system intertwining the entire body and every cell of it. When connections are disrupted, neurons very slowly, but quite easily form others, growing new processes. Using this principle, sometimes the sensitivity of the limbs or some body functions lost as a result of severe physical injury are restored.
With some damage to the brain, it happens that a person loses memory. It is restored by renewing lost neural connections. If it is not the connections that are lost, but the nerve cells themselves, then the newly formed connections of the nerve endings can help restore the overall picture from the remaining pieces of information.
But every ability has its limit. Neurons cannot endlessly grow new connections, and without the ability to restore their number, a person would die too quickly, lose his mind and sensitivity.
The process of neurogenesis in humans occurs in only two ways:
- The first way is that new neurons are produced in very small numbers in the brain. This amount is so small that it cannot even replace cells that die naturally.
- The second method is the natural regeneration of nerve tissue from the body’s stem cells. Stem cells are special cells without qualifications that can only transform once into any host cell. They are found in fairly large quantities in the bone marrow and, being formed at the level of the embryo, are not capable of dividing themselves. Not many people know that body tissues are not capable of endless division: each cell can only divide a certain number of times.
Stem cells are beginning to be used when there is extensive tissue damage or when there is a small remnant of specialized cells capable of dividing, significantly prolonging human life.
Modern science is working on ways to transplant stem cells obtained from unborn babies in early pregnancy. Stem cells do not have any characteristics that determine whether they belong to a particular person, so they are not rejected by the recipient and continue to properly perform their functions as if they were their own. Relatively recently, there was a real boom in stem cell transplants for healing and rejuvenation of the body, however, despite the stunning effect, the fashion quickly passed due to the incredible percentage of cancer incidence in people who received a dose of the life-giving vaccine. Science cannot yet find out whether the transplanted stem cells degenerate into cancer cells or whether the cancer is provoked by an excessive amount of them, or perhaps some other factors influence it. It also depends on the lack of sufficient information about the disease itself.
The third method has not yet been registered by science and is in the experimental phase. Its essence lies in transplanting RNA from animals with neurons capable of dividing into humans in order to transfer this ability to him. But so far the experiment is at the stage of theoretical consideration and possible side effects have not been identified.
So there is truth
Considering all the factors relating to the death of neurons in the human nervous system and methods for restoring their number, when asked whether human nerve cells are restored, scientists answer more likely no than yes.
As Leonid Bronevoy’s hero, the district doctor, said: “ the head is a dark object and cannot be examined..." Although the compact collection of nerve cells called the brain has been studied by neurophysiologists for a long time, scientists have not yet been able to obtain answers to all questions related to the functioning of neurons.
The essence of the question
Some time ago, until the 90s of the last century, it was believed that the number of neurons in the human body has a constant value and, if lost, it is impossible to restore damaged nerve cells in the brain. In part, this statement is indeed true: during the development of the embryo, nature lays down a huge reserve of cells.
Even before birth, a newborn baby loses almost 70% of its formed neurons as a result of programmed cell death - apoptosis. Neuronal death continues throughout life.
Starting from the age of thirty, this process is activated - a person loses up to 50,000 neurons every day. As a result of such losses, the brain of an old person is reduced by about 15% compared to its volume in youth and adulthood.
It is characteristic that scientists note this phenomenon only in humans– in other mammals, including primates, age-related brain decline and, as a consequence, senile dementia are not observed. This may be due to the fact that animals in nature do not live to old age.
Scientists believe that the aging of brain tissue is a natural process established by nature and is a consequence of the longevity acquired by a person. A lot of the body’s energy is spent on brain function, so when increased activity is no longer necessary, nature reduces the energy consumption of brain tissue, spending energy on maintaining other body systems.
These data indeed support the common saying that nerve cells do not regenerate. Why, if the body in a normal state does not need to restore dead neurons - there is a supply of cells that is more than enough to last a lifetime.
Observations of patients suffering from Parkinson's disease have shown that clinical manifestations of the disease appear when almost 90% of the neurons in the midbrain, which is responsible for controlling movement, have died. When neurons die, their functions are taken over by neighboring nerve cells. They increase in size and form new connections between neurons.
So, if in a person's life “...everything is going according to plan”, neurons lost in genetically determined quantities are not restored - this is simply not necessary.
More precisely, the formation of new neurons occurs. Throughout life, a certain number of new nerve cells are constantly produced. The brains of primates, including humans, produce several thousand neurons every day. But the natural loss of nerve cells is still much greater.
But the plan may go wrong. Massive neuronal death may occur. Of course, not due to a lack of positive emotions, but, for example, as a result of mechanical damage during injuries. This is where the ability to regenerate nerve cells comes into play. Scientists' studies prove that it is possible to transplant brain tissue, in which not only does the transplant not be rejected, but the addition of donor cells leads to the restoration of the recipient's nervous tissue.
The precedent of Teri Wallis
In addition to experiments on mice, the case of Terry Wallis, who spent twenty years in a coma after a severe car accident, can serve as evidence for scientists. Relatives refused to remove Terry from life support after doctors diagnosed a vegetative state.
After a twenty-year hiatus, Terry Wallis regained consciousness. Now he can already pronounce meaningful words and joke. Some motor functions are gradually restored, although this is complicated by the fact that over such a long period of inactivity, all the muscles of the man’s body have atrophied.
Scientists' studies of Terry Wallis' brain demonstrate phenomenal phenomena: Terry's brain is growing new neural structures to replace those lost in the accident.
Moreover, new formations have a shape and location different from the usual ones. The brain appears to grow new neurons where it feels most comfortable, rather than trying to replace those lost due to injury. Experiments conducted with patients in a vegetative state have proven that patients are able to answer questions and respond to requests. True, this can only be recorded by the activity of the brain system using magnetic resonance imaging. This discovery could radically change the attitude towards patients who have fallen into a vegetative state.
It is not only extreme situations such as traumatic brain injuries that can contribute to an increase in the number of dying neurons. Stress, poor nutrition, ecology - all these factors can increase the number of nerve cells lost by a person. The state of stress also reduces the formation of new neurons. Stressful situations experienced during intrauterine development and the first time after birth can cause a decrease in the number of nerve cells in future life.
How to restore neurons
Instead of wondering whether it is possible to restore nerve cells at all, perhaps it is worth deciding - is it worth it? The report of Professor G. Hüther at the World Congress of Psychiatrists spoke about the observation of novices of a monastery in Canada. Many of the women observed were over a hundred years old. And all of them showed excellent mental and mental health: no characteristic degenerative changes of senility were found in their brains.
According to the professor, four factors contribute to maintaining neuroplasticity – the ability for brain regeneration:
- strength of social ties and friendly relations with loved ones;
- the ability to learn and the implementation of this ability throughout life;
- balance between what is desired and what is in reality;
- stable worldview.
The nuns had all these factors.
The popular expression “Nerve cells do not regenerate” has been perceived by everyone since childhood as an immutable truth. However, this axiom is nothing more than a myth, and new scientific data refute it.Nature builds a very high margin of safety into the developing brain: during embryogenesis, a large excess of neurons is formed. Almost 70% of them die before the child is born. The human brain continues to lose neurons after birth, throughout life. This cell death is genetically programmed. Of course, not only neurons die, but also other cells of the body. Only all other tissues have a high regenerative capacity, that is, their cells divide, replacing dead ones. The regeneration process is most active in epithelial cells and hematopoietic organs (red bone marrow). But there are cells in which the genes responsible for reproduction by division are blocked. In addition to neurons, these cells include cardiac muscle cells. How do people manage to maintain intelligence until very old age if nerve cells die and are not renewed?
Schematic representation of a nerve cell, or neuron, which consists of a body with a nucleus, one axon and several dendrites
One possible explanation: in the nervous system, not all neurons “work” at the same time, but only 10% of neurons. This fact is often cited in popular and even scientific literature. I have repeatedly had to discuss this statement with my domestic and foreign colleagues. And none of them understands where this figure came from. Any cell simultaneously lives and “works”. In each neuron, metabolic processes occur all the time, proteins are synthesized, and nerve impulses are generated and transmitted. Therefore, leaving the hypothesis of “resting” neurons, let us turn to one of the properties of the nervous system, namely, its exceptional plasticity.
The meaning of plasticity is that the functions of dead nerve cells are taken over by their surviving “colleagues,” which increase in size and form new connections, compensating for the lost functions. The high, but not unlimited, effectiveness of such compensation can be illustrated by the example of Parkinson's disease, in which gradual death of neurons occurs. It turns out that until about 90% of the neurons in the brain die, the clinical symptoms of the disease (trembling of the limbs, limited mobility, unsteady gait, dementia) do not appear, that is, the person looks practically healthy. This means that one living nerve cell can replace nine dead ones.
Neurons differ from each other in size, dendritic branching, and axon length.
But the plasticity of the nervous system is not the only mechanism that allows one to maintain intelligence into old age. Nature also has a backup option - the emergence of new nerve cells in the brain of adult mammals, or neurogenesis.
The first report on neurogenesis appeared in 1962 in the prestigious scientific journal Science. The article was titled "Are New Neurons Forming in the Adult Mammalian Brain?" Its author, Professor Joseph Altman from Purdue University (USA), used an electric current to destroy one of the structures of the rat’s brain (the lateral geniculate body) and injected it with a radioactive substance that penetrates into newly emerging cells. A few months later, the scientist discovered new radioactive neurons in the thalamus (a region of the forebrain) and the cerebral cortex. Over the next seven years, Altman published several more papers demonstrating the existence of neurogenesis in the brain of adult mammals. However, then, in the 1960s, his work aroused only skepticism among neuroscientists; their development did not follow.
The term "glia" includes all cells of nervous tissue that are not neurons.
And only twenty years later, neurogenesis was “discovered” again, but in the brain of birds. Many songbird researchers have noticed that during each mating season, the male canary Serinus canaria performs a song with new "knees." Moreover, he does not adopt new trills from his brothers, since the songs were updated even in isolation. Scientists began to study in detail the main vocal center of birds, located in a special part of the brain, and discovered that at the end of the mating season (for canaries it falls in August and January), a significant part of the neurons of the vocal center died, probably due to excessive functional load . In the mid-1980s, Professor Fernando Notteboom from Rockefeller University (USA) was able to show that in adult male canaries the process of neurogenesis occurs in the vocal center constantly, but the number of neurons produced is subject to seasonal fluctuations. Peak neurogenesis in canaries occurs in October and March, that is, two months after the mating seasons. That is why the “record library” of male canary songs is regularly updated.
Neurons are genetically programmed to migrate to one or another part of the nervous system, where, with the help of processes, they establish connections with other nerve cells.
In the late 1980s, neurogenesis was also discovered in adult amphibians in the laboratory of the Leningrad scientist Professor A.L. Polenov.
Where do new neurons come from if nerve cells do not divide? The source of new neurons in both birds and amphibians turned out to be neuronal stem cells from the wall of the ventricles of the brain. During the development of the embryo, it is from these cells that the cells of the nervous system are formed: neurons and glial cells. But not all stem cells turn into cells of the nervous system - some of them “lurk” and wait in the wings.
Dead nerve cells are destroyed by macrophages that enter the nervous system from the blood.
Stages of formation of the neural tube in the human embryo.
New neurons have been shown to arise from adult stem cells in lower vertebrates. However, it took almost fifteen years to prove that a similar process occurs in the mammalian nervous system.
Advances in neuroscience in the early 1990s led to the discovery of “newborn” neurons in the brains of adult rats and mice. They were found mostly in evolutionarily ancient parts of the brain: the olfactory bulbs and the hippocampal cortex, which are mainly responsible for emotional behavior, the response to stress and the regulation of sexual functions in mammals.
Just as in birds and lower vertebrates, in mammals neuronal stem cells are located close to the lateral ventricles of the brain. Their transformation into neurons is very intense. In adult rats, about 250,000 neurons are formed from stem cells per month, replacing 3% of all neurons in the hippocampus. The lifespan of such neurons is very high - up to 112 days. Neuronal stem cells travel a long distance (about 2 cm). They are also able to migrate to the olfactory bulb, turning into neurons there.
The olfactory bulbs of the mammalian brain are responsible for the perception and primary processing of various odors, including the recognition of pheromones - substances that are close in their chemical composition to sex hormones. Sexual behavior in rodents is regulated primarily by the production of pheromones. The hippocampus is located under the cerebral hemispheres. The functions of this complex structure are associated with the formation of short-term memory, the realization of certain emotions and participation in the formation of sexual behavior. The presence of constant neurogenesis in the olfactory bulb and hippocampus in rats is explained by the fact that in rodents these structures bear the main functional load. Therefore, the nerve cells in them often die, which means they need to be renewed.
In order to understand what conditions affect neurogenesis in the hippocampus and olfactory bulb, Professor Gage from Salka University (USA) built a miniature city. The mice played there, did physical exercise, and looked for exits from the mazes. It turned out that in “city” mice new neurons appeared in much greater numbers than in their passive relatives, mired in routine life in the vivarium.
Stem cells can be extracted from the brain and transplanted into another part of the nervous system, where they turn into neurons. Professor Gage and his colleagues conducted several similar experiments, the most impressive of which was the following. A piece of brain tissue containing stem cells was transplanted into the destroyed retina of a rat's eye. (The light-sensitive inner wall of the eye has a “nervous” origin: it consists of modified neurons - rods and cones. When the light-sensitive layer is destroyed, blindness occurs.) The transplanted brain stem cells turned into retinal neurons, their processes reached the optic nerve, and the rat regained his sight! Moreover, when brain stem cells were transplanted into an undamaged eye, no transformations occurred with them. Probably, when the retina is damaged, some substances are produced (for example, so-called growth factors) that stimulate neurogenesis. However, the exact mechanism of this phenomenon is still unclear.
Scientists were faced with the task of showing that neurogenesis occurs not only in rodents, but also in humans. To this end, researchers led by Professor Gage recently carried out sensational work. In one of the American oncology clinics, a group of patients with incurable malignant tumors took the chemotherapy drug bromdioxyuridine. This substance has an important property - the ability to accumulate in dividing cells of various organs and tissues. Bromodioxyuridine is incorporated into the DNA of the mother cell and is retained in daughter cells after the mother cell divides. A pathological study showed that neurons containing bromodyoxyuridine are found in almost all parts of the brain, including the cerebral cortex. This means that these neurons were new cells that arose from the division of stem cells. The finding unconditionally confirmed that the process of neurogenesis also occurs in adults. But if in rodents neurogenesis occurs only in the hippocampus, then in humans it can probably involve larger areas of the brain, including the cerebral cortex. Recent research has shown that new neurons in the adult brain can be formed not only from neuronal stem cells, but from blood stem cells. The discovery of this phenomenon caused euphoria in the scientific world. However, publication in the journal Nature in October 2003 largely cooled the enthusiastic minds. It turned out that blood stem cells actually penetrate the brain, but they do not turn into neurons, but merge with them, forming binucleate cells. Then the “old” nucleus of the neuron is destroyed, and it is replaced by the “new” nucleus of the blood stem cell. In the rat's body, blood stem cells mainly merge with the giant cells of the cerebellum - Purkinje cells, although this happens quite rarely: only a few fused cells can be found in the entire cerebellum. More intense fusion of neurons occurs in the liver and heart muscle. It is still completely unclear what the physiological meaning of this is. One hypothesis is that blood stem cells carry with them new genetic material, which, when entering the “old” cerebellar cell, prolongs its life.
So, new neurons can arise from stem cells even in the adult brain. This phenomenon is already quite widely used for the treatment of various neurodegenerative diseases (diseases accompanied by the death of brain neurons). Stem cell preparations for transplantation are obtained in two ways. The first is the use of neural stem cells, which in both embryos and adults are located around the ventricles of the brain. The second approach is the use of embryonic stem cells. These cells are located in the inner cell mass at the early stage of embryo formation. They can transform into almost any cell in the body. The greatest difficulty in working with embryonic cells is getting them to transform into neurons. New technologies make this possible.
Some medical institutions in the United States have already formed “libraries” of neural stem cells obtained from embryonic tissue and are transplanting them into patients. The first attempts at transplantation give positive results, although today doctors cannot solve the main problem of such transplants: uncontrolled proliferation of stem cells in 30-40% of cases leads to the formation of malignant tumors. No approach has yet been found to prevent this side effect. But despite this, stem cell transplantation will undoubtedly be one of the main approaches in the treatment of neurodegenerative diseases such as Alzheimer's and Parkinson's diseases, which have become the scourge of developed countries.
