The theme of the possibilities and consequences of neuronal regeneration remains one of the most controversial spheres of neuroscience for more than 50 years. A new study presented by scientists from the University of California, San Francisco, USA, substantiates the view that the neurogenesis processes observed in the human hippocampus during childhood periods undergo gradual changes as they grow older and are not determined in adults individuals. These results are the result of a scientific search based on the previously suggested assumption that in the hippocampus - the region responsible for the functions of learning and memory - the processes of neurogenesis are characterized by continuity throughout the life of a person. In a paper published in Nature on March 7, 2018. , the research team argued the opinion positing neurogenesis processes in the adult hippocampus as an extremely rare phenomenon that causes legitimate questions about its contribution to brain restoration or the normal functioning of central nervous structures.

Arturo Alvarez-Buylla, Ph.D., a member of the Eli and Edith Research Center for Renewal and Stem Cell Research, and Eli and Edythe Broad Center of Regeneration Medicine and Stem Cell Research at UCSF, USA, is a leading expert in the field of studying brain development, over the past 30 years has played a key role in the scientific substantiation and recognition of the facts of the continuity of neurogenesis in such animals as songbirds and rodents. However, in recent years, laboratory A. Alvarez-Buill and other parallel studies, alternative facts were presented on the continuity of neurogenesis in areas of the human olfactory bulb. Thus, it has been established that, in spite of the integration of new neurons into the frontal lobes of a person after birth, this process ends in early childhood.

A new study of the leading experts was based on a detailed study of 59 human hippocampal samples conducted in laboratories of the University of California, San Francisco, and was supported by parallel studies of co-authors from the University of Valencia (Universidad de Valencia), Spain, Fudan University, China, and the University of California Los Angeles (University of California Los Angeles), USA. The results of synchronous versatile morphofunctional analysis allowed the authors to assume that the possibility of neurogenesis in adult individuals may be absent altogether. This conclusion is a clear challenge to the vast number of earlier studies that put forward alternative ideas that forcing neurogenesis mechanisms could become the basis of therapy for diseases such as Alzheimer's and depressive disorders. However, the authors of the current work propose to focus attention on the fact that new conclusions about the mechanisms of neurogenesis allow to come even closer to understanding the unique significance of how the human brain learns and adapts, attracting the possibilities of an intensive path of development bypassing the extensive mechanisms based on the launch of neurogenesis, occurs, for example, in rodents.

So, in much earlier experiments conducted on laboratory mice, a gradual reduction of neurogenesis in the area of ??the dentate gyrus of the hippocampus was established along with the possibility of modulation and expansion of such possibilities due to training exercises, as well as the slowing of this process under stress conditions. Similar conclusions became the basis of the further popular statements that strengthening of mechanisms of brain regeneration will directly depend on the possibilities of a healthy lifestyle of a person. Experiments carried out on laboratory animals also served as a rationale for theories about strategies for treating Alzheimer's disease, focusing efforts on stimulating neurogenesis. In addition, leading researchers have suggested that the mechanism of effectiveness of antidepressant drug preparations, for example, such as fluoxetine, can be based on the activation of neurogenesis in the area of ??the dentate gyrus of the hippocampus.

In the current scientific project, a team of scientists has revealed numerous evidence of neurogenesis in the dentate gyrus during prenatal development of the brain and in newborns, when in the brain tissues at the time of child's birth an average of about 1618 neurons per 1 mm2. However, the number of newborn cells decreased sharply in samples obtained from infants. Thus, the dentate gyrus samples in children aged 1 year contained 5 times less than the young neurons, which was revealed in the samples of the tissues of newborns. The decline continued in childhood, when the number of new neurons decreased by 23 times between the ages of 1 to 7 years, and then at the age of 13 decreased by another 5 times. In addition, at this age, morphologically young neurons were more mature than the cells observed in the tissues of newborns. Already in early adolescence, only about 2.4 new cells were recorded per 1 mm2 of the dentate gyrus of the hippocampus. Moreover, it was not possible to identify young neurons in any of 17 postmortem samples of adult brain tissue, nor in surgically extracted tissue samples of 12 adult patients with epilepsy.

At the next stage of the work, the researchers turned to the study of stem cells, which are the precursors of young neurons. It is established that such cells are numerous during the early antenatal development of the brain, but their number is progressively reduced in early childhood. It has also been shown that these cells are characterized by a loss of the ability to early clusterization in the concentrated "niche" of the zone of the dentate gyrus of the hippocampus, known as the subgranular zone. The researchers hypothesized that a similar configuration observed in rodents may cause the possibility of prolonged neurogenesis, and this indicates a potential explanation of why neurogenesis in humans in adulthood is differentiated by modulations.

Summarizing the results of the large-scale study, the authors recognized that, despite careful searching, it is impossible to conclusively confirm the absence of the probability of neurogenesis in the hippocampus of adults.

Along with this, scientists emphasize that the uniqueness and the rarity of identifying this process may indicate a lesser evolutionary significance of it in the mechanisms of neuroplasticity, learning and memory throughout the life of a person.

Thus, experts come to the conclusion that the absence of neurogenesis in the brain of an adult can not be considered in a negative aspect, since it indicates the difference between a person and other representatives of the animal world and brings the researchers of the future closer to the development of appropriate methods for treating brain diseases.

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