To date, there are two basic concepts of reasons for aging. The first one is death as a result of damage accumulation, and the second is death as a suicide program. There are multiple arguments in favour of both concepts.
A new – astrocytic – hypothesis has been put forward by Aleksei Boyko, Ukrainian researcher, specialist of the National Agrarian University of Ukraine. In the framework of this hypothesis, aging is treated as a result of changes in cerebrum cells. The key role is played by transmutation of cells of the radial neuroglia into stellate cells - astrocytes. Since such cell transmutation is a programmed process, the researcher is inclined to the opinion that aging and following death have been programmed.
The researcher analyzed and compared animals with different aging rates, and noticed that the majority of reptile, fish and amphibian species which grow old rather slowly have a very high regenerative activity of neural tissue. In other words, these species are able to quickly eliminate nervous system damage and replace extinct nerve cells (neurons).
Mammals and birds have almost fully lost the regenerative capacity of the neural tissue, with new neurons formed only in certain parts of the cerebrum, for example, in the walls of the lateral ventricles of the hippocampus. Neuron predecessors – neuroblasts – migrate into various parts of the brain via radial neuroglia fibres, and turn into full-fledged neurons to replace extinct or damaged cells.
The researchers have found out that the mammals undergo changes during their life, with gradual transformation of radial neuroglia cells and transmutation into astrocytes. The astrocytes do not allow neuron predecessors to migrate to the required areas. Thus, renovation of brain cells becomes impossible, and brain cells, having lost their vital resource, ultimately mortify.
Such age-specific brain changes undoubtedly cause changes to other internal and organ systems of the organism, with vascular derangements, hormonal status change, brygmus, alopecia, intercellular collagen accumulation, among the common signs of aging. In contrast to mammals, amphibians such as fish and reptiles preserve their radial neuroglia cells throughout the life cycle, and aging happens more slowly.
According to the researchers' astrocyte hypothesis, this is a probable aging mechanism. They believe that it is the presence or absence of radial neuroglia cells that determines the life span.
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