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In biology, senescence (Latin senex , “old man” or “old age”), or aging, is a physiological process that causes a slow deterioration of body functions.

Aging exists for most animal and plant species, but not all. Very rare species show no observable aging, or are even able to reverse their aging process and return to the larval stage. These include krill and a species of jellyfish: Turritopsis nutricula .

Aging of an organism begins after the maturity phase, and then progresses irreversibly to death. It is most often characterized by a decline in overall capacity of the organization : psycho -motor , immune or reproductive.

In plants, senescence may receive only one part of an organization. This is the case of leaf senescence, for example, characterized by their yellowing then through the fall, or when falling the fruits of the plant.

Why do we age?

The issue of aging has long intrigued philosophers and naturalists, as it is difficult to find a biologically explanation.

There is a fairly widespread view tends to consider aging as a long process of physical wear fabrics that would be of the same nature as the wear of inert matter by the simple effect of the passage of time, that is, ie by applying the second principle of thermodynamics.

Other approaches involve cellular physiology, including the study of oxidation phenomena (theory or mitochondrial free radical theory of Denham Harman) or wear telomeres (telomere theory of aging Alexei Olovnikov).

None of these explanations support the detailed analysis, especially given the differences in longevity that can exist between different species.

Gene regulation

In plants, far from being a simple degradation of the living conditions of the cell, or even a simple degeneration thereof, senescence is a genetically controlled process. In fact, some genes are expressed only at senescence while others become silent.

Cellular senescence

Cellular senescence has been described in vitro for the first time by Leonard Hayflick in 1961 by studying primary fibroblasts (WI-38). He observed that culture, these fibroblasts proliferate for a limited number of population doublings before getting stuck in a irreversibly stop growth in a plateau. This tray named Hayflick limit or replicative senescence plate is observed for most normal cells (eg, fibroblasts, endothelial cells …) but not to cancer cells.

Translated and adapted from Wikipedia.

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