The main characteristic of the aging process is self-accelerating chain reaction (kind of avalanche progressing). This self-accelerating chain reaction has no steady pattern, but may vary while the aging process slows down or moves faster, influenced by many different intrinsic, environmental or medical factors moving in opposite directions.
There are diverse types of aging occurrences within a variety of species and diverse kinds of aging mechanisms for different organs and tissues inside of the same individual single organism.
Physiological mechanisms of aging are specific for each class, category, and species of living organisms as well as for each type of cells, tissues and organs in each organism.
More specialized cells of multicellular organisms have less ability of reproduction until completely losing the ability to reproduce themselves. This tendency is clearly displayed in both lines of rising complicity of specialization: on the line of evolutionary hierarchy of organisms and on the line of hierarchy of rising complicity of cells in the same organism. We could see in evolutionary hierarchy of organisms that in primitive multicellular organisms, the level of cell specialization is low and in many cases is interchangeable and reversible. Such cells with low differentiation or specialization could have the ability to reproduce themselves. The next step in development is higher specialization and losing ability for reproducing themselves. From one side, higher specialization of cells benefits the entire organism, but from the other side makes such cells more fragile with a lower survival rate, which would damage the entire organism. This dilemma was resolved by developing the mechanism of constantly reproducing new specialized cells to replace damaged exhausted and used specialized cells (could be described as keeping some storage of unspecialized cells as stem cells for specialized cells).
We could generally divide mechanisms of aging into three basic categories:
A. Aging of “dividing” or replaceable cells (blood, epithelia cells)
All cells of so-called “dividing” and non-“dividing” tissues actually have no ability to divide or reproduce. Historically called “dividing,” cells in living organisms actually have no ability to divide because they are specialized cells. Instead of dividing, they are replaced by new cells, derived from their progenitor stem cells. It would be more suitable to define them as “replaceable” cells. Those mostly highly specialized cells are replaced many times during an organism’s lifespan, having their lifespan much shorter than the whole organism’s lifespan.
B. Aging of “non-dividing” or non-replaceable cells (muscle, neurons)