As humans grow older, the structures and
functions of their bodies deteriorate. As a
consequence, their risks of disability, disease,
and death increase. It is an omen of
the various confusions and controversies
existing in the research on this process
that the process itself has no universally
accepted designation.
In line with renowned gerontologists, we
distinguish between ageing and senescence.1-5
Ageing refers to the mere passage
of time. It encompasses all changes that
occur in the body during time, whether
their effects are detrimental, beneficial, or
negligible. The progress of ageing is indicated
by one’s chronological age, which can
be easily deduced from a birth registry. Senescence
is part of ageing. It refers to the
deterioration of the body’s structures and
functions and encompasses specifically
the detrimental changes that appear with
ageing. The progress of senescence is indicated
by one’s biological age, although it is
still elusive how one’s biological age can be
precisely determined.6-10
Apart from senescence, ageing is accompanied
by changes that are beneficial to
the body’s structures and functions. Such
changes take place in a programmed order
early in life as growth and development,
are brought about by the body as regeneration
when it repairs its damaged parts, for
example during the healing of a fractured
bone,11 and can be effectuated by medical
interventions, like the replacement of stem
cells, which is called rejuvenation.12 Ageing
is also accompanied by changes that are,
as far as we know, neither detrimental nor
beneficial to the body. A classic example is
the greying of hair. These changes are simply
spoken of as age-related changes.13
Some researchers are accustomed to denote
the senescence of cells in particular as
senescence and to denote the senescence
of individuals or populations as ageing.14-18
However, as will be substantiated hereafter,
there is no reason to fundamentally
separate cellular senescence from senescence
of individuals or populations.