a summary of the topics - Université de Lausanne

I. C r o s s-vi e w s fr o m t h e O u t si d e :
Bi ol o g y a n d S o ci e t y
I.1 The point of view of the simple cell
For the cell biologist, the aging and death of a cell result from the progressive accumulation
of lesions of molecules which integrity is critical to life. Such molecules may include
nucleic acids (RNA or DNA), proteins, lipids and carbohydrates. The lesions are mostly due
to unavoidable environmental aggressions such as oxidation, irradiation (e.g. UV light), or
naturally existing compounds causing molecular alterations such as mutations. Typically,
mutations in essential genes may lead to impairment of important gene products, which
in turn affect the normal functioning of the cell and ultimately its ability to generate new
progenies. In other words, a cell that is becoming incapable of sustaining a normal function
and generating new progenies is an aging cell in the process of dying.
Moreover, the aging and the death of cells may also result from in-built molecular clocks
that can trigger “programmed cell death” (called apoptosis); for instance when the function
of a particular cell or group of cells is not required anymore by the body. This mostly
happens in multicellular organisms where some cells may function as a scaffold during
the development of the organism and are naturally eliminated at the end of the process.
An intuitive example of this elimination is the degeneration of the placenta at the end
of gestation.
This raises the question of the status of a cell that would infinitely keep a normal metabolic
activity but would still lose its ability to reproduce. Would it be immortal ? Most scientists
would argue that the question is rather philosophical than scientific. Indeed, time-dependent
physical transformation of matter, which is the definition of physical aging, is an
inexorable feature of nature. Both organic and mineral compounds are constantly modified
by interactions with their physical environment. Thus they all age, even though in the case
of minerals this might take millions of years. Even the experimental dream of cloning the
sheep Dolly has revealed the inexorability of aging. Dolly was generated from artificial
fertilization of somatic cells, taken from the mammary gland of her mother. However, as a
natural process these cells had already accumulated aging mutations, which were propagated
in the cloned offspring. As a result, Dolly developed degenerative arthritis and lung
disease at an early age. Thus Dolly was aging faster than a sheep generated by normal
fecundation between germ cells.
The beauty of nature is that this accelerated aging does not happen in the compartments of
reproductive cells, because sperms and oocytes are constantly recombined and sorted out
to eliminate the less fit and select the best possible genetic assembly between the genomes
of the two parents. In biology, renewal and progeny are thus part of the definition of life.
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