Biology Reference
In-Depth Information
of cell differentiation. Having escaped from the selective constraint
that the internal environment exerts over the organism
in vivo
, cells
transform spontaneously owing to stochastic molecular interactions
which are no longer channelled. In this context, what poses the
problem is not so much explaining why two cells are different from
each other, but rather understanding how, despite this inherent ten-
dency to differentiate, homogeneous tissues of identical cells can
form in the organism. The problem is not one of cell differentiation
but of their 'identification'. To resolve this, what cell selection con-
sists of must be more precisely explained.
6.3.2
From metabolic selection to stabilisation
by the 'signal-food'
The increase in complexity of organisms arises not only from struc-
tural and functional innovation but also from their biochemical evo-
lution which is characterised by the appearance of molecules with
new properties. In ontophylogenesis, the origin of multicellularity lies
in metabolic selection, but that obviously does not exclude the
appearance of more sophisticated mechanisms in the course of evolu-
tion. Cell regulation in organisms existing nowadays is accomplished
by molecules such as hormones or growth factors. These molecules
have no nutritive value
per se
and seem to operate as signals. As with
increase in the complexity of functions, this biochemical evolution
does not invalidate the Darwinian model but improves it.
The instructive model comes up against non-specificity of the
signals and leads to the same contradiction as genetic determinism.
On the other hand, in a selective mechanism, the molecules which
constitute the cell's microenvironment do not need to exert a
specific effect since it is an adaptive mechanism, based on the
stochastic behaviour of the cell. Sergei Atamas performed an analysis
of the greatest importance on this subject. We know that antibody
synthesis takes place according to a selective mechanism (see the
introduction to this chapter). From a strictly formal point of view,
there is therefore no difference between the function of a nutrient
which allows selection of an animal in an ecosystem and that of an
antigen which produces the same effect on a lymphocyte in the
