Biology Reference
In-Depth Information
the population. The larger the size of the population, the less the
variability (see chapter 2). Secondly, selection also refines precision
because it imposes organisation between the types of cells. Thus in
the model of the heap of cells (Fig. 16), multiplication of the dark
and light cells is interconnected through indispensable metabolic
exchange. In the same way, in Fig. 20, cells A and B are interde-
pendent; one cannot exist in a stable manner without the other.
Jim Till (1981) suggested a Darwinian model for the differentia-
tion of haematopoietic cells. We also suggested this, from a theo-
retical analysis, as a general model of cell differentiation based on
the stochastic expression of genes, first of all during a colloquium
(Kupiec, 1981), then in an article (Kupiec, 1983). At that time,
suggesting that cell differentiation and gene expression might be
probabilistic phenomena was considered by most biologists as
extremely eccentric, to put it mildly! Let us however concentrate
on our discussion.
This model does resolve the difficulties of the deterministic model.
It is based on the non-specificity of molecules which induces random
events, it explains why a diversity of cells is produced from a homo-
geneous population and it integrates stochastic gene expression.
However, it in turn raises a question because it inverts the prob-
lem of differentiation, as it is usually posed. The Darwinian model
implies that, owing to random interactions between molecules which
are inherent in them, cells can change state and initiate differentia-
tion without the intervention of an inducer signal. This prediction is
compatible with experimental data which has shown that cells are
spontaneously transformed in the absence of a signal triggering
transformation when they are cultured outside the internal envi-
ronment of the organism (Rubin, 1990). This phenomenon is well
known to those cultivating cells and obliges them to clone cells reg-
ularly so that they keep their original characteristics; otherwise,
they transform and change phenotype uncontrollably. It is usually
thought that this phenomenon is of no significance, that it is an
artefact connected with cultivating cells, and does not contribute
any relevant information to differentiation produced
in vivo
in the
organism. For the Darwinian model it assumes, in contrast, essen-
tial significance. It reveals the non-specific and probabilistic nature
