Hetero-organisation - The Origin of Individuals - page 132

as well as on embryo cells at the blastula and neural crest stages
(Till et al ., 1964; Nakahata et al ., 1982; Godsave and Slack, 1991;
Barrofio and Blot, 1992; Yamada et al ., 2007; Bennett, 1983; Lin
et al ., 1994; van Roon et al ., 1989; Bohme et al ., 1995; Davis et al .,
1993; Paulus et al ., 1993). This list is not exhaustive.
6.2.5 The instructive model does not account
for stochastic gene expression
A great many molecular processes could be the origin of variability
in cell differentiation. Thanks to techniques which can analyse gene
expression in individual cells, a vast amount of experimental data
has now accumulated which demonstrate that this phenomenon is
itself a stochastic process and that it is here that variations giving
rise to cell differentiation could be produced.
According to the deterministic model, identical cells in a uni-
form environment ought to express the same genes, but that is just
not the case. There are always differences of expression between
individual cells. A gene expressed in one cell is not necessarily
expressed in another cell of the same population. This was origi-
nally demonstrated with regard to the expression of the proteins of
many cellular or viral genes (Ko et al ., 1990; Ross et al ., 1994;
Fiering et al ., 1990; White et al ., 1995; Takasuka, 1998). In muscle
cells with several nuclei, the different nuclei, while nevertheless
sharing a single cytoplasm, do not express the same genes. It has
been demonstrated in this case that the variability of expression is
located directly at the level of the gene transcription into RNA
(Newlands et al ., 1998). We have also detected variability in the
transcription into RNA of the insulin receptor gene (Heams and
Kupiec, 2003).
These results have been confirmed by even more spectacular
experiments revealing differences in expression between the two
chromosomes of a diploid cell which each carry an allele of the same
gene. Such heteroallelic difference in expression has been observed
for many genes (Chess et al ., 1994; Wijgerde et al ., 1995; Held et al .,
1999; Holländer, 1999; Rivière et al ., 1998; Jouvenot et al ., 1999;