Biology Reference
In-Depth Information
contradiction is to evoke the effect of morphogenetic gradients pre-
existing in the egg. Owing to their heterogeneous distribution, there
is said to be unequal distribution in each daughter cell of the mol-
ecules present in the egg after each of its divisions. This mechanism
is supposed to create initial differentiation of the cells which would
set differentiation in motion according to the instructive model.
Although morphogenetic gradients are an indisputable reality and
play a certain role in embryogenesis, this explanation is still a pal-
liative measure which does not really resolve the contradiction.
It bases the whole of embryogenesis on the egg's initial gradients.
A great many experiments, however, including those performed by
Driesch (see chapter 5), demonstrate that embryo cells that have
undergone several successive divisions are still capable of reconsti-
tuting complete organisms, even though the morphogenetic gradi-
ents have been destroyed in these cells. Regulatory mechanisms
must therefore exist that are capable of creating heterogeneity of
cell types in the course of embryogenesis.
6.2.4
The instructive model does not account
for variability in cell differentiation
For a long time cell differentiation for a great many cell lines has
been analysed using a variety of techniques. These analyses have
revealed stochastic variability in the differentiation of numerous tis-
sues which is not compatible with a deterministic model. This vari-
ability may be manifested in various ways: either each individual
cell of a single population differentiates with a chronology that
varies from one cell to another, or the descendants of the individ-
ual cells vary as regards their differentiated cell content. Such vari-
ability, which can only be modelled using stochastic models, has been
observed in a variety of organisms,
ex vivo
and
in vivo
, with a num-
ber of experimental techniques and for many cell lines. Jim Till and
his team (1964) put forward the first stochastic model to account
for variability in the differentiation of haematopoietic stem cells.
Following this, similar work has been performed on cells of the
immune system, blood, skin, liver, bone, intestinal and heart cells,
