Biology Reference
In-Depth Information
A
B
d
instructions
c
C
D
(adapted from Kupiec, 1983, 1997)
F
IGURE
19. The instructive model of cell differentiation.
Cells A and B differenti-
ate specifically into C and D in response to the signal they receive (
c
and
d
).
Embryonic development is supposed to be a succession of elementary stages identical
to this, with each stage corresponding to the expression of specific genes encoding
differentiation signals.
In this deterministic context, the differentiation of tissues during
embryogenesis is seen as a series of elementary stages similar to that
described in this figure, and because each stage involves the expres-
sion of genes encoding for instructive proteins, the entire process
expresses the genetic programme (see chapter 3).
There are several variants of the instructive model. If the sig-
nals are carried by membrane molecules, there needs to be direct
contact between the cells at their membranes. If the signals are car-
ried by diffusible molecules, they can act from a distance. In cer-
tain cases, instructive molecules may also form concentration
gradients and exert their specific effect only at a specific concen-
tration. The signal may equally correspond to a combination of
molecules, not one only, but whatever the variant of the model, its
principle remains the same: cells differentiate in a way determined
by the signals they receive.
There are decided advantages to this model. Firstly, it integrates
the very large amount of data which reveal interactions between
