Biology Reference
In-Depth Information
The regulator is not stabilised as long as
the interaction AB has not occurred
?
?
A
A
a
b
a
b
?
?
B
B
b
a
a
b
A
B
b
a
a
b
stabilisation in
b
stabilisation in
a
(adapted from Kupiec, 1996)
F
IGURE
22. Stabilisation of cell types by signals.
Only the interaction of the mem-
brane proteins A and B corresponding to the simultaneous presence of
R
on genes
a
and
b
in two different cells triggers its phosphorylation and stabilisation on
genes
a
and
b
respectively. The result is coordinated stabilisation of the cell types
A and B.
Signalling pathways also lead to other epigenetic reactions such
as acetylation and methylation. All the epigenetic modifications of
proteins and DNA could therefore be involved in stabilisation by
signals (Paldi, 2003).
6.3.5
Experimental data relating to cell selection
and stabilisation
In the Darwinian model, interactions between cells are selective or
stabilising, which agrees with many experimental observations.
Embryogenesis of most organs is effected through differential mul-
tiplication of the cells, those relating to the organ being formed
multiplying more quickly than neighbouring cells in the embryo.
This has been demonstrated for organs as varied as those of the
genito-urinary system, the nervous system, the heart, the liver, the
ear, the limbs, the spinal cord etc. During embryogenesis, episodes of
massive cell death occur which 'sculpt' the organs and are essential
