Hetero-organisation - The Origin of Individuals - page 150

Autostabilisation is therefore necessary for producing tissues
with a certain degree of spatial extension. In a probabilistic context
where it is a question of stabilising cell types, this result confirms
the importance for their propagation of positive retroaction loops
(Lewis et al ., 1977).
However, in the two cases of inter- and autostabilisation, the
shape of the areas cannot be reproduced from one simulation to the
next, therefore we modified the model.

6.4.2 Cell selection creates organised structures

Following these preliminary results, we improved the model by
adding a cell selection mechanism. In the first version of the model,
the cells did not multiply nor did they die. In this second version,
the cells function in line with the autostabilisation model so that a
certain expanse of tissues is obtained, and in addition, they are
interdependent for survival and multiplication. This means that, to
survive and be able to divide, a cell must metabolise a certain quan-
tity of molecules produced by the other cell type. The red and green
molecules thus have pleiotropic effects. They are involved in the
autostabilisation of their original cell type and in selection of the
heterologous cell type. They are analogous in this respect to real
growth factors which are proliferation, survival, or differentiation
factors depending on the cell context (see for example: Fortunel
et al ., 2000; Tjwa et al ., 2003).
To sum up, therefore, in this improved model, in each RED or
GREEN type of cell and at each stage of simulation:

(a) red or green molecules are synthesised;
(b) some of the molecules are broken down;
(c) the molecules not broken down diffuse;
(d) the identity of the cell is determined by a law of probability
which is a function of the red or green molecular concentra-
tions, and is identical to that used in the first version of the
model and systematically used depending on the mode of
autostabilisation;