Biology Reference
In-Depth Information
(e) if the cell is RED, it metabolises green molecules. If it is GREEN
it metabolises red molecules. Depending on the concentration of
these molecules, the cell either dies, if there are not enough,
divides if it can metabolise enough of them, or stays alive with-
out dividing until the next simulation step. This is the aspect of
the model that corresponds to cell selection. The only cells that
survive and proliferate are those of a type adapted to their
microenvironment determined by the concentrations of red and
green molecules.
The results obtained with this model are very much more con-
clusive than those obtained with the first version without cell selec-
tion. An organised tissue structure is produced in a way which is
reproducible.
Figure 25 shows the typical result of a simulation. To start with,
the matrix is seeded with 16 cells the types of which are chosen at
random (Fig. 25A). The growth of these cells gives rise to a longi-
tudinal structure composed of two cell layers, RED and GREEN
(Fig. 25B). The limit between these two layers is well defined
throughout the length of the structure and the thickness of the lay-
ers is regular. This bilayer of cells continues to grow longitudinally
(Fig. 25C) until it reaches an 'adult' state when growth ceases
(Fig. 25D). From this moment, the simulation may be allowed
to continue but the cells will not multiply further and the bilayer
of cells will remain as it is. This structure possesses an invariable
characteristic — its organisation in a bilayer, and a variable charac-
teristic — its longitudinal shape. In other words, if we perform other
simulations starting with 16 new cells selected at random, with iden-
tical parameter values, the bilayer created will always have the same
characteristics (thickness of the layers of RED and GREEN cells)
but the longitudinal form of it will be different (see for example
the structure of Fig. 26A). However, in some simulations, the
bilayer does not form and if this is the case, in general all the cells
die. More rarely, they remain in a disordered state. This mortality
is compatible with experimental reality. In all species there is a
considerable level of embryonic mortality which is in large part
