Biology Reference
In-Depth Information
model. The results we obtained show that it had certain properties
essential to embryogenesis.
6.4.1
Interstabilisation and autostabilisation
produce different effects
In the first instance, we produced a simulation with which we could
only test the effect of cell stabilisation. In this simplified version,
two cell types, RED and GREEN, fill a two-dimensional matrix.
Each box of the matrix represents one cell which may be either of
the two types, its identity being chosen by chance. At each simula-
tion step, the cell can change type randomly. This is the proba-
bilistic component of the model. However, the probability of its
changing type depends on its environment which itself depends on
the activity of the other cells of the population. The RED type of
cells synthesises red molecules and the GREEN type of cells
synthesises green molecules, these molecules diffusing in the matrix.
At any moment therefore, a cell is in an environment characterised
by concentrations of these red and green molecules.
The value for the probability (p), between 0 and 1, that a cell
will change type is a function of these concentrations of red and
green molecules. When p
0, the cell is stabilised. This is the sta-
bilising component of the model (Fig. 23). We tested two possible
methods of stabilisation. Interstabilisation is stabilisation of a cell
type by the molecules produced by the other cell type. In this case,
cells of the RED type are stabilised (p
=
0) when there is a large
concentration of green molecules in their environment (when they
exceed a certain threshold) and, in parallel, cells of the GREEN
type are stabilised (p
=
0) above a threshold concentration of
red molecules. Autostabilisation is stabilisation of a cell type by the
molecules which it synthesises. Here, RED is stabilised by red mol-
ecules and, likewise, GREEN by green molecules.
Figure 24 shows a typical result obtained for each method of sta-
bilisation. At the start of these simulations, the matrix is filled with
cells the type of which is selected at random (p
=
=
½ for each cell
