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chemical reaction-diffusion system capable of exhibiting intelligent behaviors such
as quorum sensing in bacteria as the
Trentonator
. Thus, quorum sensing in bacteria
is the first Trentonator to be characterized in molecular terms. I would not be
surprised if human intelligence will turn out to be the result of a set of elementary
Trentonators
that are spatiotemporally organized within our brains, nor if there
exists a minimum number of neurons (10
2
-10
3
?) that is needed to form the basic
Trentonator responsible for human intelligence.
15.8 Morphogenesis as a Form of Quorum Sensing
Morphogenesis or shape development is a multicellular phenomenon. Therefore,
there is the distinct possibility that cells will cooperate in morphogenesis just as
they do in quorum sensing.
Drosophila melanogaster
is the best studied model
organism for morphogenesis.
Drosophila
morphogenesis starts with a fertilized
single-celled egg which becomes a multicellular embryo with structured tissues and
specialized cells and organs. This process occurs through many cell divisions, shape
changes, and cell migrations called gastrulation. What is interesting about morpho-
genesis from a theoretical point of view is this: The initially symmetrical embryo
undergoes a series of
symmetry breaking processes
to become a less symmetric
structure. In other words, the
Drosophila
embryo undergoes state transitions from
disordered
to
ordered
states, to use the terminology of condensed matter physics of
critical phenomena (Landau and Lifshitz 1990). Thus,
symmetry breakings
and
disorder-order transitions
may be regarded as two of the most fundamental
processes in morphogenesis. The first mathematical model of symmetry breaking
in morphogenesis was proposed by A. Turing in 1952 (Gribbin 2004) which is
based on three elementary processes - (1) A catalyzes the formation of A and B, (2)
B inhibits the formation of A, and (3) A and B have different diffusion constants.
These three elements were necessary and sufficient to produce
symmetry breakings
in chemical concentration fields (Gribbin 2004, p. 127).
The simplest symmetry-breaking process in
Drosophila
morphogenesis involves
what is known as “convergent extension,” in which cells in a two-layer configura-
tion migrate vertically so as to form one-layered cells with an increased horizontal
length. Such cellular rearrangements are well within the capability of cells, since
they are self-organizing chemical reaction diffusion systems endowed with the
ability to execute the c-triad, i.e.,
communication
,
computation
, and
construction
(see Table
15.6
) (Ji and Ciobanu 2003). Consequently,
Drosophila
cells in this
stage of development can (1) communicate with one another by synthesizing and
secreting one or more morphogens (akin to autoinducers in bacteria) which diffuse
away from the cells that produce them, (2) can compute the number of neighboring
cells based on the combined concentrations of the morphogens secreted by neigh-
boring cells, and (3) when the morphogen concentrations exceed a threshold value,
can trigger the signal transduction cascade, leading to turning on or off of a set of
genes needed to rearrange cells to produce the right embryological structure, for
example,
convergent extension
. These same processes are involved in the phenom-
enon of quorum sensing in bacteria presented in Sect.
15.7
.
