3.1 Chemical communication between cells. Motion and morphogenesis

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It is easy to understand the network of bio-oscillatiors when we concentrate on a relatively simple model. We introduce a chemical oscillator that drives a change of behaviour and, later,a change of form, during the life cycle of the cellular slime mould: Dictyostelium discoideum. This useful Amoeba lives in our soil, and has been extensively studied by Bonner (1993) and recently by A.F.M.Marée (2001) who produced animations of their morphogenesis (Stan Marée, 2001). An amoeba colony develops from a few thousand separately living, vegetatively reproducing cells. When one of the individual cells has reached a phase of maturity, it starts to produce cyclic AMP. It is a relatively simple metabolite, and it signals to other cells to produce and release more of the same substance. Moreover it appeals to the cells to move in the direction of the source of the chemical signal, thus causing aggregation of cells. When a certain level of AMP has been reached the cells start to produce phospho-diesterase, an enzyme that destroys cyclic AMP. By its presence the signal of the newly formed AMP is rapidly turned off again. A prey-predator type of oscillation ensues. The release of the signal substance spreads as a pulse-wave in concentrical circles over the population and causes the cells to move in shifts towards the centre where the substance was first released. In this way colonies of cells are called together around a 'pacemaker' cell (aggregation). Where the waves meet those of other pacemaker centres the colonies compete with their neighbours for allegiance of uncommitted amoeba in this no man's land.

In a second phase the now compact mass of cells moves over the surface in a manner that resembles a crawling creature; it is in fact called a slug. The motion is periodic and has about the same frequency as that of the pulsing wave motion of aggregation that was observed in the first phase. One sees peristaltic waves of contraction travelling antero-posteriorly. The same metabolic substance, cyclic AMP, that in conjunction with chemotactic response, has caused a spatially organized aggregation field, now generates movement in what has become a multicellular individual.

Two antagonistic chemicals alternate in oscillatory fashion. In synergy they produce a new behaviour, a new form, a development towards a more complex strategy for survival. That becomes evident in a third phase. The same chemical oscillatory signal causes a further differentiation of parts of the colony. It establishes a morphogenetic field first in one large partition, then in successively smaller divisions that grow out into stalks. The stalks finally lead to the development of fruiting bodies and spore cells. All this happens by alternating messages: chemical message (1) says: follow the scent; chemical message (2) says delete the scent-trail. The signal spreads in the form of waves . Depending on the phase in which the waves approach each other a behaviour emerges: the colony moves by projecting pseudopodes, or the mass of cells forms stalks or limbs. There is a continuity of development and behaviour: the spreading of a metabolic process, in the form of pulse-like waves over an excitable medium, becomes manifest in one context as morphogenesis and in another as motion.

Thus it appears that the difference between emerging body-forms and body-functions is their position on the time-scale. More than one philosopher will be surprised that the eternal question of the relation between matter and mind, form and function, has a relatively simple answer. Because function has the shorter response time, it can induce change in form. And, because form has the longer response time, it can determine function. Form and Function are allies in a two-way hierarchic relationship to each other.

Chemical communication shows a linguistic structure.

B.J.Eshel and colleagues (2004) explain that also colonies of bacteria have developed intricate capabilities for information exchange between individual cells and between colonies. Cells use chemotactic signals, they can sense when a colony has reached a critical mass to start spore-production, they communicate by exchanging plasmids. By cooperating they self-organize into highly structured colonies that adapt intelligently to new environments.

Because the genome and the intracellular signal transduction are flexible, the quasi-organism forms into an adaptive network. The authors propose this to have a linguistic structure:

the shared interpretations of chemical cues follow rules of grammar, the exchange of meaningful chemical messages is the semantic content, the probing dialogues with the environment are the equivalent of pragmatics.

Meaningful communication is based on colonial identity. It allows intentional behaviour (e.g. pheromone guided courtship before mating), purposeful revision of colony structure (e.g. for the formation of fruiting bodies). Recognizing and identifying other colonies might be considered a form of social intelligence

3.2 Resonance in a morphogenetic field. Dancing to the chemical beat.