Fig. 4.

How to determine what pattern cells will build to?

This schematic describes a thought experiment to focus attention on the stop condition for regeneration: how do cells decide what pattern constitutes ‘correct, finished’ repair so that they can stop growth and remodeling? (A) Different species of planaria have (and regenerate) different shapes of heads, for example round or flat. (B) If half of the stem cells (neoblasts) of one species are destroyed (by irradiation), and some neoblasts from another species are transplanted (C), we can amputate the resulting worm (D) and ask: what head shape will it regenerate? Perhaps it will be an in-between (averaged) shape, or perhaps one of the sets of neoblasts is dominant, or perhaps the head will undergo continuous and unceasing deformation as neither set of neoblasts is ever satisfied with the current shape of the head. It is important to note that none of the excellent molecular-genetic work in this field has given rise to a model which can make a prediction (or constrain) the outcome of this kind of question. This thought experiment illustrates the fact that questions of control theory, representation (encoding), and algorithmic control over regeneration have been so far largely left out of mechanistic work in pattern control.