So, we have our model of a single cell: a greatly simplified model consisting of a two-dimensional outline in which are strung six elastic springs, one of which has the special property of responding to an externally imposed stretch by actively contracting. That is stage one of the modelling process. In stage two, Oster and his colleagues cloned up a few dozen of their model cells and arranged them in a circle, like a (two-dimensional) blastula. Then they took one cell and tweaked its contractile filament to provoke it into contracting. What happened next is almost too wonderful to bear. The model blastula gastrulated! Here are six screenshots showing what happened (a to f below). A wave of contraction spread sideways from the cell that was provoked, and the ball of cells spontaneously invaginated.
Oster's model blastula gastrulating
It gets even better. Oster and his colleagues tried the experiment, on their computer model, of lowering the 'firing threshold' of the contractile filaments. The result was a wave of invagination that went further, and actually pinched off a 'neural tube' (screenshots a to h, overleaf). It is important to understand what a model such as this really is. It is not an accurate representation of neurulation. Quite apart from the fact that it is two-dimensional and simplified in many other ways, the ball of cells that 'neurulated' (screenshot a) was not a two-layered 'gastrula' as it should have been. It was the same blastula-like starting point as we had for the model of gastrulation above. It doesn't matter: models are not supposed to be totally accurate in every detail. The model still shows how easy it is to mimic various aspects of the behaviour of cells in an early embryo. The fact that the two-dimensional 'ball' of cells responded spontaneously to the stimulus even though the model is simpler than the real situation makes this a more powerful piece of evidence. It reassures us that the evolution of the various procedures of early embryonic development need not have been all that difficult. Note that it is the model that is simple, not the phenomenon that it demonstrates. That is the hallmark of a good scientific model.
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