"The scale of pattern formation"

Date:
1970-1971
Reference:
PP/CRI/H/4/24
Catalogue details

Licence: In copyright

Credit: "The scale of pattern formation". Source: Wellcome Collection.

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    - for example, whether the morphogen moves in the intracellular space, or, alternatively, only within cells by means of special cell-to-cell junctions - it is impossible to set up a concentration gradient faster than a certain rate, the value of which can be estimated approximately. The actual figures are given in the paper quoted. In broad terms they show that if there is only a few hours available in development to set up a gradient system, it cannot be much larger than a millimetre or two. Shorter times would imply smaller distances. The basic postulate One can thus propose a basic postulate which states: 'when a pattern is first set up the size of the tissue embodying it is always small', ' Pattern ' is restricted to cases of positional information ; ' small ' is defined numerically in terms of the time available. A very rough and ready rule would be ^ u X ^ *Jt where x is in millimetres and t is in hours. Here x is the distance between the source and the sink, and t is the time available to set up the gradient, it should be stressed that it is the time needed for setting up which is limiting. After a gradient has been established it is possible to conceive mechanisms by which, during the growth of the tissue, it could be extended to cover quite considerable distances. It has been pointed out by Wolpert (Wolpert, 1969), independent of the above argument, that in fact the experimental data does suggest that a rule of this sort may be true. He notes that 'most embryonic fields seem to involve distances of less than 100 cells, and often less than 50'. His examples (his Table 1) are all taken from animals. A quick glance at botany would suggest that the same statement may be true there, but I speak without expert knowledge. Even if it turns out that this basic postulate is (almost) always correct, It would still not decisively favour the gradient hypothesis over the phase mechanism of Goodwin and Cohen. It is certainly true that signalling mechanisms are ideal for sending information over large distances. That is why they are used in the nerve axon. However, in order to convey posi tional information, as opposed to merely sending a sequence of pulses, two distinct signals must be sent. Each cell then obtains its position by com paring the phases of the two signals. It could be argued that if there is a limit to the precision with which this comparison can be made, there will be a limit to the distance (or number of cells) which can usefully be covered before the pattern starts to repeat again. If so, the phase différence mechan ism might not work efficiently over big distances, at least if position had to be spcified rather precisely. 28 S|eb 35
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