Suppression of positional errors in biological development

Cells in developing embryos behave according to their positions in the orga nism, and therefore seem to be receiving 'positional information'. A widesp read view of the mechanism for this is that each cell responds locally to t he concentration level of some extracellular chemical which is distributed in a spatial gradient. For molecules conveying and receiving the positional signal, concentrations are likely to be low enough that, per individual ce ll, only a few thousand molecules may be involved. Fluctuations to be expec ted in these numbers (Poisson distribution) could readily lead to errors up to a few percent of embryo length in the reading of position. This is an i ntolerable level of error for some developmental pattern-forming events. Em bryos must have means of suppressing such errors. We maintain that this req uires communication between cells, and illustrate this by using the reactio n part of two well-known Turing-type reaction-diffusion models as the local gradient reader. We show that switching on diffusion in these models leads to adequate suppression of positional errors. (C) 1999 Elsevier Science In c. All rights reserved.