Abstract
Cells in developing embryos behave according to their positions in the organism, and therefore seem to be receiving `positional information'. A widespread view of the mechanism for this is that each cell responds locally to the 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 cell, only a few thousand molecules may be involved. Fluctuations to be expected 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 intolerable level of error for some developmental pattern-forming events. Embryos must have means of suppressing such errors. We maintain that this requires communication between cells, and illustrate this by using the reaction part of two well-known Turing-type reactionâ€"diffusion models as the local gradient reader. We show that switching on diffusion in these models leads to adequate suppression of positional errors.,Peer reviewed,Published. Received 31 December 1997, Revised 20 July 1998, Accepted 21 July 1998, Available online 24 March 1999.