Positional Information and Positional Signalling in Hydra

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Abstract
A model is proposed for regulation and regeneration of the head end of hydra in terms of positional information, which involves two gradients. One is a diffusible substance made at the head end, which may be regarded as a positional signal. The other is a more stable cellular parameter which is the positional value. The rule for head end formation is that the concentration of the diffusible substance falls a threshold amount below the positional value. This model, for which some computer simulation is provided, can account for head end formation in a wide variety of grafts. Evidence for a diffusable signal is provided by experiments in which the time/distance relationships for head inhibition by a grafted head are determined. Changes in positional value during regulation have been assayed and are much slower away from the boundary. Polarity is interpreted in terms of the interaction between the two gradients. The biochemical basis of the gradients is not known, but an approach to the problem has been made by treating hydra with a variety of chemical agents.

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