Positional information and pattern regulation in hydra: the formation of boundary regions following axial grafts.

A two-gradient model is proposed for regulation and regeneration of the head end of hydra in terms of positional information. One gradient is set up by a diffusible substance made at the head end, which may be regarded as a positional signal, and 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. A variety of instantaneous axial grafts have been carried out which are discussed in terms of this model. Some simple computer simulations of diffusion are provided. There is some evidence that the source of the positional signal is localized at the head end. Some experiments on changes in polarity are given. It also seems that the positional signal from the head end inhibits foot end formation.