Head and foot formation: two organizing regions at opposite sites of a field:
A Hydra is under control of two organizing regions located at opposite ends. This is a common feature of many morphogenetic fields. How can it be achieved that two organizing centres reliably appear at opposite positions? A simple cross-inhibition is not appropriate since in small (young) animals head and foot must appear very close together. At such short distances a direct mutual inhibition between the two systems lead would lead to the suppression of a foot by the nearby head. As mentioned, hydra has a polarity
. The head activation appears at the position where the cells are most competent, and the head signal stabilizes this competence in the surroundings. If the foot system has the opposite behavior, i.e., if the foot appears in the region of lowest head-forming competence (lowest source density, blue in the simulation), the foot signal (pink) forms at the maximum distance from the head signal (green) without a direct inhibition. Therefore, in small animals head and foot system can coexist at a close neighborhood. In the simulation further below this is demonstrated by a simulation of regeneration in a small fragment. The graded source density only generates a preference for the terminal positions.
Hypostome and tentacles: formation of structures adjacent to each other:
During development many structures emerge close to each other. A controlled neighborhood of structures is enforced if one structure activates the other on long range and excludes it locally (see also segmentation
and left-right patterning
) In Hydra, the tentacles appear around the opening of the gastric column, the hypostome. Many experiments can be accounted for by the assumption that tentacles are under control of a separate activator-inhibitor system that also depends on the source density. Since the primary head system causes an increase in the source density in a wider area, the head system generates the precondition for tentacle initiation. Locally, however, the head signal suppresses tentacle formation. Thus, tentacle formation (red) occurs next to the head signal. The following simulations show that the model describes not only the generation but also the correct regeneration of these signals: