Simulations demonstrating regulatory properties of organizing regions
Mutual competition of nearby organizing regions
The induction of new organizing regions by implantation of tissue fragments derived from the endogenous organizing region has been investigated in the chick  (Khaner and Eyal-Giladi, 1989) and extensively in hydra [2,3]. In general, implantation of such tissue at a distance from an existing organizer can be successful, while a more proximal implantation may not. Removal of the endogenous organizing region greatly enhances the probability to form a new organizing region. This behavior is a straightforward consequence of the general pattern forming mechanism proposed, as demonstrated by the following simulations:
Implantation of somewhat activated tissue at a position distant to the endogenous organizer can induce a full activation. The inhibition of the latter is small enough that the somewhat elevated activation can develop into a full maximum (green: activator, read: inhibitor, blue: asymmetry, i.e., source density or competence).
When somewhat activated tissue is implanted closer to the endogenous organizing region, even a stronger activation can be down-regulated due to the elevated inhibitor level.
After partial removal of the existing organizer, the implanted activation has a better chance to survive. Whether one or two maxima survive depends on their distance and the total size of the field into which the inhibitor can escape. In this case, the still high competence in the surrounding of the organizer leads to a survival of both maxima.
One of the problems in the early search for molecules involved in organizer formation was that very unspecific manipulations, such as implantation of denatured tissue or injury turned out to be sufficient to trigger the formation of a secondary embryonic axis. Waddington et al.  proposed that this non-specificity results from the removal of an inhibitor. The tendency for unspecific induction is species-dependent. It is high in Triturus, the model system most studied in the early days but is low in Xenopus.
According to the model, a local activator maximum is necessarily surrounded by a field of inhibition. The inhibition decreases with distance from the maximum. Therefore, at higher distances, the inhibition may become insufficient to repress the onset of a new auto regulatory center. A region opposite to an organizer region is thus expected to be especially prone to any artificial decrease of the inhibition.
A leakage of the (rapidly diffusing) inhibitor at an injury or a release of degrading enzymes can be sufficient. A secondary organizing region generated by such unspecific means is expected to be indistinguishable of a normal maximum since it leads to a maximum of the genuine activator (below). Therefore, the model accounts for the fact that was most puzzling and discouraging for a long time.
- Khaner, O. and Eyal-Giladi, H. (1989). The chick's marginal zone and primitive streak formation. I. Coordinative effect of induction and inhibition. Dev. Biol. 134, 206-214.
- Wilby, O.K. and Webster, G. (1970). Experimental studies on axial polarity in hydra. J. Embryol. exp. Morphol. 24, 595-613.
- Wilby, O.K. and Webster, G. (1970). Studies on the transmission of hypostome inhibition in hydra. J. Embryol. exp. Morphol. 24, 583-593.