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(Maternally supplied) asymmetries in the competence can suppress supernumerary organizing regions

In many egg, strong asymmetries have been found that result either from sperm entry and cortical rotations or gravity. In the model, a strong asymmetry shortens the time required that one part dominates over the other. Moreover, in larger embryos the danger exists that multiple organizing regions are formed. This is illustrated in the simulations below. A substantial asymmetry in the ability of the cells to perform the self-enhancement (blue) can lead to a single maximum (green, the red ring is the inhibitor distribution).
A somewhat weaker asymmetry can lead to a second maximum at the most disfavoured site, a characteristic feature of many systems when generating a secondary organizer:

Without an asymmetry, up to three organizing regions could be formed with the same set of parameters. In the simulation shown above, an incipient third maximum can be seen that finally looses the competition with the more favoured maximum. Such multiple organizing regions have been observed by Nieuwkoop [1] in composite aggregates of animal and vegetal Xenopus cells.
Nevertheless, the formation of a single organizing region is possible if the total area in which the pattern formation can take place is small.
In a ring with only half as many cells, only a single maximum emerges even when initiated by random fluctuations only: Thus, from the model it is expected that initial asymmetries such as maternal determinants are of advantage especially for large embryos. They make sure that only a single organizer arises and that this is a rapid process.


  1. Nieuwkoop, PD (1992). The formation of the mesoderm in urodelean amphibians. VI. The self-organizing capacity of the induced meso-endoderm. Roux's Arch. Dev. Biol. 201,18-29

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