Mutual activation of cell states that locally exclude each otherThe possibility to generate a sequence of structures by gene activation under the influence of a gradient has been outlined above (see gene activation ). In such a mechanism, the cells do not communicate directly with each other to obtain the correct determination. They measure the local concentration of a substance and behave accordingly. Due to this lack of communication, as a rule, a discontinuity cannot be repaired; especially when at later stages the signaling gradient is no longer available. The cells maintain the once obtained state of determination. A gradient mechanism is therefore inconvenient to account for a dynamic regulation of a correct neighborhood.
Segmentation: the repetitive pattern of (at least) three cell statesThe simplest periodic structure would consist of an alternation of two cell states, let us say, A and P. The two compartments found in Drosophila appear to support such a view. However, such a binary sequence has no polarity, in contrast to the biological observation. No signal would be available that indicates at which AP-border will form the segment border; the grouping could be AP/AP/AP or ..A/PA/PA/P.. Both sequences would have opposite polarity. This problem has two possible solutions: