During gastrulation the notochord is formed. This long thin structure marks the midline within the mesoderm and inherits organizer function. Measuring the distance from this line (and not earlier from the patch-like organizer) can provide positional information for the mediolateral or DV axis. At early stages, the prospective notochord is a regulating pattern forming system: a removal leads to a re-expression of corresponding markers in chick and in Xenopus
(Psychoyos and Stern, 1996; Yuan et al, 1995; Levin and Mercola, 1998). This suggests that the cells next to the notochord are competent, but that the notochord suppresses its own further extension by some sort of lateral inhibition. However, if lateral inhibition is involved, why does the notochord (or more generally speaking the midline pattern) not decay into separated patches? Why does the lateral inhibition work only to the sides and not along the long extension? According to the model, a stripe-like activation is stable if the self-enhancing reaction has an upper bound (see Periodic Structures
). However, it is not possible to generate a single straight and long-extended structure using this mechanism alone since stripe formation requires a restricted lateral inhibition and other nearby stripes would not be suppressed. This is in contrast to the formation of a single spot-like activation in which the lateral inhibition need not be restricted by an upper bound of the activator concentration. A “hot stripe” system can be generated by a unique hot spot system (organizer) if the spot system induces the stripe system or its elongation while, in turn, the hot stripe repels and induces a shift of the hot spot.