Models for organizer and notochord formation

In the development of higher organisms, small groups of cells can play an important role by directing the fate of the surrounding cells. Models are discussed that account for the generation of such organizing regions. The generation of local high concentrations of signalling substances was proposed to depend on local self-enhancement combined with a long-range inhibition. The model accounts for pattern regulation, for instance, for the formation of multiple embryos after fragmentation of the early blastodisc in chickens or for head regeneration in the fresh water polyp Hydra. The model has found support from more recently discovered interactions involved in organizer formation. The mutual down-regulation of noggin/chordin and BMP-4 is proposed to function as an indirect self-enhancement, establishing in this way an essential prerequisite for primary pattern formation. Self-enhancement and long-range inhibition is also crucial for the generation of substructures such as bristles or tracheae. A poisoning of an organizing region by a second antagonistic reaction of a short range but a long time constant can lead to its displacement. Long extended structures can be formed as a trace behind the moving organizer. The notochord and the tracheae of insects are discussed as examples.