Formation and maintenance of the organizer among the vertebrates.

The organizer is established at the blastula stage of development, under the influence of a special region of cells known as the Nieuwkoop center in amphibians, where Vg1/activin-like signals overlap with activity of the Wnt-pathway. Despite differences in their mode of early development, a similar region can be identified in other vertebrates. It has widely been assumed that once the organizer property is assigned to cells at this early stage, it is fixed so that by the gastrula stage, no new cells acquire organizer properties. However, when the organizer is ablated, it can regenerate for a limited period during gastrulation, a process regulated by both positive and negative signals emanating from various domains in the embryo. Here we compare the mechanisms that initially establish the organiser in the blastula with those that maintain it during gastrulation in different vertebrate classes, and argue that similar molecular mechanisms may be involved in the two processes. We also suggest that these mechanisms are required to ensure the appropriate location of the organizer property in the gastrula, where cells are continuously moving.