Formation and maintenance of the organizer among the vertebrates

The organizer is established at the blastula stage of development, under th e influence of a special region of cells known as the Nieuwkoop center in a mphibians. 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 t hat once the organizer property is assigned to cells at this early stage, i t is fixed so that by the gastrula stage, no new cells acquire organizer pr operties. However, when the organizer is ablated, it can regenerate for a l imited period during gastrulation, a process regulated by both positive and negative signals emanating from various domains in the embryo. Here we com pare the mechanisms that initially establish the organiser in the blastula with those that maintain it during gastrulation in different vertebrate cla sses, and argue that similar molecular mechanisms may be involved in the tw o processes. We also suggest that these mechanisms are required to ensure t he appropriate location of the organizer property in the gastrula,where cel ls are continuously moving.