Emergence of Organizer function: A lot of “stuff” involved

Spemann's Organizer is a critical signaling center for patterning the embryo. It arises during blastula stages through the combined influences of dorsal modifying signals and general mesendoderm inducers. Dorsal modifying signals require the nuclear accumulation of β-catenin, but how this is initiated remained a mystery until recently. New findings now demonstrate that maternal Wnt11 activates the canonical Wnt signaling pathway and is essential for organizer formation. Furthermore, two of the earliest identified mesendoderm inducers, activin and Vg-1, have now been shown to be required for induction of a fully functional organizer. Finally, while it has been clear for a number of years that the Organizer secretes a cocktail of growth factor antagonists, their necessity for organizer function has been in question. Their requirement has now been demonstrated through a multiple “knockdown” approach in frog embryos. Here, we discuss the impact these recent findings have on our understanding of formation and function of the Organizer.