New twists on embryonic patterning. EMBO workshop: embryonic organizer signaling: the next frontiers.

The term ‘organizer’ was coined by Hans Spemann and Hilde Mangold in 1924, based on the seminal observation that a small region of the amphibian gastrula, the dorsal lip, when transplanted to an ectopic location, had the ability to induce, or organize, the formation of a twinned embryo, recruiting neighboring cells from the host into a variety of cell differentiation pathways. The gastrula organizer has been reviewed in extenso recently (special issue of Int. J. Dev. Biol., 45). It has become apparent that organizing centers, which secrete signals that pattern adjacent regions, are a recurring theme in developmental biology. Organizing centers function during neural tube formation in vertebrates, compartment boundary formation in Drosophila, limb development and the establishment of left–right asymmetry. Intercellular signaling is effected through a surprisingly small number of cell surface receptor-mediated signaling pathways, in particular Hedgehog, Wnt, FGF, Delta/Notch and TGF-β family members. Molecular and genetic studies on the various organizing centers have revealed that these signaling pathways are highly modulated, by agonist and antagonist signals and a variety of post-translational modifications, to generate the intricate patterns observed in developing embryos. The complexity, mechanisms of action and the conceptual convergence/divergence of distinct embryonic organizer centers was discussed at the EMBO workshop on ‘Embryonic organizer signaling: the next frontiers’ held at the EMBL on April 28–30, 2001 in Heidelberg. In this review, we highlight some of the salient themes that emerged.