The Spemann-Mangold organizer: the control of fate specification and morphogenetic rearrangements during gastrulation in Xenopus.

Vertebrate embryonic development is controlled by sequentially operating signalling centres that organize spatial pattern by inductive interactions. The embryonic body plan is established during gastrulation through the action of the Spemann-Mangold or gastrula organizer, a signalling source discovered 75 years ago by Hans Spemann and Hilde Mangold. Transplantation of the organizer to a heterotopic location in a recipient embryo results in the formation of a secondary embryonic body axis, in which several tissue types, most notably somites and the neural tube, are derived from ventral host cells. Because of these non-cell autonomous recruiting or inducing activities the organizer has become a paradigm for studying intercellular communication in the vertebrate embryo. Here, I review some of the recent advances in understanding 1) the initiation of the Spemann-Mangold organizer, 2) its function in pattern formation along the dorsal-ventral and anterior-posterior axes and 3) the integration of cell fate specification events and downstream execution of morphogenetic movements during gastrulation in Xenopus laevis.