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The grafting experiments of Spemann and Mangold have been a textbook classic for years, but as with many conclusions from experimental embryology, the idea that the dorsal lip of the blastopore ;organized' the early patterning of the embryo has sometimes come under question. In their 1983 paper in JEEM, Smith and Slack extended these classical experiments in newts to the now-standard amphibian model Xenopus laevis. By using injected lineage tracers, they distinguished the fates of graft and host, and showed unambiguously that the organizer is responsible for neural induction and that it dorsalizes the mesoderm.
Fig. 1. Organizer grafts result in induction of a secondary axis. (A) Schematic of the organizer graft created by Spemann and Mangold, using a light-gray newt donor (Triturus cristatus) grafted into a dark-gray host (Triturus taeniatus). The gastrulae are shown in hemisection for illustrative purposes only (dorsal is towards the right, and the dimensions of these embryos are more Xenopus-like than Triturus-like). (B) The famous result of an optimal grafting experiment (Spemann and Mangold, 1924), showing a section through the trunk of a twinned embryo. The light-gray graft has contributed to the notochord, medialsomite and floor plate of the secondary axis. The graft has an induced neural tube, somites, a pronephros and a secondary archenteron cavity. (C, D) Contemporary organizer grafts from Andrea E. Wills (UC Berkeley, CA, USA). (C) The section shows a rafted organizer labeled with lacZ mRNA and stained with Red-Gal; the section is taken through the trunk of a stage 28 Xenopus laevis embryo, where the axial tissues are also stained with Tor70 antibody. (D) Twinned Xenopus embryo, resulting from an organizer graft carried out at stage 10.
