In 1924, Spemann and Mangold demonstrated the induction of Siamese twins in transplantation experiments with salamander eggs. Recent work in amphibian embryos has followed their lead and uncovered that cells in signalling centres that are located at the dorsal and ventral poles of the gastrula embryo communicate with each other through a network of secreted growth-factor antagonists, a protease that degrades them, a protease inhibitor and bone-morphogenic-protein signals.
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I would like to thank the multiple generations of postdoctoral and graduate students that helped unravel the Spemann's organizer in our laboratory. B. Reversade, H. Kuroda and H. Lee generously provided material for the figures. Many thanks also to the members of the University of California, Los Angeles (UCLA) Embryology Club, which celebrates its twentieth anniversary this year, for providing the atmosphere that made developing these ideas possible. Our work is supported by the Norman Sprague Endowment, a National Institutes of Health MERIT Award and the Howard Hughes Medical Institute.
The author declares no competing financial interests.
Re-enactment of the Spemann–Mangold experiment by Eddy De Robertis. (AVI 12207 kb)
The movie starts with photos of Hans Spemann and Hilde Mangold circa 1924 (Ref. 1). Next, it shows the author at the dissection microscope. Two embryos can be seen, one of which has the dorsal blastopore lip, the Spemann's organizer, clearly visible as a crescent. With the help of a tungsten needle and forceps, a square of organizer tissue is excised — the operation is done free-hand. The organizer is pushed into the ventral side of a recipient gastrula with an eyebrow hair. One hour after transplantation, the graft has, almost miraculously, healed into the host embryo. Two days later, a Siamese twin has developed with two perfect body axes. The Spemann's organizer graft induced complete central nervous systems and mesodermal somites in tissues of the host that would otherwise have become ventral tissue.
1. De Robertis, E. M. & Aréchaga, J. (eds). Special issue: the Spemann–Mangold Organizer Int. J. Dev. Biol. 45, (2001).
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De Robertis, E. Spemann's organizer and self-regulation in amphibian embryos. Nat Rev Mol Cell Biol 7, 296–302 (2006). https://doi.org/10.1038/nrm1855
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