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However, complete ectopic body axes, like the ones seen during transplantation of the dorsal blastopore lip 1 or the Nieuwkoop center cells 6, nearly never form. In the frog Xenopus, the Spemann organizer expresses Wnt and BMP antagonists 3, whose microinjection in single vegetal blastomeres can induce ectopic rostral or dorsal structures correspondingly 4, 5. Whether this similarity reflects the homologous or convergent origin of the cnidarian and vertebrate blastoporal axial organizers remained unclear, since the molecular nature of the signal conveying axial organizer properties to an embryonic tissue has not been determined outside deuterostomes. Although axis-forming blastoporal organizers were long thought to be a chordate-specific feature, transplantation of a fragment of the mid-gastrula blastopore lip in the sea anemone Nematostella vectensis, a member of the early branching non-bilaterian phylum Cnidaria (which, in addition to sea anemones, includes corals, hydroids and jellyfish), also resulted in the formation of an ectopic body axis 2, just like the Mangold-Spemann organizer in amphibians. The ability of the amphibian dorsal blastopore lip to induce ectopic body axes upon transplantation has fascinated biologists for nearly a century 1.
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