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![]() The EGO, acting through what can be considered as a “Nieuwkoop activity” (Rossant see Figure 1), was shown to be a source of neural inducing signals and sufficient to initiate neural patterning. Rossant (Mount Sinai, Toronto) proposed the existence of an e arly g astrula o rganizer (EGO Tam) present in the embryo well before the appearance of the node. Tam (CMRI, Wentworthville, Australia) and J. Through a series of extremely skillful grafting and recombination experiments in the early mouse gastrula embryo, P. Is the Spemann's Organizer the Only Organizing Center in Early Embryos? Remarkably, animal caps treated with a combination of Activin and retinoic acid give rise to renal tubules in culture that can function in vivo when transplanted into Xenopus embryos previously subjected to kidney extirpation (Asashima). Animal caps treated with activin and sandwiched between two nontreated caps, behaved as organizer-like structures giving rise to embryo-like explants (M. Indeed, different responses to activin have been shown to occur in vitro, with the formation of structures with extraordinary organotypic characteristics. The mechanism proposed by Gurdon has been devised with the use of activin as a model. In the case of BMP, a higher degree of complexity appears, since molecules that bind to it, such as Chordin, could serve to transport it at a distance (De Robertis). Thus, cells can elicit a different response at different times after exposure to a signal emanating from a given source. Gurdon, Wellcome CRC Institute, Cambridge, UK). Furthermore, the same cells can respond differently to varying concentrations of the signal, choosing the response corresponding to the highest level they detect (J. Cells seem to be able to make continuous assessments of the concentration of the signal by the perception of the number of receptors occupied. This is in agreement with results obtained in chick embryos, where ectopic expression of Chordin and/or Noggin is not able to induce neural tissue in nonneural ectoderm ( They do however, show forebrain defects, compatible with a function in maintenance and/or differentiation, rather than in initial neural induction. Furthermore, mice double mutant for chordin and noggin present severe reductions of the prosencephalon but a fairly complete neural tube (De Robertis). Interestingly, mice mutant for chordin, noggin, or follistatin, as well as fish lacking chordin function, show complete early neural tubes. In this respect, the fish and mouse embryos are proving to be highly informative owing to the possibility of generating and analyzing mutants. Loss-of-function experiments are needed to clarify this matter, something that is not so easy to perform in amphibia. Those experiments, however, had the limitation that what they really showed was the ability of those factors to elicit a function, but they did not unequivocally demonstrate that they were necessary and sufficient for neural induction in the normal embryo. Xenopus is an extremely useful system to overexpress or ectopically express different molecules, a strategy that led to the identification of BMP antagonists as neural inducers. V, ventral D, dorsal A, anterior P, posterior. ![]() In the fish, the additional ectodermal organizer is also shown. Both the organizer and the proposed anterior signaling centers are shown in all vertebrates except in the chick, where the latter has been suggested to reside in the organizer derivatives (see text). (G) Section through a 6.5 dpc mouse embryo the arrow indicates the movement of the anterior visceral endoderm, and division between extraembryonic and embryonic tissues along the proximodistal axis is indicated. (F) Dorsal view and scheme of the lateral organization of layers in a gastrulating chick embryo. The black arrows indicate the progression of the involuted mesoderm and the pink arrow, the signaling from the anterior ectodermal cells. Embryos are shown open in order to visualize the internal tissues. Xenopus (B and D) and zebrafish embryos (C and E) are shown at both early and late gastrulation stages. The interplay between them results in the patterning of the ectoderm, the mesoderm, and the endoderm (see text). The BMP and Wnt pathways triggered in the ventral side of the Xenopus embryo are antagonized by the action of molecules secreted by the organizer in the dorsal side. (A) Scheme of the opposing signaling systems that result in the regionalization of the gastrula.
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