Xenbase Image ID: 78828

Fig. 9. A model for the induction and positioning of the isthmus organizer. (A) Gastrula. Xiro1 encompasses Gbx2 expressing domain and the presumptive midbrain territory of Otx2 and participate in the activation of both genes (arrows). In addition, Otx2 also activates Xiro1 expression in the midbrain. At this stage Otx2 and Gbx2 expression domains overlap in the prospective isthmus and the mutual repressive activities between the corresponding proteins begin (red lines) (B) Early neurula. The expression domains of Otx2 and Gbx2 start to separate although a faint overlapping is still detected. At this stage, Xiro1 is no longer able to activate Otx2. In addition, Fgf8 expression, and therefore the establishment of the isthmus, begins as a result of the overlapping domain created by Otx2 and Gbx2 (broken arrows) and the activity of Xiro1 in this region (arrow). (C) Mid neurula. A sharp boundary between Otx2 and Gbx2 arises, which is probably due to an equilibrium reached by their cross-inhibitory activities (red lines). The interaction between Otx2 and Gbx2 maintains Fgf8, which reinforces the expression of Gbx2 in the caudal face of the isthmus (arrow). In addition, Fgf8 induces En2 expression in the competent territory defined by the co-expression of Otx2 and Xiro1. a, anterior; p, posterior. Image published in: Glavic A et al. (2002) Copyright © 2002. Image reproduced with permission of the publisher and the copyright holder. This is an Open Access article distributed under the terms of the Creative Commons Attribution License. Larger Image Printer Friendly View

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