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Amphibians and Fish
Like amphibians, fish induce the epidermis by BMPs (especially BMP2B) and Wnt proteins (especially Wnt8) made in the ventral and lateral regions of the embryo (see Schier 2001; Tucker et al. 2008). The notochords of both zebrafish and Xenopus secrete factors (the homologues of chordin, noggin, and follistatin) that block this induction, thereby allowing the ectoderm to become neural (Dal-Pra 2006). As in amphibians, FGFs made in the dorsal side of the embryo also inhibit BMP gene expression (Fürthauer et al. 2004; Tsang et al. 2004; Little and Mullins 2006). In the caudal region of the embryo, FGF signaling is probably the predominant neural specifier (Kudoh et al. 2004). And as in Xenopus, insulin-like growth factors (IGFs) also play a role in the production of the anterior neural plate. Zebrafish IGFs appear to upregulate chordin and goosecoid while restricting the expression of bmp2b. Although IGFs appear to be made throughout the embryo, during gastrulation the IGF receptors are found predominantly in the anterior portion of the embryo (Eivers et al. 2004). Also, Wnt inhibitors appear to play roles in head formation. When antisense morpholinos are used to downregulate Wnt3a and Wnt8 throughout gastrulating zebrafish embryos, the trunk structures become anteriorized (Shimizu et al. 2005b).
Literature Cited
Dal-Pra, S., M. Fürthauer, J. Van-Celst, B. Thisse and C. Thisse. 2006. Noggin1 and Follistatin-like2 function redundantly to Chordin to antagonize BMP activity. Dev. Biol. 298: 514–526.
Eivers, E., K. McCarthy, C. Glynn, C. M. Nolan and L. Byrnes. 2004. Insulin-like growth factor (IGF) signaling is required for early dorso-anterior development of the zebrafish embryo. Int. J. Dev. Biol. 48: 1131–1140.
Fürthauer, M., J. van Celst, C. Thisse and B. Thisse. 2004. FGF signaling controls the dorsoventral patterning of the zebrafish embryo. Development 131: 2853–2864.
Kudoh, T., M. L. Concha, C. Houart, I. B. Dawid and S. W. Wilson. 2004. Combinatorial FGF and BMP signalling patterns the gastrula ectoderm into prospective neural and epidermal domains. Development 131: 3581–3592.
Little, S. C. and M. C. Mullins. 2006. Extracellular modulation of BMP activity in patterning the dorsoventral axis. Birth Defects Res. C Embryo Today 78: 224–242.
Schier, A. F. 2001. Axis formation and patterning in zebrafish. Curr. Opin. Genet. Dev. 11: 393–404.
Shimizu, T., Y. K. Bae, O. Muraoka and M. Hibi. 2005b. Interaction of Wnt and caudal-related genes in zebrafish posterior body formation. Dev. Biol. 279: 125–141.
Tsang, M., S. Maegawa, A. Kiang, R. Habas, E. Weinberg and I. B. Dawid. 2004. A role for MKP3 in axial patterning of the zebrafish embryo. Development 131: 2769–2779.
Tucker, J. A., K. A. Mintzer and M. C. Mullins. 2008. The BMP signaling gradient patterns dorsoventral tissues in a temporally progressive manner along the anteroposterior axis. Dev. Cell 14: 108–119.