The juxtaposition of presumptive neural plate and epidermis forms a signaling center responsible for the stereotypic pattern of dorsal neurons in the neural plate (including lateral primary neurons in amphibians and fish), roofplate, neural crest cells

The juxtaposition of presumptive neural plate and epidermis forms a signaling center responsible for the stereotypic pattern of dorsal neurons in the neural plate (including lateral primary neurons in amphibians and fish), roofplate, neural crest cells and cranial placodes (Baker and Bronner-Fraser, 2001; Moury and Jacobson, 1990; Selleck and Bronner-Fraser, 1995). In addition, signals from this region influence mesodermal structures such as somites (Lassar and Munsterberg, 1996; Pourquie, 2000) and heart (Raffin et al., 2000; Sarasa and Climent, 1987). Although considerable progress has been made recently in the molecular characterization of neuralinducing factors, relatively little is known about the molecules that determine the site of the border between neural plate and epidermis. Diffusible proteins such as BMP, Wnt and FGF isoforms play an important role in patterning neural and non-neural ectoderm along the mediolateral axis in Xenopus and zebrafish. BMP antagonists, such as chordin and noggin, initiate neural induction in the dorsal ectoderm while BMP signaling in the ventral ectoderm represses neural fates and promotes epidermal differentiation (Brewster et al., 1998; HemmatiBrivanlou and Melton, 1997; Kuo et al., 1998; Mizuseki et al., 1998; Nakata et al., 1997; Sasai et al., 1994; Smith et al., 1993; Wilson and Hemmati-Brivanlou, 1995). One model for neural plate border formation suggests that the ectoderm is differentially patterned by threshold levels of BMP signaling: high levels induce epidermal fates, low or absent signaling permits neural differentiation while intermediate levels induce border fates (Marchant et al., 1998; Morgan and Sargent, 1997; Nguyen et al., 1998; Wilson et al., 1997). However, studies showing that not all aspects of neural and neural crest induction are recapitulated by modulating BMP levels in non-neural tissues (e.g. LaBonne and Bronner-Fraser, 1998) have prompted an examination of additional factors that might synergize with BMP to control cell fate at the border region of late blastula embryos (Bachiller et al., 2000; Klingensmith et al., 1999; Streit and Stern, 1999). In particular, Wnt and FGF isoforms appear critical for induction of neural crest and cranial placodal cells (Adamska et al., 2000; Chang and Hemmati-Brivanlou, 1998; LaBonne and Bronner-Fraser, 1998; Mayor et al., 1997; Phillips et al., 2001; Saint-Jeannet et al., 1997; Streit and Stern, 1999; Vallin et al., 2001; Wilson et al., 2001). Several transcription factors have been proposed to influence neural/non-neural ectodermal patterning. For example, Xiro, Xash-3 and Zic family members are induced by preto early gastrula stage dorsalizing and neuralizing signals, such as noggin, and with time their expression becomes localized to the 331 Development 130, 331-342 © 2003 The Company of Biologists Ltd doi:10.1242/dev.00212