While progress has been made in identifying signals that may initiate the earliest steps of neural induction, apparent difference between amniotes and anamniotes remain to be resolved. Neural induction is initiated when the ectoderm is exposed to signals emanating from Spemann's organiser in amphibians, or its functional equivalents - the nodes in amniotes. Later in development, these specified cells will no longer respond to signals that induce alternative fates, and have thus committed to a neural fate. According to this model, gastrula ectodermal cells adopt a default neural state when removed from extra cellular signals. .
In Xenopus embryos, high levels of BMP signaling in the ventral ectoderm suppress its neural tendencies and cause it to adopt an epidermal fate (Wilson & Hemmati-Brivanlou 1997). BMP antagonists expressed in the organiser repress the epidermalising activity of BMP-4, and the release from inhibition is sufficient to specify neural fates. In addition, BMP-4 has been found to have a dose dependant effects on Xenopus ectoderm, inducing in epidermis at high doses and neural plate border fates at lower doses (Wilson et al 1997). Also, the neural induction occurs much earlier in development, at the blastula stage before the organiser region has formed.
The amniote node, like the Xenopus organiser expresses BMP antagonists, including follistatin, noggin and chordin (McMahon et al 1998). In amniotes, however, expression of BMP-2 and BMP-4 is not detected in ectodermal regions prior to neural induction (Streit & Stern 1999). Therefore, in these organisms there is no need for BMP antagonists to suppress BMP-mediated epidermalising influences in order to reveal the underlying neural potential of the neural plate.
Accordingly, ectopic chordin expression cannot induce expression of neural markers in chick extra-embryonic or non-neural ectoderm, even though these tissues can form and ectopic neural plate in response to a grafted node.
