Figure 1. Model for amphibian ectodermal patterning during gastrulation. (a) Dorsal view of a X. laevis gastrula embryo. During gastrulation, the organizer (red) secretes factors (including noggin, chordin, follistatin and Xnr3) that bind directly to, and inhibit epidermal inducers (including BMP2, BMP4 and BMP7). The border of the dorsal and ventral ectoderm is enlarged in (b). (b) In the dorsal ectoderm, factors secreted by the organizer inhibit epidermal differentiation by binding to epidermal inducers. In the ventral ectoderm, these BMP antagonists are absent and epidermal inducers can bind to their receptors without interference, and epidermal differentiation takes place. Signalling in ventral ectodermal cells downstream of these receptors is illustrated in (c). (c) BMP signalling in a ventral ectodermal cell. BMP signals are transduced via type I and II serine/threonine receptors. BMP-activated type I receptors activate the pathway-specific effector Smad1, which forms a complex with regulatory Smad4; this complex translocates to the nucleus where the expression of epidermal genes is induced and neural gene expression is inhibited, ultimately resulting in epidermal induction in the ventral ectoderm. In the Smad signal transduction cascade, several inhibitory Smads have been identified; Smad7 (and perhaps Smad8) inhibits signalling of TGF growth factors at the receptor level; Smad6 competes with Smad1 for Smad 4, resulting in an inactive Smad1/Smad6 heteromer.