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Fig. 7. Model of the three steps involved in PPE formation. (1) The embryonic
ectoderm is separated into presumptive epidermis (E), pre-placodal (P), neural
crest (NC) and neural plate (NP) domains in response to a combination of a
dorsoventral gradient of BMP-antagonizing factors (Noggin) and a gradient of
posteriorizing signals (Wnt, FGF). (2) six1 promotes the expression
of placode genes at the expense of neural crest and epidermal genes. (3) A
network of gene interactions further defines the ectodermal subdomains. Green
arrows indicate that the expression of the downstream gene is enhanced, and
red bars indicate that it is reduced. Lines are broken to indicate that
interactions may be direct or include intermediary genes.
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