Figure 2.

Several steps are involved in forming the pre-placodal ectoderm.

Top: The embryonic ectoderm of a neurula stage has been flattened into a sheet. On the left is represented the fields apparent at early neural plate stages: epidermis (Epi), neural border zone (NB) and neural plate (NP). On the right the NB has divided into its neural crest (NC) and pre-placodal ectoderm (PPE) derivatives.

Bottom: Different sets of transcription factors are differentially expressed in these ectodermal domains over developmental time. At gastrula stages, the expression domains of neural plate genes (blue bar; SoxB1 and Zic) and epidermal genes (yellow bars; Msx1, Dlx, GATA, TFAP2, Foxi) overlap in a region that will become the neural border zone. At early neural plate stages, SoxB1 gene expression recedes from the NB zone, Zic genes (purple bars) are no longer expressed in the medial region of the neural plate, Msx1 and Pax3 expression (dark green bars) is confined to the NB zone, whereas Dlx, GATA, TFAP2 and are expressed in both the NB zone and epidermis (yellow bars). At late neural plate stages Six and Eya genes are expressed in the PPE domain (medium green bars) and “neural crest specifying” genes (Msx1, Pax3, FoxD3, Sox9/10, Snail2) are expressed in the neural crest domain (light green bars). Dlx3 and TFAP2 also are expressed in the neural crest domain, whereas Dlx5/6, GATA and Foxi1 are expressed in the epidermis (yellow bars), and are now excluded from the PPE. Once these four domains are formed, their boundaries appear to be maintained by mutual repression between domain-specific transcription factors (red bars).