ABSTRACT
We have seen in the last chapter that all placodes originate from a common primordium. The latter is defined by the expression of transcriptional regulators of the Six and Eya families in the pre-placodal ectoderm (PPE) which is established as a distinct ectodermal domain during gastrulation. Six and Eya factors bias the PPE to develop into one of the different cranial placodes. During subsequent development, different parts of the PPE will give rise to different types of placodes. Whereas adenohypophyseal, olfactory, and lens placodes will form from its anterior portion, otic, lateral line, and epibranchial placodes will form from its posterior part, with profundal and trigeminal placodes sandwiched in between (Fig. 4.1).
