ABSTRACT
The brief survey of placode development in Chapter 2 shows that placodes are quite diverse and form different types of sense organs, ganglia, and endocrine glands at different positions in the head. However, underlying this diversity are some important similarities between the development of different placodes. In this chapter I will show that these shared aspects of placode development are due to the origin of all placodes from a common primordium, the so-called pre-placodal ectoderm (PPE). I will then review how the PPE originates in early vertebrate development in parallel to other ectodermal territories such as neural crest, neural plate, and epidermis. I will focus in particular on the role of transcription factors during this process, since these play central roles for the cell fate decisions that step by step convert pluripotent cells into the sensory and neurosecretory cell types that concern us in this book.
