ABSTRACT
The last two chapters summarized how the pre-placodal ectoderm (PPE), a common primordium of various placodes, is established in the vertebrate embryo and then subdivided into different placodes. Each of these individual placodes then gives rise to a number of different cell types. For example, primary sensory cells (with an axon) develop from the olfactory placode, secondary sensory cells (without an axon) as well as sensory neurons from the otic and lateral line placodes, and sensory neurons but no sensory cells from the profundal/trigeminal and epibranchial placodes. Additional, non-neuronal cell types develop from many of the placodes, in particular, the neurosecretory and lens fiber cells generated by the adenohypophyseal and lens placodes, respectively. In this and the following chapters, I will review how the differentiation of these various placodal cell types is regulated. In this chapter, I will first summarize common mechanisms regulating the formation of neurons and sensory cells in most placodes. In Chapter 6, I will then discuss, how the specific neurons and sensory cells of individual placodes differentiate. In Chapter 7, I will briefly review the differentiation of photoreceptors. While not derived from placodes in vertebrates, photoreceptors will turn out to share a common evolutionary history with sensory cell types derived from placodes (see Schlosser 2021) and, therefore, must be discussed here. In Chapter 8, I will then review how the non-neuronal cell types of the adenohypophyseal and lens placodes differentiate.
