ABSTRACT
After reviewing the differentiation of neurons and sensory cells from individual placodes in the last chapter, I will sketch in the present chapter how photoreceptors and other cells type differentiate from the vertebrate retina. Photoreceptors share many of the general mechanisms of sensory and neuronal differentiation discussed in Chapter 5 with these placode-derived sensory cells and neurons. As explained in Chapter 1, the photoreceptors found in the retina of the vertebrate eye or in the pineal body do not develop from cranial placodes but from the neural tube. However, I will try to show in Volume 2 (Schlosser 2021) that vertebrate photoreceptors are evolutionarily closely related to other sensory cell types, such as mechanoreceptors, that are placode derived. To make this argument, I will briefly introduce vertebrate photoreceptors here and summarize how their specification and differentiation is transcriptionally regulated. I may be excused in drawing heavily on previous reviews in the following paragraphs since it is impossible to do justice to the vast literature on this topic in the short space available. I will not cover the early development of the retina here, which has been extensively reviewed elsewhere (Livesey and Cepko 2001; Hatakeyama and Kageyama 2004; Brzezinski and Reh 2015; Cepko 2015; Reh 2018).
