The Role of the Placodes in the Development of the Glossopharyngeal, Vagal, and Trigeminal Ganglia

The epibranchial placodes combine with the neural crest to form the inferior and superior ganglia of the glossopharyngeal and vagal cranial nerves, respectively. By comparison, the single trigeminal ganglion is composed of both neural crest and placodal cells. The steps that lead up to these events include gastrulation and the embryology of the notochord, neural crest, and the placodes. Each of these steps is reviewed in some detail. In previous reviews in this series, the embryology related to the olfactory, otic, and lens placodes, and to the geniculate ganglia has been discussed.1-3 However, the somewhat unusual embryology of the 2 ganglia of cranial nerves IX and X was only briefly mentioned as was the development of the trigeminal ganglion.4 This present review revisits these events and specifically focuses on how these ganglia develop.Learning Objective: The reader will learn the unusual development of the superior and inferior glossopharyngeal and the vagal ganglia as well as review the varied steps in the embryology that proceeds these events. By comparison, the development of the single trigeminal ganglion is presented and the differences in its development from that of the ganglia of cranial nerves IX and X are emphasized.