Most of the cranial sense organs of vertebrates arise from embryonic structures known as cranial placodes. Schlosser discusses how these primordia are established in the early embryo, how individual placodes develop, and how various placodally derived sensory and neurosecretory cell types differentiate into discrete structures.

Most vertebrate cranial sense organs arise from placodes. These placodes give rise to sensory neurons that transmit information to the brain and neurosecretory cells. This book reviews the evolutionary origin of the sensory and neurosecretory cell types. It summarizes our current understanding of vertebrate evolution, clarifies conceptual issues relating to homology and evolutionary innovation of cell types, compares the sensory and neurosecretory cell types with similar cell types in other animals, and addresses the question of how cranial placodes evolved as novel structures in vertebrates by redeploying pre-existing and sometimes evolutionarily ancient cell types.

This thesis deals with the origin of the photosensory and neurosecretory cell types in the bilaterian brain. As the main experimental system, I used the annelid Platynereis dumerilii. Platynereis is an emerging protostomian model organism that is ideally suited for comparisons with vertebrates because it has retained many ancestral cell types, yet has a relatively simple morphology and mode of development. In the first section, I describe my contribution to the reconstruction of the urbilaterian photosensory system. By cloning and analyzing a novel opsin gene from Platynereis, I was able to provide molecular support for the hypothesis that Urbilateria (the last common ancestors of all Bilateria) already possessed two photoreceptor cell types (PRCs) 6 rhabdomeric and ciliary PRCs. I further corroborated this by the comparative analysis of different upstream regulators for each PRC type. Presumably only the rhabdomeric type was ancestrally involved in vision, while the ciliary type was a light detector in the inner brain (like the ciliary PRCs of the vertebrate pineal organ). Both types, however, were recruited into the vertebrate eye, which is thus a compound structure. This finding provides a novel basis for understanding both the molecular similarities and differences between the vertebrate and invertebrate eyes. I extended these analyses in the second section by investigating the molecular and morphological set-up of the median brain of the Platynereis larva and in particular of the apical organ (APO). The APO is a specialized, highly neurosecretory structure. From a detailed analysis of molecular markers and cellular morphologies I concluded that the median brain of trochophora type larvae with the APO as its core structure and the ventral/ median prosencephalon of vertebrates with the hypothalamus as its center share common heritage from their urbilaterian ancestor. Both vertebrate eyes and hypothalamus have been shown to require proper Hedgehog (Hh) signalling.

Most of the cranial sense organs of vertebrates arise from embryonic structures known as cranial placodes. Such placodes also give rise to sensory neurons that transmit information to the brain as well as to many neurosecretory cells. This book focuses on the development of sensory and neurosecretory cell types from cranial placodes by introducing the vertebrate head with its sense organs and neurosecretory organs and providing an overview of the various cranial placodes and their derivatives, including evidence of common embryonic primordia. Schlosser discusses how these primordia are established in the early embryo and how individual placodes develop. The latter chapters explain how various placodally derived sensory and neurosecretory cell types differentiate into discrete structures.

Most of the cranial sense organs of vertebrates arise from embryonic structures known as cranial placodes. Such placodes also give rise to sensory neurons that transmit information to the brain as well as to many neurosecretory cells. This book focuses on the development of sensory and neurosecretory cell types from cranial placodes by introducing the vertebrate head with its sense organs and neurosecretory organs and providing an overview of the various cranial placodes and their derivatives, including evidence of common embryonic primordia. Schlosser discusses how these primordia are established in the early embryo and how individual placodes develop. The latter chapters explain how various placodally derived sensory and neurosecretory cell types differentiate into discrete structures.

This book is an overview of primary sensory maps of vertebrates, characterized by continuous and discrete properties. The eight primary sensory maps of vertebrates have unique features and use distinct molecular cues, cell cycle exit, and activity combinations during development, regeneration, and plasticity. As an introduction and overview, the book provides a short overview for all eight sensory senses and presents through evolution and gene regulatory networks, the molecular cues needed for sensory processing. Independent contributions are included for olfactory, vision, trigeminal, taste, vestibular, auditory, lateral line, and electroreception.