Evolution of neurosecretory cell types

Chapter 5 investigates the evolutionary history of the neurosecretory cells derived from cranial placodes in vertebrates. These comprise the hormone-producing cells of the adenohypophysis and the olfactory nEuropeptidergic (GnRH) neurons. The chapter argues that specialized neurosecretory cell types evolved many times independently. Apart from an anteromedial population of neurosecretory cells (specified by Otp and other transcription factors), there is little evidence for neurosecretory cell types with a deep evolutionary history. The placodal neurosecretory cells probably originated as novel cell types only in vertebrates or the last common tunicate-vertebrate ancestor. Data from amphioxus suggest that in ancestral chordates neurosecretory cells were concentrated in rostral endomesodermal pouches (forming Hatschek’s pit in amphioxus). These probably secreted protein hormones allowing the regulation of metabolic and reproductive processes in response to environmental cues. A new regulatory environment was then established in the non-neural ectoderm anterior to the neural plate in the lineage leading to tunicates and vertebrates by the recruitment of transcription factors from the endoderm (FoxA, GATA2/3) and anterior neuroectoderm (FoxG, SP6-9, DMRT4/5). This facilitated the recruitment of neurosecretory cells (specified by Pitx, Lhx3/4, Islet, POU1f1, Six1/2, Eya) from the rostral endomesodermal pouches to this domain and the origin of a new type of neurosecretory (GnRH) neuron.