The pre-vertebrate origins of neurogenic placodes.

• Autores: Philip Barron Abitua, T. Blair Gainous, Angela N. Kaczmarczyk, Christopher J. Winchell, Clare Hudson, Kaori Kamata, Masashi Nakagawa, Motoyuki Tsuda, Takehiro G. Kusakabe, Michael Levine
• Localización: Nature: International weekly journal of science, ISSN 0028-0836, Vol. 524, Nº 7566, 2015, págs. 462-465
• Idioma: inglés
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• Resumen

  • The sudden appearance of the neural crest and neurogenic placodes in early branching vertebrates has puzzled biologists for over a century 1. These embryonic tissues contribute to the development of the cranium and associated sensory organs, which were crucial for the evolution of the vertebrate "new head" 2,3. A previous study suggests that rudimentary neural crest cells existed in ancestral chordates 4. However, the evolutionary origins of neurogenic placodes have remained obscure owing to a paucity of embryonic data from tunicates, the closest living relatives to those early vertebrates 5. Here we show that the tunicate Ciona intestinalis exhibits a proto-placodal ectoderm (PPE) that requires inhibition of bone morphogenetic protein (BMP) and expresses the key regulatory determinant Six1/2 and its co-factor Eya, a developmental process conserved across vertebrates. The Ciona PPE is shown to produce ciliated neurons that express genes for gonadotropin-releasing hormone (GnRH), a G-protein-coupled receptor for relaxin-3 (RXFP3) and a functional cyclic nucleotide-gated channel (CNGA), which suggests dual chemosensory and neurosecretory activities. These observations provide evidence that Ciona has a neurogenic proto-placode, which forms neurons that appear to be related to those derived from the olfactory placode and hypothalamic neurons of vertebrates. We discuss the possibility that the PPE-derived GnRH neurons of Ciona resemble an ancestral cell type, a progenitor to the complex neuronal circuit that integrates sensory information and neuroendocrine functions in vertebrates. (C) 2015 Nature Publishing Group