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1: PLoS Biol. 2006 Jun;4(7):e225.

Formation of the ascidian epidermal sensory neurons: insights into the origin of 
the chordate peripheral nervous system.

Pasini A, Amiel A, Rothbächer U, Roure A, Lemaire P, Darras S.

Institut de Biologie du Développement de Marseille-Luminy (IBDML), UMR6216,
CNRS/Université de la Méditerranée, Marseille, France. [e-mail]

The vertebrate peripheral nervous system (PNS) originates from neural crest and
placodes. While its developmental origin is the object of intense studies, little
is known concerning its evolutionary history. To address this question, we
analyzed the formation of the larval tail PNS in the ascidian Ciona intestinalis.
The tail PNS of Ciona is made of sensory neurons located within the epidermis
midlines and extending processes in the overlying tunic median fin. We show that 
each midline corresponds to a single longitudinal row of epidermal cells and
neurons sharing common progenitors. This simple organization is observed
throughout the tail epidermis, which is made of only eight single-cell rows, each
expressing a specific genetic program. We next demonstrate that the epidermal
neurons are specified in two consecutive steps. During cleavage and gastrula
stages, the dorsal and ventral midlines are independently induced by FGF9/16/20
and the BMP ligand ADMP, respectively. Subsequently, Delta/Notch-mediated lateral
inhibition controls the number of neurons formed within these neurogenic regions.
These results provide a comprehensive overview of PNS formation in ascidian and
uncover surprising similarities between the fate maps and embryological
mechanisms underlying formation of ascidian neurogenic epidermis midlines and the
vertebrate median fin.

PMID: 16787106 [PubMed - indexed for MEDLINE]

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