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Background: The neural crest (NC) and placode are transient neurogenic cell populations that give rise to cranial ganglia of the vertebrate head. The formation of the anterior NC- and placode-derived ganglia has been shown to depend on the single activity of either Neurog1 or Neurog2. The requirement of the more posterior cranial ganglia on Neurog1 and Neurog2 is unknown. Results: Here we show that the formation of the NC-derived parasympathetic otic ganglia and placode-derived visceral sensory petrosal and nodose ganglia are dependent on the redundant activities of Neurog1 and Neurog2. Tamoxifen-inducible Cre lineage labeling of Neurog1 and Neurog2 show a dynamic spatiotemporal expression profile in both NC and epibranchial placode that correlates with the phenotypes of the Neurog-mutant embryos. Conclusion: Our data, together with previous studies, suggest that the formation of cranial ganglia along the anterior-posterior axis is dependent on the dynamic spatiotemporal activities of Neurog1 and/or Neurog2 in both NC and epibranchial placode. Developmental Dynamics 241:229-241, 2012. © 2011 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

The parasympathetic reflex circuit is controlled by three basic neurons. In the vertebrate head, the sensory, and pre- and postganglionic neurons that comprise each circuit have stereotypic positions along the anteroposterior (AP) axis, suggesting that the circuit arises from a common developmental plan. Here, we show that precursors of the VIIth circuit are initially aligned along the AP axis, where the placode-derived sensory neurons provide a critical 'guidepost' through which preganglionic axons and their neural crest-derived postganglionic targets navigate before reaching their distant target sites. In the absence of the placodal sensory ganglion, preganglionic axons terminate and the neural crest fated for postganglionic neurons undergo apoptosis at the site normally occupied by the placodal sensory ganglion. The stereotypic organization of the parasympathetic cranial sensory-motor circuit thus emerges from the initial alignment of its precursors along the AP axis, with the placodal sensory ganglion coordinating the formation of the motor pathway. Developmental Dynamics 239:1155-1161, 2010. © 2010 Wiley-Liss, Inc. [ABSTRACT FROM AUTHOR]