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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., (Copyright © 2011 Wiley-Liss, Inc.)

The reciprocal relationship between rhombomere (r)-derived cranial neural crest (NC) and epibranchial placodal cells derived from the adjacent branchial arch is critical for visceral motor and sensory gangliogenesis, respectively. However, it is unknown whether the positional match between these neurogenic precursors is hard-wired along the anterior-posterior (A/P) axis. Here, we use the interaction between r4-derived NC and epibranchial placode-derived geniculate ganglion as a model to address this issue. In Hoxa1(-/-) b1(-/-) embryos, r2 NC compensates for the loss of r4 NC. Specifically, a population of r2 NC cells is redirected toward the geniculate ganglion, where they differentiate into postganglionic (motor) neurons. Reciprocally, the inward migration of the geniculate ganglion is associated with r2 NC. The ability of NC and placodal cells to, respectively, differentiate and migrate despite a positional mismatch along the A/P axis reflects the plasticity in the relationship between the two neurogenic precursors of the vertebrate head., (Copyright © 2011 Wiley-Liss, Inc.)

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.