Compensatory responses and development of the nodose ganglion following ablation of placodal precursors in the embryonic chick (Gallus domesticus).

The nodose ganglion is the distal cranial ganglion of the vagus nerve which provides sensory innervation to the heart and other viscera. In this study, removal of the neuronal precursors which normally populate the right nodose ganglion was accomplished by ablating the right nodose placode in stage 9 chick embryos. Subsequent histological evaluation showed that in 54% of lesioned embryos surviving to day 6, the right ganglion was absent. Most embryos surviving to day 12, however, had identifiable right ganglia. In day 12 embryos, the right ganglion which developed was abnormal, with ganglion volume and ganglion cell diameter reduced by 50% and 20%, respectively, compared to control ganglia. To investigate the source of the neuron population in the regenerated ganglion, we combined nodose placode ablation with bilateral replacement of chick with quail "cardiac" neural crest (from mid-otic placode to somite 3). These cells normally provide only non-neuronal cells to the nodose ganglion, but produce neurons in other regions. At day 9, quail-derived neurons were identified in the right nodose ganglia of these chimeras, indicating that cardiac neural crest cells can generate neurons in the ganglion when placode-derived neurons are absent or reduced in number. On the other hand, we found that "sympathetic" neural crest (from somites 10 to 20) does not support ganglion development, suggesting that only neural crest cells normally present in the ganglion participate in reconstituting its neuronal population. Our previous work has shown that right nodose placode ablation produces abnormal cardiac function, which mimics a life-threatening human heart condition known as long QT syndrome. The present results suggest that the presence of neural crest-derived neurons in the developing right nodose ganglion may contribute to the functional abnormality in long QT syndrome.