Developmental characteristics of the chick nodose ganglion

Morphometric studies were carried out on the chick nodose ganglion between day 5 of incubation and 2 weeks after hatching. Previous findings showed that ablation of the nodose placode, the locus of precursor cells of nodose ganglion sensory neurons, results in abnormal cardiac function, and that these precursors can be induced to migrate abnormally to the heart and express abnormal phenotypes there, following cardiac neural crest ablation. These results prompted us to investigate further the normal development of nodose ganglion neurons. We find that the major period of neuron generation from placodal precursor cells in the ganglion occurs prior to day 5 of incubation. The loss of more than half of these neurons takes place between embryonic days 5 and 20, while neuron and ganglion sizes increase dramatically. Myelination is not seen at day 12 of incubation, but is present on day 15. Neurons continue to develop after hatching (day 21), reaching their adult size by 2 weeks after hatching. Unexpectedly, we found that the number of neurons increases after hatching, reaching the adult level of 62% more than embryonic day-20 numbers by 2 weeks after hatching. The mechanisms underlying the increase in neuron number after hatching are unexplained and require further investigation.