Morphometric studies were carried out on the chick nodose ganglion bet
ween day 5 of incubation and 2 weeks after hatching. Previous findings
showed that ablation of the nodose placode, the locus of precursor ce
lls of nodose ganglion sensory neurons, results in abnormal cardiac fu
nction, 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 furth
er the normal development of nodose ganglion neurons. We find that the
major period of neuron generation from placodal precursor cells in th
e 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, whi
le neuron and ganglion sizes increase dramatically. Myelination is not
seen at day 12 of incubation, but is present on day 15. Neurons conti
nue to develop after hatching (day 21), reaching their adult size by 2
weeks after hatching. Unexpectedly, we found that the number of neuro
ns increases after hatching, reaching the adult level of 62% more than
embryonic day-20 numbers by 2 weeks after hatching. The mechanisms un
derlying the increase in neuron number after hatching are unexplained
and require further investigation.