DEVELOPMENTAL CHARACTERISTICS OF THE CHICK NODOSE GANGLION

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.