Neurogenesis and differentiation of sympathetic B and C cells in the bullfrog tadpole

The relation between birthdates of sympathetic neurons and their subsequent differentiation into cutaneous B cells and vasomotor C cells was examined in paravertebral ganglia 9 and 10 of the bullfrog tadpole. Neurons undergoing terminal cell division were identified by injecting tadpoles repeatedly with 5'-bromodeoxyuridine (BRDU) for one to six developmental stages between III and XXI. After allowing the tadpoles to enter late metamorphic stages (XX-XXV), the ganglia were double immunostained for BRDU and neuropeptide Y (NPY). NPY is a marker for mature C-type neurons (Horn et al., 1987). Double-labeled neurons were readily discerned through use of distinct black and brown HRP reaction products and also because immunoreactivity for BRDU was localized in nuclei while that for NPY was localized in perinuclear cytoplasm. Counts of labeled cells showed that neurogenesis occurs throughout limb bud and paddle stages, and that it ceases during early foot stages (XII-XIV), a time coinciding with the onset of NPY expression. By contrast, the labeling of non-neuronal satellite cells with BRDU was most common weeks later during metamorphic stages. Irrespective of their birthdates, about half of the BRDU-labeled neurons were also positive for NPY immunoreactivity. This proportion of NPY-positive cells is indistinguishable from that in the entire adult ganglia (Horn et al., 1987). In addition to establishing that neurogenesis and gliogenesis occurs during tadpole stages, the results indicate that the onset of NPY expression by vasomotor C neurons is unrelated to their time of origin. In other words, the last wave of neurogenesis in sympathetic ganglia does not give rise to a specific subclass of sympathetic neurons.