Development of the sympathetic system in the Mexican axolotl, Ambystoma mexicanum

Abstract

Migration of trunk neural crest cells in axolotl embryos has been followed by autoradiography using grafts of [ 3H]thymidine-labeled neural folds. Crest cells form melanocytes, dorsal fin mesenchymal cells, spinal ganglion cells, and reach the sympathetic region. Sympathetic neurons, however, are not identifiable morphologically until about 6 weeks posthatching, in 24-mm larvae. At this stage, neurons, although few, have characteristic ultrastructure and receive synapses. The diffuse ganglia also contain innervated chromaffin cells; these differentiate earlier, a few days posthatching, in 14-mm larvae. A third type of cell is of morphologically indifferent appearance. Catecholamine-specific formaldehyde-induced fluorescence first appears clearly at 14 mm; with growth, the number of fluorescent cells increases. Series of larvae were injected intraperitoneally with nerve growth factor (NGF), six 30-unit injections over 2 weeks. NGF treatment increases the number of neurons apparent in 24-mm larvae. Furthermore, differentiated neurons occur in NGF-treated 20-mm larvae (about 4 weeks posthatching), when there are none in controls. The early appearance of differentiated chromaffin cells and the relatively late appearance of differentiated sympathetic neurons suggest that adrenergic functions during the first few weeks of larval life are controlled humorally by the chromaffin cells, and that at 24 mm, neurons begin to provide faster, finer control.