EFFECTS OF NERVE GROWTH-FACTOR ON SYMPATHETIC NEURON DEVELOPMENT IN NORMAL AND LIMBLESS CHICK-EMBRYOS

Wing bud removal in chick embryos has been shown to affect the generat ion of sympathetic neurons prior to the normal period of limb innervat ion [Saltis and Rush (1995) J. auton. nerv. Sys., 51, 117-127.]. Pykno tic activity occurred earlier within the peripherally deprived ganglio n, suggesting that a precocious cell death of dividing sympathoblasts led to the reduced neuronal population. We have now sought to test whe ther the effect of limb bud extirpation can be overcome by the adminis tration of nerve growth factor (NGF). Specifically, the peripherally d eprived ganglion has been examined for mitotic activity and total neur onal numbers. In brachial ganglia from the operated side, neuron numbe rs decreased by 67% by embryonic day (E) 13, but by only 28% when NGF was administered from E9. Ganglia on the unoperated side were unaffect ed by the NGF treatment. In contrast, in embryos receiving NGF from E5 to E9, neuron numbers in the ganglia increased by more than 100%, on both the intact and operated side. This increase was accompanied by a greater proportion of H-3-thymidine-labelled neurons. We therefore con clude that NGF, in addition to its previously described role of preven ting naturally occurring neuron death, can also affect the generation of sympathetic neurons. This ability of NGF to affect gangliogenesis i s most likely achieved by increasing the survival of dividing neurobla sts, although a direct effect on mitosis has not been excluded.