Anat Embryol (1985) 171:357 363 Anatomy and Embryology 9 Springer-Verlag1985 Nerve growth factor treatment does not prevent dorsal root ganglion cell death induced by target removal in chick embryos Charles Straznieky* and Robert A. Rush Departments of Human Morphology* and Physiology, School of Medicine, The Flinders University of South Australia, Bedford Park, SA 5042, Australia Summary. In chick embryos, on the 3rd day of incubation, the developing right wing bud was removed. One group of the operated embryos was treated with a daily dose of 20 btg purified nerve growth factor (NGF) from the 5th day of incubation and sacrificed on the 12th day. The other group was sacrificed on the 12th day of incubation and served as control. NGF was also administered to intact, unoperated embryos for comparison. The size of the dorsal root ganglia in segments 13-16 innervating the wings, were estimated and the number of surviving dorsal root ganglion cells counted both on the right (operated) and left (intact) sides. Although NGF brought about an increase in the size of the ganglia and an increase in the number of dorsal root ganglion cells bilaterally, it was not able to prevent excessive cell death of dorsal root ganglion cells on the operated side. The number of surviving neurons in the dor- sal root ganglia on the operated side in embryos with or without NGF administration was only about 30-50% of the number of the intact side. These results show that cell death induced by target removal cannot be offset by NGF administration. It is con- cluded that NGF may act as a growth promoting agent for developing sensory neurons but other peripheral trophic factor/s are also needed for the maintenance and survival of dorsal root ganglion cells. Key words: Neuronal death - NGF - Trophic factor/s - Dorsal root ganglion cells - Chick embryo Introduction The normal histogenesis of dorsal root ganglion cells has been shown to be dependent on an intimate interaction with the target tissue. In particular the size of the neuron population in the ganglion is influenced by the size of the available target which these neurons actually innervate. Following Iimb bud removal during early development in chick embryos a considerable reduction in neuron number (hypoplasia) develops in the corresponding dorsal root gan- glia (Hamburger and Keefe 1944). In contrast the trans- Offprint requests to." Dr. C. Straznicky, Department of Human Morphology, School of Medicine, The Ftinders University of South Australia, Bedford Park, SA, 5042, Australia. plantation of a supernumerary limb brings about an in- crease of neurons (hyperplasia) in the adjacent dorsal root ganglia (Hamburger 1939; Hamburger and Levi-Montal- cini 1949). Nerve growth factor (NGF) effects the development of sensory and postganglionic sympathetic neurons in many ways. NGF promotes a considerable increase in surviving sensory and sympathetic neurons (Levi-Montalcini and Hamburger 1951), an increase of the nuclear volume (Levi- Montatcini and Angeletti 1968) and promotes a prolific neurite outgrowth from cultures of such neurons (Levi- Montalcini and Angeletti 1963). More recent experiments in chick embryos have demonstrated that dorsal root gan- glion cells are able to transport NGF retrogradely from their peripheral target (Brunso-Brechtold and Hamburger 1979). Furthermore, the daily administration of NGF has been shown to be able to reduce naturally occurring neuron death, resulting in a considerable increase of dorsal root ganglion cells (Hamburger et al. 198J). Hamburger and co- workers interpreted these results to indicate that NGF, present in the target tissue, is the naturally produced trophic maintenance factor for dorsal root ganglion cells. In order to test the validity of this hypothesis it must be demon- strated unequivocally that the effect of the periphery on developing sensory neurons can be substituted by NGF administration. In this study we report observations in chick embryos showing that NGF cannot maintain the normal develop- ment of dorsal root ganglion cells whose natural target, the right wing bud, was surgically removed. The results suggest that in addition to NGF other peripheral trophic factors are required for the maintenance of normal differen- tiation and survival of dorsal root ganglion cells. Before the completion of the present study Hamburger and Yip (1984) reported observations on the same experimental model which are different in many respects from our find- ing. The reason for the differences in the results and in the consequent conclusions are discussed. Methods Experiments were carried out in White Leghorn chick em- bryos. Fertilized eggs, obtained from a local supplier (Para- field Poultry, Adelaide) were incubated in a forced-draft incubator at 37.8 ~ C at 70% relative humidity.