Effects of a nerve-growth factor, embryo age and metabolic inhibitors on growth of fibres and on synthesis of ribonucleic acid and protein in embryonic sympathetic ganglia

Incorporation of labelled precursors into RNA and protein was measured in lumbar sympathetic ganglia from chicken embryos (usually 13-14 days old) in the presence . or absence of nerve-growth factor. The ganglia were incubated with labelled precursors while embedded in plasma clots, so that the outgrowth of nerve fibres could be measured in the same ganglia as the incorporation. Fibre outgrowth was estimated quantitatively by the use of a newly-devised objective measure of mean halo width. In controls without the nerve-growth factor, there was an abrupt slowing of labelling of both RNA and protein after 6-12 h. This slowing was greatly delayed or prevented by addition of the growth factor. At earlier times, small increases in incorporation of both labels accompanied addition of the growth factor, but were not always statistically significant. When ganglia were incubated for 28 h with labelled precursors in the presence of the growth factor, 11 per cent of both the labelled protein and the labelled RNA were found in the halos of outgrowing fibres and 89 per cent in the bodies of the ganglia. In ganglia from embryos of different ages, there was a maximum of labelling per unit volume of ganglion in both RNA and protein at 10 days and a minimum at 12 days of embryonic age. The growth factor increased the labelling during 22 h of incubation at most ages, regardless of whether outgrowth of fibres was great (13-14 days) or minimal (9-10 days). Almost total inhibition of RNA labelling by actinomycin-D caused only moderate impairment of fibre outgrowth. Actinomycin-D also somewhat reduced protein labelling. Cyclo-heximide, in concentrations which produced degrees of inhibition of protein labelling identical to those of actinomycin-D, caused similar impairment of fibre outgrowth. We conclude that RNA synthesis is not essential to the initiation of fibre outgrowth by the nerve-growth factor.