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Nerve growth-promoting activity in the chick embryo: quantitative aspects.

Nerve growth-promoting activity in organ extracts from the chick embryo was titrated using ganglia explanted to a collagen gel. Fibre outgrowth responses evoked in ciliary, sympathetic and spinal ganglia were well correlated. At embryonic day 8, 66% of the activity was localized in the yolk sac, 19% to the chorioallantois and the remaining 15% was widespread in the embryo. At day 18, total activity had increased 27-fold, the carcass now accounting for 90%. In parallel, the embryo extracts also promoted survival and neurite extension in dissociated ganglionic neurons seeded at low density in the gel. It is suggested that the observed effects are due to one active substance widely distributed in the embryo and increasing in amount during development. The substance has a molecular weight of over 10,000 and is distinct from nerve growth factor (NGF). A function of it may be to regulate axonal growth and survival of autonomic and sensory neurons.