Abstract
Chick neuroblasts from 6βdayβold embryos were grown on a collagen substrate in minimum Eagle's medium supplemented with 5% horse serum. We found that [^3^H]thymidine incorporation in these cultures decreased drastically from day 2 to day 3 after seeding. Addition of brain extract from 8βdayβold chick embryos (CBe8) to the nutrient medium after 24 hours of culture elicited a very important stimulation of [^3^H]thymidine incorporation. Extracts of brains from adult animals were much less effective. It was shown that the increase of tritiatedβthymidine incorporation reflected proliferation of neuroblasts in the culture, at least between day 2 and 4. The activity of the brain extracts was destroyed by trypsin treatment, which suggests that the factor responsible for the stimulation of the proliferation is a polypeptide. The CBe8 and the brain extract of adult chicken (CBa) were fractionated by gel filtration. An active fraction was eluted at a volume indicating an apparent molecular weight of roughly 70,000. At that stage of the study results suggest that a polypeptide (tentatively called neuroblast proliferation factor, NPF) present in the embryonic chick brain extract, is a proliferation factor for chick neuroblasts in primary culture.