1. Fibroblasts freshly isolated from skeletal muscle of check embryos were maintained as monolayers and used to assay the growth-promoting action of nucleoprotein fractions (NPF) from adult chicken organs and from embryos. Fractions were obtained from saline extracts by precipitation with streptomycin according to methods described previously.

  2. Adult spleen NPF induced logarithmic growth of skeletal muscle fibroblasts when added to a basal medium containing chicken serum and Medium No. 199. Cell populations remained uniform in appearance, and growth rates were undiminished over many serial passages. Unsupplemented controls in basal medium showed only slight proliferation, and the cell populations rapidly became heterotypic.

  3. Nucleoprotein fractions from adult chicken liver, brain, and heart also stimulated logarithmic growth of skeletal muscle fibroblasts. The specific activity of these fractions was lower, and the cell populations less uniform. Embryonic NPF was only weakly growth-stimulating in monolayer cultures.

  4. The relative activity of protein and nucleic acid components of spleen NPF was investigated by splitting off the nucleic acid with high concentrations of phosphate. Assays indicated that growth stimulation with the protein subfraction was equal to that of the complete NPF. Nucleic acid may act as a stabilizing factor for the associated protein or for the cell populations of the assay system.

*

Aided by grants from the Atomic Energy Commission, Division of Biology and Medicine, and Cancer Research Funds of the University of California.

This content is only available via PDF.
©1958 American Association for Cancer Research.
1958
Cancer Research, Inc.