Normal embryo fibroblasts release transforming growth factors in a latent form

Normal chicken, mouse, and human embryo fibroblasts release into their culture media transforming growth factors (TGFs) in a latent form. Their soft agar colony‐forming activity on two widely used target cells, rat NRK‐49F and mouse AKR‐2B, is essentially revealed only after prior acidification of cell‐conditioned media. These TGFs are EGF‐dependent when assayed on NRK‐49F cells and EGF‐independent on AKR‐2B cells. The TGF activity from the chicken source is released in three (apparent) molecular weight forms of 500 kd, 125 kd, and 20 kd.