ACTIVIN SIGNALING AND RESPONSE TO A MORPHOGEN GRADIENT

Using combinations of amphibian embryo tissues, it is shown that the s election of genes expressed by a cell is determined by its distance fr om a source of activin, a peptide growth factor contained in vegetal c ells and able to induce other cells to form mesoderm. This long-range signal spreads over at least 10 cell diameters In a few hours. It does so by passive diffusion, because It can by-pass cells that do not the mselves respond to the signal nor synthesize protein. These results pr ovide direct support for the operation of a morphogen concentration gr adient In vertebrate development.