Juxtacrine signaling by non-diffusible ligands The signaling consequences of cell contact are still being actively researched, and the pathways involved are probably not yet all known. Novel cell surface ligands have also been found, such as boss in Drosophila, which is recognized by a novel RTK on neighboring cells, activating a ras -linked pathway as above. The mechanism of Notch-Delta signaling is not clearly understood, but it appears that Delta interaction with Notch at the cell surface may simply cause the receptor to release a bound transcription factor, which then travels to the nucleus to regulate the target genes. The remaining pathways in the Gerhart Gallery will be considered in the next lecture, and we will further discuss most of the above ligands and pathways in connection with specific developmental events during the rest of the course. The remainder of this lecture will consider growth factors, their discovery in quite a different context, and what has been learned about them through the study of oncogenes.
Most growth factors are polypeptides, similar in many respects to hormones.
Most growth factors were discovered as requirements for cells in tissue culture. Oncogenes and growth factors Mechanisms of growth factor responses are being clarified by the study of oncogenes . Oncogenes are mutant viral or cellular genes whose expression can transform normal cells into neoplastic or cancer cells. They are of interest for developmental biologists because they cause breakdown of normal growth controls, and therefore were suspected to code for altered forms of normal components of the growth control system regulated by growth factors. As you have learned in other courses, oncogenes were discovered in studying tumor viruses.