PHOSPHOTYROSINE-CONTAINING PROTEINS ARE CONCENTRATED IN DIFFERENTIATING CELLS DURING CHICKEN EMBRYONIC-DEVELOPMENT

Protein tyrosine phosphorylation may be an important indicator of both the proliferative status and differentiation status of cells during e mbryonic development. To determine how each of these factors contribut es to the level of phosphotyrosine-containing proteins detectable in e mbryonic tissues we have used immunohistochemistry with anti-phosphoty rosine antibodies on sections of developing chicken embryos. In contra st to an earlier study (Takata and Singer, 1988) we found proteins pho sphorylated on tyrosine residues to be present in many different cells of the developing chicken embryo. The successful detection of phospho tyrosine-containing proteins in many cell types required the presence of sodium orthovanadate, a phosphotyrosine phosphatase inhibitor, duri ng fixation. Despite the fact that the majority of tyrosine kinases id entified to date are growth factor receptors, the highest levels of ph osphotyrosine-containing proteins in many tissues were localized to po pulations of cells which were differentiating or migrating rather than dividing.