A barrier to diffusion in pseudoplasmodia of Dictyostelium discoideum

Several enzymatic activities have been reported to be preferentially localized in the anterior portions of pseudoplasmodia of Dictyostelium discoideum. Since anterior cells are responsible for formation of the stalk during fruiting body construction, it has been suggested that accumulation of these enzymes may direct the cells toward stalk differentiation. However, the evidence for enzyme localization has come only from histochemical studies. We have assayed for succinoxidase and several dehydrogenase activities in cell free extracts of isolated anterior and posterior fragment and found no significant differences in specific activities, although by histochemical techniques each is apparently localized in the anterior cells. The preferential staining appears to be a consequence of a barrier to diffusion that is more effective at the back than at the front in limiting entry of the histochemical chromogen into pseudoplasmodia. The barrier appears to be the glycoprotein surface sheath that surrounds the pseudoplasmodium. The consequences of a barrier to diffusion of compounds into and out of pseudoplasmodia are discussed in relation to a mechanism that could give cells information concerning their position in this organism.