Orientation of cells during slug formation inDictyostelium.

Scanning electron microscopic observations ofDictyostelium discoideum cell masses during slug formation revealed two populations around the anterior tip; one group of cells resembled elongated aggregation stream cells and their orientation suggested that they move to the tip, whereas the other group of cells were isodiametric and showed no obvious orientation. In seeking further evidence for a role of differential cAMP chemotaxis in the orientation and movement of slug cells the anterior prestalk cells were compared to the posterior prespore cells in two chemotaxis tests. When a cell mass is placed on cAMP agar the prestalk cells exhibited better movement to cAMP sources but when the gradient was generated in a diffusion chamber the prestalk cells did not. This evidence suggested that the cells which are better able to generate a cAMP gradient might form part of the anterior zone of the slug.