The orientation of nucleus, nucleus-associated body and protruding nucleolus in aggregating Dictyostelium discoideum.

Dictyostelium discoideum growing or developing on cellulose dialysis membranes were fixed with acrolein vapour for electron microscopy. In interphase amoebae, nucleoli began to protrude from the nuclei. The percentage of cells with protruding nucleoli increased during aggregation by a value approximately twice as high in aggregation streams as in centers. Cells in pseudoplasmodia showed only a low percentage and protrusions disappeared at early culmination stage. The protrusions did not reappear when cells from dissociated pseudoplasmodia migrated toward cAMP. Thus the formation of the protrusions did not depend solely on chemotaxis; rather, it was specific to the aggregation stage. In aggregation streams, the nucleus was anterior in the cell, with the protrusion at its anterior periphery. In contrast, the nucleus associated body (NAB) was evident at the cell's mid-point. This orientation of nucleus and NAB in the aggregating slime mould amoeba is contrary to that seen in human neutrophils or cultured mouse 3T3 cells.