A precise group size in Dictyostelium is generated by a cell-counting factor modulating cell-cell adhesion

A remarkable aspect of Dictyostelium development is that cells form evenly sized groups of similar to2 X 10(4) cells. A secreted 450 kDa protein compl ex called counting factor (CF) regulates the number of cells per group. We find that CF regulates group size by repressing cell-cell adhesion. In both experiments and computer simulations, high levels of OF (and thus low adhe sion) result in aggregation streams breaking up into small groups, while no CF (and thus high adhesion) results in no stream breakup and large groups. These results suggest that in Dictyostelium and possibly other systems a s ecreted factor regulating cell-cell adhesion can regulate the size of a gro up of cells.