The tip of the Dictyostelium discoideum pseudoplasmodium as an organizer

We have extended Raper's original work on the organizing ability of the tip of the Dictyostelium discoideum slug. Our new results are that tips from all multicellular (pseudoplasmodial) stages act as organizers; that the structure organized by a tip depends on the developmental stage of the cells responding to the tip's signal; that tips from all stages release a qualitatively similar signal which is continuous, most probably a gradient of c-AMP; and that the signal from fruiting-body tips appears stronger than that from conus tips. We discuss these results with reference to the control of morphogenetic movement and patterned differentiation and point out that the D. discoideum tip is analagous to a classical organizer, whose signal is interpreted according to the state of determination of cells in its field of influence.