Morphogenesis in the cellular slime mould Dictyostelium discoideum; the formation and regulation of aggregate tips and the specification of developmental axes.

A general discussion of 'organizing regions' and the specification of biological patterns is followed by introducing the idea that the tip of the slime mould cell mass is such an 'organizer'. This view is supported by a discussion of the developmental ubiquity of the tip and its effects. A staging system is described which assigns numbers to sequential morphological changes during development. A set of experiments investigating the role of the tip are described, using techniques of cell labelling, grafting and bisection of cell masses with barriers, and the manufacture and use of cylindrical barriers of permeable cellulose. The results of such experiments show: (1) That the tip of the cell mass is made of the same group of cells from stage 10 (late aggregate) to stage 20+ (culmination). (2) That a stage 10 aggregate will regenerate a new tip in an average time of 32 min. (3) That if a stage 10 aggregate is bisected by an impermeable barrier two tips, indicating two new developmental axes, develop in an average time of 34 min. (4) That if a stage 10 aggregate is bisected for 40 min, the barrier removed and one of the tips removed, the remaining tip inhibits the re-formation of the second tip, and the polarity of the aggregate is again reorganized with respect to the remaining tip. (5) That if experiments (3), (4) and (5) are repeated with a stage 9 aggregate, which is an hour younger, all the regulation times are increased by about 60 min. Similarly a stage 8 aggregate takes over 120 min longer to show the effect. (6) That if part or all of a cell mass from any stage is placed inside a cellulose tube, all the enclosed cells differentiate into stalk cells. These results are then discussed in relation to pattern formation and the role of the tip in polarization and the specification of new developmental axes in cell masses. A model for culmination in the slime mould is proposed which takes account of the above results. The essence of this model is that at no time are stalk and spore cells 'determined' in the classical sense, and that, by a non-signalling positional information system, the size invariance of the ratio of stalk to spore cells seen in the fruiting body is a result of the mechanical process of culmination.