A way of following individual cells in the migrating slugs of Dictyostelium discoideum

Abstract

In the development of the cellular slime mold Dictyostelium discoideum there is a stage in which the aggregated amoebae form a migrating slug that moves forward in a polar fashion, showing sensitive orientation to environmental cues, as well as early signs of differentiation into anterior prestalk and posterior prespore cells. Heretofore it has been difficult to follow the movement of the individual cells within the slug, but a new method is described in which small, flat (one cell thick) slugs are produced in a glass-mineral oil interface where one can follow the movement of all the cells. Observations of time-lapse videos reveal the following facts about slug migration: (i) While the posterior cells move straight forward, the anterior cells swirl about rapidly in a chaotic fashion. (ii) Turning involves shifting the high point of these hyperactive cells. (iii) Both the anterior and the posterior cells move forward on their own power as the slug moves forward. (iv) There are no visible regular oscillations within the slug. (v) The number of prestalk and prespore cells is proportional for a range of sizes of these mini-slugs. All of these observations on thin slugs are consistent with what one finds in normal, three-dimensional slugs.

Figure 1

A large (A) and a small (B) 2-D slug photographed from a TV monitor showing a videotape. The large slug is 470 μm long and contains ≈2,020 cells; the small one is 80 μm long and has ≈110 cells.

Figure 2

The movement of individual cells in a migrating 2-D slug. (A) A photograph of an individual frame (the slug is 175 μm long and is made up of ≈226 cells). (B) The tracings of selected cells. 50 frames from a video of the slug, spanning 6 min in real time, were caught on a computer and played repeatedly so a few individual cells could be followed by tracing them on the screen. Note the difference in the speed and the direction between the anterior and posterior amoebae. [This was done on a Silicon Graphics (Mountain View, CA) O2 workstation and a Silicon Graphics media recorder utility to capture a sequence of the individual video frames.]

Figure 3

A computer image of a single video frame of a 2-D slug in the process of splitting in two. The arrows represent 6 min of cell movement, using the same methods described in the caption of Fig. 2. Note the two active regions of cell motion that are moving away from one another. (The cell mass is 139 μm long and is made up of ≈330 amoebae.)

Figure 4

A log-log graph showing that the number of prestalk cells is proportional to the number of spore cells in slugs of different sizes. •, Normal slugs taken from an early study (23). They were stained with a vital dye to distinguish the two zones and their respective volumes estimated and converted to cell numbers. ○, 2-D slugs of this study.