Mound-cell movement and morphogenesis in Dictyostelium.

To examine the mechanisms of cell locomotion within a three-dimensional (3-D) cell mass, we have undertaken a systematic 3-D analysis of individual cell movements in the Dictyostelium mound, the first 3-D structure to form during development of the fruiting body. We used time-lapse deconvolution microscopy to examine two strains whose motion represents endpoints on the spectrum of motile behaviors that we have observed in mounds. In AX-2 mounds, cell motion is slow and trajectories are a combination of random and radial, compared to KAX-3, in which motion is fivefold faster and most trajectories are rotational. Although radial or rotational motion was correlated with the optical-density wave patterns present in each strain, we also found small but significant subpopulations of cells that moved differently from the majority, demonstrating that optical-density waves are at best insufficient to explain all motile behavior in mounds. In examining morphogenesis in these strains, we noted that AX-2 mounds tended to culminate directly to a fruiting body, whereas KAX-3 mounds first formed a migratory slug. By altering buffering conditions we could interchange these behaviors and then found that mound-cell motions also changed accordingly. This demonstrates a correlation between mound-cell motion and subsequent development, but it is not obligatory. Chimeric mounds composed of only 10% KAX-3 cells and 90% AX-2 cells exhibited rotational motion, suggesting that a diffusible molecule induces rotation, but many of these mounds still culminated directly, demonstrating that rotational motion does not always lead to slug migration. Our observations provide a detailed analysis of cell motion for two distinct modes of mound and slug formation in Dictyostelium.