Cell shape and organization of F‐actin and microtubules in randomly moving and stationary amebae of <i>Dictyostelium discoideum</i>

Abstract

Abstract We investigated, by video‐light microscopy and fluorescence microscopy with probes specific for microtribules (MTs) and F‐actin, the relationship between cytoskeletal elements, cell shape and behavior of vegetative, undifferentiated amebae of Dictyostelium discoideum , strain NC‐4. In an unconstrained situation, as on the underside of a coverglass in a thin layer of liquid medium, interphase cells moved around randomly in a polypodial or monopodial fashion. Locomotion was characterized by the formation of pseudodigits, rounded or pointed pseudopodia, and retraction fibers. F‐actin occurred in all these structures, as well as in a thin cortical layer. Microtubules extended into some of the cellular extensions rich in F‐actin. Pseudopodial activity, but not locomotion, also took place at the interface between medium and air, demonstrating that ameboid movement requires contact with a solid substrate. Stationary mitotic amebae on glass were studded with continuously changing, peripheral spike‐shaped filopodia that also contained F‐actin. During telophase and cytokinesis the spikes were gradually replaced by pseudopodia in transition to the fully motile phase. In live cells, the nucleus‐associated body (NAB), which is at the center of the complex of cytoplasmic MTs [CMTC; term from Brinkley, Fuller, and Highfield, 1975] was in a rather fixed position; it did not orient in a concerted fashion to follow changes in the direction of cell movement. In amebae fixed and processed for fluorescence microscopy after a period of recorded movement, the NAB was not preferentially positioned with respect to the nucleus and the direction of movement. It is unlikely that the NAB exerts a directional control during randon ameboid movement. The complex of cytoskeletal MTs must be very dynamic or flexible to adjust to the rapid changes of cell shape.