Assembly, disassembly, and movements of the microfilament-rich ridge during the amoeboflagellate transformation in Physarum polycephalum.

The amoeboflagellate transformation in Physarum polycephalum involves a series of dramatic changes in cell shape and motile behavior. This report describes the morphological and behavioral changes through which a synchronously transforming population of cells passes, stressing that, although there are a series of distinguishable stages, cells at all stages display striking plasticity. Our previous studies showed that amoeboflagellates transiently display a flattened motile extension--the ridge--that projects from a specific location on the cell surface and contains a laminar core densely packed with a series of crisscrossing arrays of actin microfilaments. Details are presented here concerning the movements of the ridge as well as the dynamics of ridge formation and disassembly in relation to other morphogenetic events of the transformation. The ridge forms at about the same time as transforming cells begin to elongate, propagates undulations parallel to the long axis of the cell as the transformation proceeds, and disassembles late in the transformation. Staining of fixed cells with the fluorescent probe rhodaminephalloidin shows that the actin of amoeboid cells is strikingly redistributed as the transformation proceeds. Amoeboflagellates contain most of the stainable actin in the ridge and in a ventral-posterior spot that may be a site of cell-substratum adhesion. These results provide additional insights into the possible functions of the ridge and the roles of actin during the amoeboflagellate transformation.

Improved negative staining of microfilament arrangements in detergent-extracted Physarum amoeboflagellates.

A motile, lamellipodium-like structure, the ridge, forms as amoeboflagellate cells of Physarum polycephalum release from a substratum and begin swimming in fluid. Actin microfilaments form a distinct laminar core within the ridge; they are seen as a sparse, disordered meshwork in cytoskeletons prepared by conventional methods using uranyl acetate negative staining [10]. Preservation and visualization of these filaments and their arrangements improved considerably when cytoskeletons were imaged with phosphotungstic acid buffered with ammonium hydroxide (PTA(NH4]. Microfilaments within ridge cytoskeletons were found to form loose bundles and criss-crossing, 'meshwork' arrays several layers deep. Differences could be detected in morphology and detailed arrangement of microfilaments within cytoskeletons prepared in the presence of phalloidin. PTA(NH4) may be useful for studies of cytoskeletal elements and their rearrangements in dynamic, motile regions of cells.