Previous experiments have shown that amoebae can migrate at a liquid–liquid or air–liquid interface; however, cells might be able to gain traction at specialized physical features of these interfaces. Therefore, Nicholas Barry and Mark Bretscher sought to create conditions under which they could unequivocally determine whether amoebae can swim. They designed an observation chamber filled with an isodense Ficoll (a polysaccharide) solution to provide buoyancy for the amoebae. The chamber contained a point source of the chemoattractant cyclic AMP (cAMP) so that they could detect directed chemotaxis, rather than random movement. The authors found that Dictyostelium discoideum amoebae could migrate towards the cAMP source and did not show directed migration through the solution in the absence of cAMP. The speed at which the amoebae moved through the liquid was similar to that of cells that were attached to a solid surface in the same medium (4.2 μm per min compared with 3.8 μm per min). Can other cells swim too? The authors found that human neutrophils can also swim towards a chemotactic source, but do so more slowly, presumably because the experiments were carried out at 23 °C, which is much below the cells' normal temperature of 37 °C.
Barry and Bretscher used confocal microscopy to examine swimming amoebae that were labelled with a fluorescent plasma membrane marker. The cells project a leading lobe, and sometimes, microspikes in a forward direction, and these spikes and lobes then move towards the back of the cell. The projections and/or a rearward surface flow could be used to provide the force for the cells to move forwards. This pattern of movement is reminiscent of that observed when cells migrate over a substratum, leading the authors to suggest that similar mechanisms come into play during cell migration on solid surfaces and in solutions. These results might have implications for the way we view cell migration, and the authors suggest that it could be helpful to consider these similarities to better understand the process of migration.
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