Cell movements and mechanical force distribution during the migration of Dictyostelium slugs

Migration of Dictyostelium discoideum slugs results from coordinated movement of their constituent cells. It is generally assumed that each cell contributes to the total motive force of the slug. However, the basic mechanisms by which mechanical forces (traction and resistive forces) are transmitted to the substrate, their magnitude and their location, are largely unknown. In this work, we performed detailed observations of cell movements by fluorescence microscopy using two-dimensional (2D) slugs. We show that 2D slugs share most of the properties of 3D ones. In particular, waves of movement propagate in long 2D slugs, and slug speed correlates with slug length as found in 3D slugs. We also present the first measurements of the distribution of forces exerted by 2D and 3D slugs using the elastic substrate method. Traction forces are mainly exerted in the central region of the slug. The large perpendicular forces around slug boundary and the existence of parallel resistive forces in the tip and/or the tail suggest an important role of the sheath in the transmission of forces to the substrate.