Changes in fine structures during the development of the cellular slime molds D. discoideum and D. mucovoides were studied, with emphasis on the regional differentiation between the prestalk and prespore cells of the slug.Cells in the prestalk region were in closer contact than those in the prespore region. Some differences were also noticed in the structure of plasma membrane between the two types of cells. An endoplasmic reticulum, vesicle, autophagic vacuole, and cytoplasmic fibril were found more abundantly in the prestalk cell than in the prespore cell. In the prespore cells there were observed a number of prespore specific vacuoles of ca. 0 . 6 1~ diameter which consist of membraneous and fibrous structures. The vacuole was never found in the prestalk cells, and was a sole structure that existed only in one of the two types of cells. A possible function of such a vacuole was discussed in relation to spore differentiation. No differences in structure and distribution of mitochondria and crystal bodies were noticed between the prestalk and prespore cells, although these structures underwent considerable changes during the development. The nucleolus underwent considerable structual differentiation between the prestalk and prespore cells as well as during the course of development.
We use the flexible substrate method to study how and where mechanical forces are exerted during the migration of Dictyostelium slugs. This old and contentious issue has been left poorly understood so far. We are able to identify clearly separate friction forces in the tip and in the tail of the slug, traction forces mostly localized in the inner slug/surface contact area in the prespore region and large perpendicular forces directed in the outward direction at the outline of contact area. Surprisingly, the magnitude of friction and traction forces is decreasing with slug velocity indicating that these quantities are probably related to the dynamics of cell/substrate adhesion complexes. Contrary to what is always assumed in models and simulations, friction is not of fluid type (viscous drag) but rather close to solid friction. We suggest that the slime sheath confining laterally the cell mass of the slug experiences a tension that in turn is pulling out the elastic substrate in the direction tangential to the slug profile where sheath is anchored. In addition, we show in the appendix that the iterative method we developed is well adapted to study forces over large and continuous fields when the experimental error is sufficiently low and when the plane of recorded bead deformations is close enough to the elastomer surface, requirements fulfilled in this experimental study of Dictyostelium slugs.
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