SummaryContraction at the cellular level is vital for living organisms. The most prominent type of contractile cells are heart muscle cells, a lesswell-known example is blood platelets. Blood platelets activate and interlink at injured blood vessel sites, finally contracting to form a compact blood clot. They are ideal model cells to study the mechanisms of cellular contraction, as they are simple, having no nucleus, and their activation can be triggered and synchronized by the addition of thrombin. We have studied contraction using human blood platelets, employing traction force microscopy, a single-cell technique that enables time-resolved measurements of cellular forces on soft substrates with elasticities in the physiological range (,4 kPa). We found that platelet contraction reaches a steady state after 25 min with total forces of ,34 nN. These forces are considerably larger than what was previously reported for platelets in aggregates, demonstrating the importance of a single-cell approach for studies of platelet contraction. Compared with other contractile cells, we find that platelets are unique, because force fields are nearly isotropic, with forces pointing toward the center of the cell area.

Injuries in blood vessels are accompanied by disrupted endothelial cell layers. Missing or destroyed endothelial cells lead to rough, structured surfaces on the micrometer scale. The first cells to arrive at the site of injury and to cover the wound are platelets, which subsequently drive blood clot formation. Therefore, investigating the interactions of platelets with structured surfaces is essential for the understanding of blood clotting. Here, we study the effects of underlying topography on platelet spreading using microstructured model substrates with varying area fractions of protein coating. We thereby distinguish the effects of (physical) topography and of (biochemical) protein availability. By analyzing the cell area and morphology, we find that the extent of protrusion formation - but not the total spread area - is determined by the area fractions of coating. The extent of filopodia formation is influenced by the availability of binding sites and the reaction of cells to the substrate's topography. The cells react to the structured substrate by avoiding topographic holes at the cell periphery and thus adapting their outer shape. This finding leads us to the conclusion that both chemically blocked and fibrinogen-coated holes represent "energetic obstacles" to the cells. Thus, the shape of the cell is governed by the interplay between spreading to an optimized area and adaption to the substrate topography.

Abstract. The size polydispersity distribution of synaptic vesicles (SVs) is characterized under quasiphysiological conditions by dynamic light scattering (DLS). Highly purified fractions of SVs obtained from rat brain still contain a small amount of larger contaminant structures, which can be quantified by DLS and further reduced by asymmetric-flow field-flow (AFFF) fractionation. The intensity autocorrelation functions g 2(τ ) recorded from these samples are analyzed by a constrained regularization method as well as by an alternative direct modeling approach. The results are in quantitative agreement with the polydispersity obtained from cryogenic electron microscopy of vitrified SVs. Next, different vesicle fusion assays based on samples composed of SVs and small unilamellar proteoliposomes with the fusion proteins syntaxin 1 and SNAP-25A are characterized by DLS. The size increase of the proteoliposomes due to SNARE-dependent fusion with SVs is quantified by DLS under quasi-physiological conditions.

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