Strong and dynamic blood platelets
In order to gain more insight into the role of human platelets for blood clot formation, project A12 (Köster) in cooperation with theorists U. Schwarz and D. Probst from Heidelberg and A. Zemel from Jerusalem investigated the dynamics of force generation by platelets spreading onto elastic substrates of variable stiffness (Hanke et al., Soft Matter, accepted for publication). Despite their small size, platelets generate high and rapidly varying traction forces on their extracellular environment, which we reconstruct with adapted implementations of Fourier Transform Traction Cytometry. While the final spread area is reached within few minutes, the build-up of forces typically takes 10 - 30 minutes. In addition, the researchers identified two distinct behaviors of individual cells, namely oscillating and non-oscillating platelets. An eigenvalue analysis of the platelet dipole tensor reveals a small anisotropy of the exerted force, which is compatible with a random distribution of a few force transmitting centers, in agreement with the observed shapes and traction patterns. They found a correlation between the maximum force level a platelet reaches and its spread area, which is explained by a thin film model for the actively contracting cell. The model reveals a large internal stress of hundreds of kPa. Experimentally there is no statistically relevant relation between the force level reached and the substrate stiffness within the stiffness range from 19 to 83 kPa, which might be related to the high platelet activation level used in the study. In addition, the model suggests that due to the uniquely small thickness of platelets, their mechanosensitivity might be limited to a lower stiffness range.
Steps of investigation (left to right): Human blood platelets were stained for actin and imaged on soft hydrogels; fluorescent markers embedded in the gels were imaged during platelet contraction; force fields were derived from the marker displacement; a theoretical model was applied to the data describing the platelet as thin elastic sheet.