Granula motion and membrane spreading during activation of human platelets imaged by atomic force microscopy.

The redistribution of platelet constituents during activation is essential for their physiological function of maintaining hemostasis. We report here about real time investigations of the activation of native human platelets under physiological conditions from the initial formation of filopodia to the fully spread form by atomic force microscopy. We followed the trafficking of granules and their interaction with the plasma membrane within single cells. Our results show movement of certain granula towards the lamellipodia. Analysis of this rearrangement and the subsequent enlargement of the platelet surface reveals details of the membrane spreading process. Images of living cells are presented that show the distribution of cytoskeletal components and membrane-bound filaments at a resolution of better than 50 nm. The local minimum forces between the tip and the platelets were estimated to be smaller than 60 pN. A model for the elastic contributions of the glycocalix to the tip/membrane interaction was developed using the theory of grafted polymers.