[Ultrastructural changes of platelets during ADP- stimulated aggregation (in the presence of fibrinogen)].

: The ultrastructure of resting and stimulated human blood platelets (P) was studied by the transmission electron microscopy. The cells were chemically fixed (using tannic acid and OsFeCN mixture) 1, 3, 5 and 15 min after the addition of ADP and fibrinogen (F). Early changes in P ultrastructure consist in drastic reduction of the electron-dense layer of glycocalyx and in an increase of the plasma membrane permeability. At the early stages of P aggregation the cells contact with each other due to rapidly arising pseudopodia. Later, the extracellular network containing an exogenous F participates in the aggregation process.