Ultrastructural Visualization of the Thrombin‐Induced Platelet Release Reaction

The present study distinguishes the morphological changes associated specifically with the thrombin‐induced platelet release reaction as isolated from platelet aggregation. At a time when maximal amounts of calcium and nucleotides are discharged from the cell, there also occurs an almost complete loss of alpha granules from the cytoplasm. Partial inhibition of the thrombin‐induced release reaction by incubation of the platelet suspensions at different temperatures is correlated with the discharge of fewer alpha granules from the cells. A complex lamellar‐vacuolar network gradually develops in the cells as release becomes maximal. Portions of this system may be derived from the limiting membranes of alpha granules that have fused with the lamellae and deposited their contents. Because this membrane system is continuous with the surface of the cell during release, it may provide a conduit whereby the alpha granule contents are transported to the surrounding medium. At 37°, when maximal thrombin‐induced release occurs, a heretofore undescribed electron‐dense fibrous material appears within the lamellar spaces. This material is morphologically similar to the fibrin seen in blood clots and may well be derived from the previously described alpha granule fibrinogen that is secreted during thrombin‐induced nucleotide and calcium release.