Summary
In this study the influence of thrombin activation on human blood platelets has been
followed by freeze-fracturing electron microscopy using rapid freezing in order to
catch the initial changes in shape and the morphological alterations during the process
of exocytosis of secretory granules. We found that isolation of the platelets by itself
leads to some degree of shape change, which made it impossible to study the resting
discoid platelet by rapid freezing.
Activation of the platelets by thrombin induced dilation of the “surface connecting
system (SCS)” with formation of large vacuoles as a result of fusion of the secretory
granules with SCS. No intermediary fusion stages or structures were observed even
using rapid freezing. Volcano-like protrusions and the corresponding complementary
pits were seen at the SCS. These structures were interpreted by us as fractures through
protoplasmic channels crossing the SCS. These channels originate during the swelling
of the SCS as a result of the fusion of secretory granules with the SCS.
Keywords
Blood platelets - Freeze - fracture - Secretion
