The formation of compound granules from different types of secretory organelles in human platelets (dense granules and alpha-granules). A cryofixation/-substitution study using serial sections.

The secretory pathway of dense granule contents in cryofixed and cryosubstituted human platelets was investigated by electron microscopy of serial sections. The spherical dense granules in resting platelets were separated by cytoplasm. No contact between these granules and the plasmalemma or the membranes of alpha-granules or other membranes could be observed. Stimulated platelets (thrombin, 0.1-0.5 IU/ml for 20-30 sec) contained dense granules with long protrusions. Tight contacts between the membranes of these granules with the plasmalemma and with membranes of alpha-granules were detected (apposition). Fusion events took place at these sites. After fusion of the dense granules with the plasmalemma the organelles were swollen and contained remnants of their dense matrix. After fusion between dense and alpha-granules compound granules were formed which included remnants of matrices from both types of organelles. It is concluded that the secretory pathway is similar for both types of organelles.