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 cryos ubstituted human platelets was investigated by electron microscopy of serial sections. The spherical dense granules in resting platelets wer e separated by cytoplasm. No contact between these granules and the pl asmalemma or the membranes of alpha-granules or other membranes could be observed. Stimulated platelets (thrombin, 0.1-0.5 IU/ml for 20-30 s ec) contained dense granules with long protrusions. Tight contacts bet ween the membranes of these granules with the plasmalemma and with mem branes of alpha-granules were detected (apposition). Fusion events too k place at these sites. After fusion of the dense granules with the pl asmalemma the organelles were swollen and contained remnants of their dense matrix. After fusion between dense and alpha-granules compound g ranules were formed which included remnants of matrices from both type s of organelles. It is concluded that the secretory pathway is similar for both types of organelles.