α-Granule decondensation is a kinetic precursor of granule exocytosis. Mouse platelets were stimulated, or not, with thrombin for the indicated times, and reactions were stopped by the addition of glutaraldehyde/paraformaldehyde fixative. Aldehyde fixation can stop secretion in <1 second (cultured hippocampal neurons).³⁸  (A) In electron micrographs of thin sections from unstimulated mouse platelets (0 seconds), α-granules appeared as round to ovoid structures marked by an electron-dense matrix. Upon thrombin stimulation (B-C; 0.1 U/mL), there was distinct accumulation of expanded (ie, decondensed) α-granules marked by residual loose fibrous matrix components. The arrow in panel A points to a condensed α-granule. The arrowheads in panels B-C point to examples of compound-fused, decondensed α-granules. The white star in panel B marks a decondensed α-granule showing variations in apparent matrix density. (D) Quantification of condensed and decondensed granules in platelets stimulated for increasing times. Numbers of platelet profiles scored: 0 seconds/0 time, n = 50; 10 seconds, n = 50; 20 seconds, n = 47; 30 seconds, n = 45; 60 seconds, n = 19; 90 seconds, n = 145; 120 seconds, n = 45; 300 seconds, n = 41. Means and standard errors of the mean are presented, and the graphs are color coded: condensed (red) and decondensed (blue). P values relative to 0 seconds were determined at 90 and 300 seconds. Values for both condensed and decondensed granules were statistically significant for both sets of comparisons. Asterisks indicating statistical significance, as determined with Student t test are as follows: **P ≤ .01.