Abstract
We interrogated in 3D-space the arrangement of alpha-granules and their proteins within 30 human platelets in order to address two central structure/function questions. First, are alpha-granules based on their size, shape and protein content a single population or not, and second, what implications could organelle frequency and organization within the resting platelet have for platelet biogenesis and membrane fusion events in activated platelets? We used serial block face scanning electron microscopy (SBF-SEM) to render in full platelet volume the ultrastructure of alpha-granules, dense granules, mitochondria, canalicular system (CS), and plasma membrane (PM) in 30 platelets, 10 each from 3 donors. Size and shape were measured for 1488 a-granules. Compositional data were assessed for multiple proteins over hundreds of granules by 3D-structured illumination microscopy (SIM) and serial section cryo-immunogold electron microscopy. Data analysis led to 3 conclusions: 1) Based on size, shape and protein composition, there was one major population of alpha-granules in resting human platelets, 2) Alpha-granules clustered tightly together while dense granules were more peripherally located and distal from one another suggesting a spatial arrangement that in the one case supports compound granule fusion and in the other case rapid fusion with the PM, and 3) Based on the weak relationship between platelet size and organelle number and volume, we inferred that platelet biogenesis is likely relatively imprecise with alpha-granule inclusion being more precisely metered than that of other organelles,. These results provide a strong, informative baseline for a-granule structural properties and suggest a spatial clustering of organelles within the resting platelet that may be functionally significant during platelet activation.
No relevant conflicts of interest to declare.
