IEM characterization of granule subtypes. Platelets were fixed for immunoelectron microscopy as described in “Blood collection and platelet preparation.” Immunogold labeling was performed on 50-nm-thick cryosections, and the samples were analyzed on a JEOL 1200CX electron microscope. (A-F) Tubular granules were identified as α-granule after immunogold labeling with specific α-granule markers. Labeling of soluble (A-D) and integral membrane proteins (E-F) as indicated on the figures. (G) Quantification of the immunogold label over tubular and spherical profiles. Label densities were determined on electron micrographs at 30 000× nominal magnification, by dividing the total number of gold particles attributed to each subtype by the number of random intersections over the structure. For each marker, a total of 20 electron micrographs was evaluated. Label density is expressed in arbitrary units. Bars represent 200 nm.