EM tomography of the intracellular membrane organization in nonstimulated platelet. (A-C) Tomographic slices of directly fixed platelet. Dual axis tilt series were generated from chemically fixed platelets as described in “Tomography and data analysis.” The membranes and other structures of interest were manually traced to generate a 3D representation of the structure. Dark blue represents OCS; light blue, platelet secretory α-granule; and red, dense granule. (D) Microtubules are located at the platelet periphery. The OCS membranes are continuous with the cell surface at multiple sites (panel E and arrowheads in panel I), and span the platelet cell surface from one side to the other, revealing numerous branching areas. (An animated model of the OCS is included as supplemental Video 1.) (F-H) Manual tracking of the peripheral microtubular coil (yellow dots at the cell periphery) reveal a microtubule ending at an OCS invagination (supplemental Video 2). α indicates α-granule; δ, dense granule; mt, microtubules; and dts, dense tubular system. (A) Bars represent 200 nm. (B-G) Bars represent 100 nm.