Three-dimensional reconstruction of human blood platelets using serial sections.

The typical reconstruction model of an unstimulated human platelet is presented. It shows a strict spatial order of organelles and enodmembrane systems. The surface-connected vesicle system is predominantly found in the periphery. The dense tubules constitute a continuous, interconnected system found just under the surface of the platelet. They are particularly pronounced in the vicinity of the marginal microtubules. All organelles are in contact with the dense tubular system (DTS). Granules, vesicles and plasmalemma fuse into the tubules. The platelet organelles are centrally located. Lysosomal granules are primarily shperical and all of them contain a nucleoid. This speaks in favor of a single type of storage organelle in the platelet. The dense bodies and their remnants (large, solitary vesicles) are not joined by the other endomembrane systems, as in the case of the granules. A particular pathways for the release of substances from these organelles is now morphologically plausible. Mitochondria are small, solitary granules surrounded by dense tubules. Owing to its unique position with the platelet, the DTS's role as a mediating and regulatory structure is discussed.