High frequency of elongated platelet forms in guinea pig blood: ultrastructure and resistance to micropipette aspiration
Smith, C.M.; Burris, S.M.; White, J.G.
Journal of Laboratory and Clinical Medicine 115(6): 729-737
1990
ISSN/ISBN: 0022-2143 PMID: 2366033 Document Number: 358735
Elongated cylindric and beaded platelet forms are infrequently observed in the circulation, and have been termed "proplatelets," as presumed precursors to discoid platelets. Some 1% of the blood platelets obtained from the guinea pig by cardiac puncture were elongated forms (1/5 beaded and 4/5 cylindric) under conditions of steady state thrombopoiesis, thus providing an opportunity to contrast the structure and deformability of platelets of potential graded maturation. Ultrastructural studies were consistent with graded maturation. Longitudinal bundles of microtubules in cylindric platelets gave way to the appearance of basket-weave arrays in the nodal thickenings of beaded forms, and, finally, to circumferential coils in a portion of the discoid platelets. Individual platelets were aspirated into micropipettes of 0.8 .mu.m internal diameter by application of incremental pressure up to a maximum pressure of 10 cm water. None of the guinea pig platelets fragmented into smaller forms even when aspiration was performed under physiologic environmental conditions. At all guinea pig platelets passed through the micropipette in aspiration studies performed under conditions minimizing platelet activation and adhesion. Elongated platelets aspirated at the very ends of the cells passed through the pipette at lower pressure than discoid forms, whereas elongated platelets aspirated along the length of the cell were more resistant to deformation and cell passage than were discoid platelets. Beaded forms were similar in deformability to cylindric platelets at low aspiration pressure but were more resistant to deformation at high pressure. In conclusion, elongated cylindric and beaded platelet forms did not fragment into smaller cells under application of a large deforming force and no graded continuum of mechanical behavior was expressed from cylindric to beaded to discoid form. Circulating elongated platelets may not be true proplatelets, or, alternatively, progression from an elongated to discoid cell involves an abrupt cytoskeletal reorganization without a stable mechanical transitional state.