High frequency of elongated platelet forms in guinea pig blood: ultrastructure and resistance to micropipette aspiration.

Elongated cylindric and beaded platelet forms are infrequently observed in the circulation, and have been termed "proplatelets," as presumed precursors to discoid platelets. Some 15% 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 microns 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. All the 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.(ABSTRACT TRUNCATED AT 250 WORDS)