Evidence for the involvement of microtubules in the ADP-induced secretion in platelet 14C-5-hydroxytryptamine

Evidence for a theory of physical fragmentation of megakaryocytes, implying that all platelets are produced in the pulmonary circulation

The mathematical description of a fragmentation mechanism which leads to log normal volume distribution is applied to platelet production from megakaryocyte cytoplasm. Large cytoplasmic fragments and whole megakaryocytes are released from the bone marrow into the circulation. Physical fragmentation of the cytoplasm occurs in the pulmonary circulation producing the observed heterogeneity of platelet volume. Experimental evidence is used to provide quantitative verification that physical fragmentation is the process which produces all platelets. With this theory the currently accepted concept of the predesignation of the platelet population by the demarcation membrane system in megakaryocyte cytoplasm becomes redundant.

Histological demonstration of platelet thrombi with a double silver impregnation argyrophil stain

Images

Word and data processing in histopathology laboratories

Images

Ultrastructure of clots during isometric contraction

We explored the retraction or contraction of platelet-fibrin clots under isometric conditions. In the presence of micromolar calcium clots of normal platelet-rich plasma developed tension at an initial rate of 0.1 to 0.2 g/min per cm2 (initial cross-sectional area). Electron microscopy of clots fixed after attaining a force of 1.6 g/cm2 revealed platelets with elongated bodies and pseudopods in close apposition to fibrin strands which were oriented in cablelike fashion in the direction of tension. The development of tension could not be explained simply on the basis of platelet-platelet association and interaction alone. First, factor XIII-dependent cross-linking of fibrin fibers was critical to normal isometric contraction. Second, tension decreased linearly, rather than exponentially, when the platelet count in the platelet-fibrin clot was decreased, suggesting that platelets must be interacting with another component (i.e. fibrin). Thrombasthenic platelets, deficient in fibrinogen receptors, failed to develop tension or to align fibrin strands or pseudopods in the clot. Platelet-fibrin clots treated with vincristine to disassemble microtubules or cytochalasin B to disrupt microfilaments failed to develop tension and relaxed if these agents were added after tension had developed. Relaxation under these conditions, however, was not associated with loss of orientation of fibrin strands. Our findings suggest that platelet-fibrin interaction in clots under isometric conditions leads to orientation of fibrin strands and platelets in the direction of force generation. Tension develops as platelets simultaneously attach to and spread along fibrin strands, and contract. The contraction draws some fibrin into platelet-fibrin clumps and aligns other strands in the long axis of tension. The achievement and maintenance of maximum tension appears to depend on the development of platelet-fibrin attachments and extension of platelet bodies and long pseudopods containing bundles of microfilaments and microtubules along the oriented fibrin fibers.