Abnormalities of platelet function and ultrastructure in macrothrombocytic thrombopathia

Megakaryocytes and inherited thrombocytopenias

Inherited thrombocytopenias may be divided into two groups. In the first group, there is no marked thrombocytopathy. Although numerous in the bone marrow, megakaryocytes often are abnormal cytologically. A dysmegakaryocytopoiesis with defective platelet production is suggested but remains to be evidenced. In the second group, thrombocytopenias are accompanied with variable thrombocytopathy. The functional and biochemical platelet abnormalities responsible for these different thrombocytopathies often are well elucidated. The study of the relations with the occurrence of thrombocytopenia constitutes an interesting field of investigation.

Case report: familial thrombo-cytopenia in seven members of three generations

All members in one generation and two of six members of the next generation were found to have familial thrombocytopenia. The disorder was transmitted as an autosomal dominant trait. Easy bruising and a bleeding tendency were the common features. No member had splenomegaly. Platelet counts varied from 8 to 75 x 10-9/liter, and there were no apparent morphologic abnormalities. Platelet survival studies using sodium chromate Cr 51-labeled autologous platelets were done for three members and showed shortened survival times. Two members were treated with corticosteroids and splenectomy without benefit. Three members tolerated surgery (splenectomy, polypectomy) without undue difficulty. Defective or ineffective thrombocytopoiesis appeared to be the basis of this hereditary disorder.

Hereditary giant platelet syndrome. Absence of collagen-induced coagulant activity and deficiency of factor-XI binding to platelets

The platelets from two related patients with the hereditary giant platelet syndrome were examined. They were larger than normal but otherwise ultrastructurally normal; they contained increased storage pools of adenine nucleotides and heparin-neutralizing activity and took up serotonin at an increased rate. They aggregated normally with ADP and collagen but failed to aggregate with bovine factor VIII and Ristocetin. Some change in shape occurred with ADP, and the reduction in adenylate energy change after addition of ADP to platelet-rich plasma was smaller than normal. Platelet coagulant activities including contact product forming activity, intrinsic factor-Xa forming activity and platelet factor 3 activity were normal or increased, but collagen-induced coagulant activity was absent whether tested in washed platelet suspensions or platelet-rich plasma. Platelet washing experiments showed decreased binding of factors V and VIII to hereditary giant platelets and no detectable factor XI in washed platelet suspensions. It is concluded that (1) the hereditary giant platelets studied lacked a binding mechanism for factors, V, VIII and XI; (2) the normal development of collagen-induced coagulant activity apparently depends upon the binding of factor XI to the platelet membrane; and (3) the defective prothrombin consumption observed in these patients may have resulted from the failure of their platelets to bind factor XI.