[Megakaryocytic myelosis--cause of "idiopathic" thrombocythemia]

Primary thrombocythaemia is to be distinguished from the secondary type by higher counts of megakaryocytes especially of atypic and gigantic forms of these cells, showing up in adequate histological preparations of bone marrow biopsies. From the analysis of those preparations the autonomous proliferation of the megakaryocytopoiesis clearly is to be understood as the reason of the socalled primary thrombocythaemia, occurring in the forms of isolated or mixtcellular megakaryocytic myelosis of the well differentiated type. Both of these forms can convert into thrombocytopenia when atypical and immature megakaryocytes start to overcrowd the picture or when myelofibrosis develops. The latter is to be considered in causal connections with the ineffective dislocated thrombopoiesis, a common finding among megakary-ocytic myelosis complicated with myelofibrosis. Megakaryocytic myelosis therefore is the clue of three different clinical syndromes, which are alternatively marked by the haemostaseologic or the histologic consequences of the overproduction of platelets, or by the proliferation of the megakaryocytes themselves.