Bone marrow histopathology and biological markers as specific clues to the differential diagnosis of essential thrombocythemia, polycythemia vera and prefibrotic or fibrotic agnogenic myeloid metaplasia.

Clinical, hematological and morphological peripheral blood and bone marrow characteristics, in particular, megakaryopoiesis and bone marrow cellularity, reveal diagnostic clues and pathognomonic features, which enable a clear-cut distinction between essential thrombocythemia (ET), polycythemia vera (PV) and prefibrotic and fibrotic agnogenic myeloid metaplasia (AMM). The characteristic increase of enlarged mature megakaryocytes with mature cytoplasm and multilobulated nuclei and their tendency to cluster in a normal or slightly increased cellular bone marrow represent the hallmark of ET. The characteristic increase and clustering of enlarged mature and pleiomorphic megakaryocytes with multilobulated nuclei and proliferation of erythropoiesis in a moderate to marked hypercellular bone marrow with hyperplasia of dilated sinuses are the specific diagnostic features of untreated PV. ET may precede PV for many years to more than one decade. Prefibrotic and fibrotic AMM appears to be a distinct dual proliferation of abnormal megakaryopoiesis and myelopoiesis. The histopathology of the bone marrow in prefibrotic and fibrotic AMM is dominated by atypical enlarged and immature megakaryocytes with cloud-like immature nuclei, which are not seen in ET and PV at diagnosis and during follow-up. Myelofibrosis is not a feature of ET at diagnosis and during long-term follow-up. Myelofibrosis, which is secondary to the megakaryocytic/granulocytic myeloproliferation, and extramedullary myeloid metaplasia constitute a prominent feature and usually progress more or less rapidly during the natural history of PV and AMM. Life expectancy is normal in ET, normal in the first and decreased in the second decade of follow-up in PV, but significantly shortened in thrombocythemia associated with prefibrotic AMM as well as in the various fibrotic stages of AMM. These clinical and pathological characteristics of the Ph-negative MPDs, by including bone marrow histopathology, enable a clear-cut distinction between ET, PV and prefibrotic and fibrotic AMM. The use of established and new biological markers of MPDs, like spontaneous EEC, PRV-1 gene expression etc, should be validated in large prospective multicenter studies of newly diagnosed and previously treated MPD patients using the proposed European clinical and pathological (ECP) criteria as the only gold standard available for the proper diagnosis and differential diagnosis of ET, PV and AMM.