Megakaryopoiesis in chronic myeloproliferative disorders: immunohistochemical evaluation of endoreduplicative activity by PCNA-staining reaction.

A morphometric analysis has been performed on bone marrow trephine biopsies following sequential double-immunostaining with monoclonal antibodies PC10 (anti-proliferating cell nuclear antigen--PCNA) and Y2/51-CD61 (anti-platelet glycoprotein IIIa) to evaluate endoreduplicative activity of megakaryopoiesis. In addition to a control group, patients included different subtypes of chronic myeloproliferative disorders (CMPDs) like chronic myeloid leukaemia (CML), polycythaemia vera (P. vera), primary thrombocythaemia (PTH) and finally primary (idiopathic) osteomyelofibrosis (OMF). In comparison with the normal bone marrow and also with P. vera and PTH a significant increase in PCNA-labelling (late G1 and S phases) of megakaryocytes was recognizable in OMF, contrasting with a striking reduction of this marker in CML. Particularly in advanced stages of OMF, secondary folate deficiency leading to a megaloblastoid appearance of erythroid precursors is a frequent finding. In pernicious anaemia previous cytokinetic studies have demonstrated an arrest in the S phase (DNA synthesis) of the cell cycle due to vitamin B12/folate (haematinic) deficiency. A similar pathomechanism may also be effective in OMF. Consequently, a block in the S phase of the cell cycle is assumed which is in keeping with the increased numbers of PC10-positive megakaryocytes. Significant correlations were calculable between megakaryocyte sizes and PCNA-staining capacity in the normal bone marrow and CMPDs. According to morphometry small-sized (hypoploid) megakaryocytes showed a prevalence of PCNA labelling. This finding is confirmative with a hypothesis on the dynamics of endoreduplicative activity of megakaryocytes, i.e. the prolongation of G1/G2 phases in larger (polyploid) elements. On the other hand, some of the giant polyploid megakaryocytes may cease endoreduplication and enter into G0 phase, which could partially explain the predominance of PCNA-negative large-sized cells of this lineage.