The significance of endogenous erythroid colonies (EEC) in haematological disorders.

The myeloproliferative disorders are the result of an underlying abnormality of the pluripotential stem cell. One feature of this abnormality is a greatly increased sensitivity of the committed erythroid progenitors (BFU-E and CFU-E) to the hormone erythropoietin. Culture in vitro of these bone marrow or peripheral blood cells results in the growth of a proportion of colonies in the absence of added erythropoietin. These endogenous erythroid colonies (EEC) are seen in the great majority of cases of polycythaemia vera, as well as in some cases of thrombocythaemia, chronic myeloid leukaemia and idiopathic myelofibrosis. The presence of EEC appears to be a marker for the stem cell mutation and may serve to distinguish the neoplastic disorders from reactive increases of red cell mass or platelet numbers. Their absence in idiopathic erythrocytosis may also distinguish this condition from early polycythaemia vera and be useful in deciding on appropriate treatment. In patients with even a modest increase in the platelet count endogenous colonies provide firm evidence for a myeloproliferative disorder. Provision of myelosuppressive treatment can avert or improve vaso-occlusive or haemorrhagic complications. The mechanism of erythropoietin hypersensitivity is unknown but it has been shown to be a feature acquired rather late in maturation and by only a proportion of the progeny of the mutated clone. Normal erythroid progenitors co-exist with these abnormal cells in polycythaemia vera and the way in which their growth in vivo is inhibited has yet to be determined.