Human megakaryocytes: finding the root

16 Aug 2017

In this issue of Blood, Miyawaki et al identify the most primitive progenitor cell population that makes only megakaryocytes and platelets in adult humans and show it is expanded in myeloproliferative neoplasms such as essential thrombocythemia (ET).1 Approximately 1011 platelets are produced on a daily basis in humans, but their exact journey from undifferentiated hematopoietic stem cells (HSCs) is still highly debated. Platelets have the shortest half-life of all blood components and are rapidly recruited when injury occurs, yet have long been thought to be among the cell types to be specified as the furthest from the HSCs in the hematopoietic hierarchy. For several decades, it was understood that differentiation proceeds by a series of binary fates choices, in particular with a common myeloid progenitor (CMP) downstream of HSCs that would give rise to a restricted myeloid progenitor (granulocyte-macrophage progenitor) and to a megakaryocyte-erythrocyte progenitor (MEP). Only downstream of MEPs would unilineage megakaryocyte and unilineage erythrocyte progenitors arise. Recently though, several groups have reported that megakaryocyte and platelet production may not follow this strict hierarchical branching path. Instead, committed megakaryocyte precursors could be found much earlier, either within the HSC2-4 or the multipotent progenitor compartment.5 An early precursor that exclusively produces human megakaryocytes in humans, however, had not been described.