Schematic representation of the traditional and current models of hematopoiesis and developmental changes in progenitor composition. (A) In the traditional model of hematopoiesis, the initial separation segregated myelopoiesis and lymphopoiesis, and MKs emerged exclusively from MEPs. (B) Current literature supports a model in which the initial step separates MK-E/myeloid and lymphomyeloid progenitors, and MKs arise directly from the HSC compartment, as well as from classically defined MEPs (adapted from Psaila and Mead²⁸). (C) There is a gradual decrease in the ratio of multipotent to unipotent progenitors in the course of human development. FL CD34⁺ cells give rise to multipotent, oligopotent, and unipotent progenitors, whereas adult BM CD34⁺ cells primarily generate multipotent and unipotent progenitors, with a striking paucity of oligopotent progenitors. CLP, common lymphoid progenitor; CMP, common myeloid progenitor; EoMP, eosinophil-basophil-mast cell progenitor; Ery, erythroid cells; GMP, granulocyte-monocyte progenitor; Gran, granulocytes; LMPP, lymphomyeloid progenitors; Ly, lymphocytes; Mono, monocytes; MPP, multipotent progenitor; MyeP, myeloid progenitor. Professional illustration by Somersault 18:24.