EMBI in mouse bone marrow are composed of a central macrophage or “nurse” cell surrounded by a rosette of maturing granulocyte and erythroid precursors. At steady state, there are similar numbers of these 2 lineages. In mice treated with granulocyte colony-stimulating factor or in models of inflammatory disease, there is a shift toward granulocyte precursors, leading to anemia. Conversely, in mice treated with Epo, erythroid cells are increased relative to their granulocyte counterparts within EMBI. Epo also promotes an absolute increase in EMBI numbers. Professional illustration by Somersault18:24.