Two’s company but three’s a blast!

In this issue of Blood, Romano et al discover that in murine bone marrow, both granulocyte and erythroid precursors develop around a single central macrophage, within a specialized niche termed erythromyeloblastic islands (EMBI).1 Although it is well established that mammalian erythropoiesis occurs in erythroblastic islands (EBI), comprising 5 to 30 maturing red blood cell precursors surrounding a macrophage,2 like an orchestra around a conductor, Romano et al identify that the "orchestra" constitutes granulocyte as well as erythroid precursors. Furthermore, they introduce a new concept in hematopoietic cell homeostasis: an inverse relationship between erythroid and granulocyte lineage cell numbers in EMBI that vary in response to microenvironmental cues. These findings provide insight into disorders such as anemia of inflammation/chronic disease.