In response to stress: quicker, yet bigger

In this issue of Blood Advances, Winward et al 1 determined that (1) maturing erythroid progenitors normally undergo a progressive shortening of the cell cycle and increased cell size and (2) these characteristics are accelerated in response to stress (ie, exposure to erythropoietin or hypoxia).More than 8 out of 10 cells in the human body are red blood cells. 2 Given their limited life span, maintaining these prodigious cell numbers requires an expansive and responsive system of blood production.Erythropoiesis is comprised of 3 progressive compartments (see figure).The first is comprised of lineage-committed progenitors, termed burst-forming unit erythroid (BFU-E) and colonyforming unit erythroid (CFU-E) based on their in vitro ability to generate large and small colonies, respectively, of erythroid cells.The second compartment is a series of morphologically identifiable erythroblast precursors that execute a program of terminal maturation that includes accumulation of hemoglobin and loss of internal organelles.The final compartment is comprised of reticulocytes, derived from the enucleation of late-stage erythroblasts, that ultimately complete their maturational program in the bloodstream to become mature red blood cells.Cells within all 3 compartments have in recent years been defined by their surface immunophenotype, which facilitates their quantitative analysis using flow cytometry-based approaches. 3,4