In vivo Epo dose–dependent expression of PODXL by immature reticulocytes. (A) Epo dose–dependent increases in reticulocytes, PODXL⁺ immature reticulocytes, and PODXL expression levels in wild-type mice. At day 3 after Epo injection (0, 600, 1800 U/kg), increases in peripheral blood reticulocytes and in PODXL⁺ immature reticulocytes (immature reticulocyte fraction; IRF) were assayed by flow cytometry (left and center panels). Average PODXL expression levels within the IRF also were assayed based on fluorescence intensity (right panel). (B) PODXL staining intensities specifically within the IRF compartment and among stage mature R4 and immature R5 reticulocytes are illustrated. Here, wild-type mice were injected with Epo (1200 U/kg). For peripheral blood sampled at day 3, frequencies of PODXL⁺ cells within reticulocyte compartments were then determined. (C) Epo dose–dependent increases in PODXL⁺ expression levels (cell surface densities) by R5-stage reticulocytes also are illustrated.