Epo rapidly increases cell-surface densities of PODXL expression in R5 and R4 bone marrow reticulocytes. (A) In Epo-injected wild-type mice, cell-surface levels (ie, densities) of PODXL expression were assayed among bone marrow–resident R5 and R4 reticulocytes. Values (relative fluorescent intensities) are means (± SE) for n = 3 independent mice. Arrows indicate up-modulation of PODXL expression by Epo within 0.5-1.5 days. Bottom panels illustrate representative flow cytometry profiles at days 0, 0.5, and 1.5. (B) R5 and R4 reticulocytes in peripheral blood also were assayed in Epo-treated mice, including total and PODXL⁺ reticulocytes populations. For R4 reticulocytes, note the rapid pulse of PODXL positivity at day 0.5 (top right panel, arrow). Error bars represent SEM for n = 3 mice.