Splenic erythroid progenitor and precursor recovery post–4 Gy TBI. (A) Erythroid progenitor kinetics in the spleen post–4 Gy TBI. Splenic erythroid progenitor recovery does not begin until 7 to 8 days after radiation and peaks at 9 days after TBI. (B) Splenic erythroid precursor kinetics post–4 Gy TBI. Erythroid precursor recovery begins by 8 to 9 days in spleen and remains at high levels, especially in later precursors, at 13 days after TBI. Erythroid progenitors and precursors are expressed as the total number of each cell type per spleen. Error bars represent SEM of at least 3 experiments, and 3 or more independently assayed mice were used to determine each data point. (C) Representative sections of unirradiated spleen and spleen at 6 and 10 days post–4 Gy TBI (H&E staining; 50-micron bars). The low level of steady-state erythropoiesis in spleen is rapidly lost and remains absent at 6 days after radiation; robust erythropoiesis occurs exclusively in the red pulp by 10 days post–4 Gy TBI. White arrows represent areas of erythroid activity in the spleen; rp indicates red pulp; and wp, white pulp. Images were captured with a Nikon Digital Sight Ds-Fi1 camera using Nikon NIS-Elements software on a Nikon Eclipse 80i upright microscope using a 10× objective.