B-cell differentiation in the bone marrow and spleen revealed by pulse labeling with BrdU of the pro-B/pre-B population. (A) A contemporaneous and wavelike progression of BrdU-labeled B cells was observed in the bone marrow and spleen. (B) Representative flow cytometric histograms following BrdU labeling in splenic and bone marrow B-cell fractions at various time points. For the 2-day time point, n = 7 mice. All other time points include results from 4 mice. Error bars represent ± SEM. (C) BrdU⁺ NF/T1 B cells are readily observed in the peripheral blood (left panel) but minimally in lymph nodes (center) or the peritoneal cavity (right).