B lymphopoiesis in TSLP transgenic mice is arrested at an early stage of development. (A) The top row shows a comparison of the percentages of progenitor (CD19⁺CD43⁺IgM^–) and precursor (CD19⁺CD43^–IgM^–) B-cell subsets in the marrow of transgenic (TG) mice and their littermates (LM) killed at 7 weeks of age. The plots were generated by gating on viable IgM^– cells in the lymphocyte light-scatter region. The percentages for each subset are shown. The bottom row shows a comparison of percentages of IgM-bearing lymphocytes in the marrow of TG and LM mice. The histogram represents the events from the total CD19⁺ lymphocyte population. Analysis of 5 additional TG and 6 additional LM mice yielded similar results to those depicted. (B) Total numbers of B-lineage cells from TG and LM bone marrow. *TG mice have significantly fewer (P ≤ .002) pro-B (left; the y-axis break is 40 000-100 000), pre-B (center), and B lymphocytes (right) compared with the LM controls. The results are presented as the average number of B-lineage cells ± SEM (n = 6 and n = 7 for TG and LM mice, respectively).