High frequency of B-progenitors in first- and second-trimester fetal BM (FBM). Frequency of fetal liver (FL) and FBM HSCs/MPPs (A) and LMPPs (B) from 6 to 21 pcw. Data are expressed as percentage of Lin⁻CD34⁺ (mean ± SEM, n = 59 for FL and n = 32-35 for FBM). Frequency of FL and FBM ELPs (C), PreProB-progenitors (D), and ProB-progenitors (E) from 6 to 21 pcw. Data are expressed as percentage of Lin2⁻CD34⁺ (mean ± SEM, n = 39 for FL and n = 32-38 for FBM). (F) Frequency of FL and FBM B cells (CD34^–CD19⁺) from 6 to 21 pcw. Data are expressed as percentage of CD34^– (mean ± SEM, n = 37 for FL; n = 38 for FBM). (G) Representative flow cytometric analysis of FBM Lin2⁻CD34⁺ cells showing the gating strategy used to identify CD10^–CD34⁺CD19⁺ PreProB-progenitors and CD10⁺CD34⁺CD19⁺ ProB-progenitors. (H) Frequency of FBM HSCs/MPPs, LMPPs, ELPs, PreProB-progenitors, and ProB-progenitors from 8 to 21 pcw. Data are expressed as percentage of Lin2⁻CD34⁺ cells for B-progenitors and Lin⁻CD34⁺ cells for ELPs, HSCs/MPPs/LMPPs (mean ± SEM; n = 32-38). (I) Frequency of second-trimester (12-21 weeks) FL and FBM HSPC populations expressed as percentage of Lin⁻/Lin2⁻CD34⁺ cells (mean + SEM, n = 17-49 for FL and n = 16-27 for FBM). (J) Matched FL and FBM from the same biological samples, showing mean frequency of PreProB-progenitors and ProB-progenitors expressed as percentage of Lin2⁻CD34⁺ cells. Individual samples represented as circles for FL and squares for FBM. Lin: CD2, CD3, CD14, CD16, CD56, CD19, and CD235a. Lin2: CD2, CD3, CD14, CD16, CD56, and CD235a. All graphs show mean ± SEM where n ≥ 3. Statistical significance determined according to 2-way analysis of variance. *P < .05; ** P < .01; ***P < .001; ****P < .0001. SSC, side scatter.