PreProB-progenitors are enriched in fetal life, and B-progenitor expansion is favored over differentiation in a narrow developmental time window. (A) Frequency of ELPs, PreProB-progenitors, and ProB-progenitors in the Lin2⁻CD34⁺ compartment of second-trimester fetal BM (n = 29-32, 12-21 pcw) and adult BM (n = 6). Representative flow plots for PreProB-progenitors and ProB-progenitors are shown on the right. Statistical significance determined by unpaired Student t test: *P < .05; ***P < .001; ****P < .0001. (B) Cell cycle analysis by flow cytometry: representative flow plots of fetal BM HSPC populations showing the proportion of cells in G0, G1, and S-G2-M phases. (C) Percentage of cells in S-G2-M phase in each fetal BM HSPC population. Data represented as mean + SEM (n = 3). Statistical significance determined by paired Student t test. (D) Changes in frequency of fetal BM PreProB-progenitors, ProB-progenitors, and B cells from 8 to 21 pcw. B cells are shown as percentage of CD34^– cells and B-progenitors as percentage of Lin2^–CD34⁺ (n = 32-38). Shaded area highlights the developmental time window when B-progenitor expansion occurs before B-cell differentiation. Lin2 antibody cocktail: CD2, CD3, CD14, CD16, CD56, and CD235a.