B lymphocyte development in fetal liver

Single cell cultures of fetal liver cells (FLC) from day 13 of gestation onwards until birth develop lipopolysaccharide (LPS)‐reactive B cells which can be stimulated by LPS to IgM‐secreting plaque‐forming cells (PFC). When the cell concentration of FLC is lowered from 4 × 10 6 cells/ml to 4 × 10 s cells/ml and below, the “in vitro” development to IgM‐secreting PFC is lost. Less and less FLC initiate growth to IgM‐secreting PFC as the cell concentration is lowered. Addition of 3 × 10 6 thymus cells/ml from either syngeneic mice or from xenogeneic Lewis rats dramatically increases the efficiency with which FLC develop into LPS‐reactive cells and initiate, after stimulation by LPS, clonal growth and maturation to IgM‐secreting PFC. Thus, day 13 FLC can be diluted to 1 × 10 6 cells/culture, day 16 FLC to 1 × 10 4 cells/culture and day 19 FLC to 3 × 10 1 cells/culture before the responding cells become limiting. The frequencies of precursor B cells developing to LPS reactivity and then to IgM‐secreting PFC in culture have been determined by limiting dilution analyses of FLC from day 13 to 19 (birth) of gestation and of liver cells at days 2, 4 and 7 after birth in cultures containing thymus cells and LPS. They are 1 precursor B cell in ∼ 1 × 10 6 FLC at day 13 of gestation, 1 in ∼ 1 × 10 s at day 14, 1 in ∼ 3 × 10 4 at day 15, 1 in ∼ 1 × 10 4 at day 16, 1 in ∼ 2 × 10 3 at day 17, 1 in ∼ 1 × 10 2 at day 18 and 1 in ∼ 3 × 10 1 at day 19 of gestation. After birth, these frequencies decline rapidly to 1 in ∼ 3 × 10 2 at day 2, 1 in 3 × 10 3 at day 4 and 1 in less than 10 s liver cells at day 7 after birth.