Murine embryonic blood between day 10 and 13 of gestation as a source of immature precursor B cells.

Pre-B cells which show a delayed development of responsiveness to the B cell mitogen lipopolysaccharide (LPS) are found in murine embryonic blood between day 10 and 14 of gestation. The highest frequency of pre-B cells in embryonic blood is found at day 12 of gestation. All pre-B cells of embryonic blood, isolated at different days of gestation, develop responsiveness in vitro to LPS at the same time, i.e. around day 14 of gestation, yielding a peak of LPS-induced IgM plaque-forming cells at the equivalent of day 19 of gestation. Dextran sulfate supports the development of pre-B cells into LPS-reactive B cells; by itself, however, it does not stimulate these B cells into IgM plaque-forming cells. The wave of pre-B cells in embryonic blood occurs parallel in time to that found previously in placenta. Fetal liver and yolk sac, at this time of embryonic development, do not contain significant numbers of pre-B cells, although they are connected with the placenta and in contact with embryonic blood by circulation.