Appearance of functional lymphocytes in fetal liver.

During ontogeny of the mouse, cells bearing surface Ig first appear at approximately 16 days of gestation. To characterize B cell differentiation during embryogenesis further, we have measured several functional activities of B cells and their precursors. B cell function was measured in two assays, response to sheep red blood cells in a transplantation assay, and response in vitro to a B cell mitogen. By both assays, B function is first detectable at 16 days, the same time that Ig-positive cells appear. The immediate precursors of B cells, PB cells,3 can also be measured by a transplantation assay. This activity is first detectable in fetal liver at 13 days. The yolk sac at 13 days does not contain significant PB activity even though both fetal liver and yolk sac contain pluripotent stem cells, spleen colony-forming units, at that time. Tese data are consistent with the hypothesis that the development of B lymphocytes occurrs early in ontogeny and that the fetal liver may provide the inductive stimulus for B cell maturation.