DIFFERENTIATION AND CHARACTERIZATION OF B-CELL PRECURSORS DETECTED INTHE YOLK-SAC AND EMBRYO BODY OF EMBRYOS BEGINNING AT THE 10-SOMITE TO12-SOMITE STAGE

The embryonic sites in which progenitors of the hematopoietic lineages first emerge are ideal regions to characterize both the cells and env ironment needed to initiate blood cell development. For a number of ye ars both the murine yolk sac and embryo have been recognized to contai n progenitors of B lymphocytes. However, clonal, quantitative in vitro assays, which allow precise observation of precursors and their proge ny, have been lacking. Moreover, the site of origin of the initial eve nts remains controversial. In this report we document the presence of B-cell progenitors in yolk sac and embryonic tissue obtained from mous e fetuses beginning at the 10-somite stage, day 8.5. We determine the frequency, cell-surface phenotype, and growth properties of these prog enitors. We show that these cells can differentiate into immunoglobuli n-secreting cells and that the progeny derived from single progenitors are diverse with respect to immunoglobulin heavy-chain allotype expre ssion, diversity-joining region use, and heavy-chain variable-region u tilization.