IN-VITRO DIFFERENTIATION OF B-CELLS AND MYELOID CELLS FROM THE EARLY MOUSE EMBRYO AND ITS EXTRAEMBRYONIC YOLK-SAC

The yolk sac is the first site of hematopoiesis during ontogeny. Howev er, the source of early embryonic hematopoietic stem cells remains unr esolved. Early studies have shown that cells obtained from day-8 and - 9 extraembryonic yolk sacs can give rise to T cells and myeloid cells, whereas the embryo itself appears to lack such cells. Controversy rem ains as to whether it is the embryo itself or the extraembryonic yolk sac that contains the initial precursors capable of differentiating in to B cells. This study used the approach of enriching hematopoietic st em cells by immunocytoadherence and studying cells isolated from withi n the embryo itself or from the yolk sac obtained at days 8 and 9 of m ouse embryonic development. We report that on day 9, both yolk sac-der ived and embryo-derived cells can give rise to B cells and myeloid cel ls in vitro. On day 8, however, cells isolated from the yolk sac but n ot from the embryo produce myeloid colonies in vitro; neither source o f stem cells generates B cells. Our study suggests that myeloid precur sors migrate from yolk sac to embryo earlier than has previously been reported but that the origin for B cell. precursors remains to be dete rmined.