EMERGENCE OF MULTIPOTENT HEMATOPOIETIC-CELLS IN THE YOLK-SAC AND PARAAORTIC SPLANCHNOPLEURA IN MOUSE EMBRYOS, BEGINNING AT 8.5 DAYS POSTCOITUS

We show by an in vitro approach that multipotent hemopoietic cells can be detected in the body of the mouse embryo between the stages of 10- 25 somites (8.5-9.5 days of gestation)-i.e., prior to liver colonizati on (28-32 pairs of somites). Interestingly, hemopoietic cells appear i n parallel in this location, the paraaortic splanchnopleura, and in th e yolk sac, where they represent a new generation by reference to the primitive hemopoietic stem cells. Lymphoid cell clones, which could di fferentiate into mature B cells, were obtained from yolk sac and paraa ortic splanchnopleura cell preparations but not from other tissues of the embryonic body. These B-cell precursors were first detected around the stage of 10 somites; thereafter, their initial minute numbers inc reased in parallel in the yolk sac and the paraaortic splanchnopleura, suggesting that their emergence in the two sites was simultaneous. By single cell manipulation, we show that these precursors can generate B and T lymphocytes and myeloid cells; these precursors can thus be de fined as multipotent hemopoietic cells.