LYMPHOID POTENTIAL, PROBED BEFORE CIRCULATION IN MOUSE, IS RESTRICTEDTO CAUDAL INTRAEMBRYONIC SPLANCHNOPLEURA

Emergence of hemopoietic stem cells in the mammalian embryo has yet to be definitively allocated. Previously, we detected multipotent hemopo ietic precursors in the region surrounding the dorsal aorta (paraaorti c splanchnopleura) beginning at 8.5 days postcoitum (dpc). However, as circulation is already established, it remained unclear whether hemop oietic precursors arise in situ or are blood-delivered. By adding an o rganotypic step to our former culture system, we now detect lymphocyte and multipotent myeloid precursors from the intraembryonic splanchnop leura as early as 7.5 dpc. Under identical conditions, yolk sacs from the same embryos are unable to generate lymphoid progeny and have a re duced potential for myeloid differentiation and maintenance. Thus, if isolated before circulation, the yolk sac does not produce multipotent precursors and therefore does not contribute to definitive hemopoiesi s in the mouse.