The aorta-gonad-mesonephros (AGM) region is a potent hematopoietic site in
the midgestation mouse conceptus and first contains colony-forming
units-spleen day 11 (CFU-S(11)) at embryonic day 10 (E10). Because
CFU-S(11) activity is present in the AGM region before the onset of
hematopoietic stem cell (HSC) activity, CFU-S(11) activity in the complex
developing vascular and urogenital regions of the AGM was localized. From
E10 onward, CFU-S(11) activity is associated with the aortic vasculature,
and is found also in the urogenital ridges (UGRs). Together with data
obtained from organ explant cultures, in which up to a 16-fold increase in
CFU-S(11) activity was observed, it was determined that CFU-S(11) can be
increased autonomously both in vascular sites and in UGRs. Furthermore,
CFU-S(11) activity is present in vitelline and umbilical vessels. This,
together with the presence of CFU-S(11) in the UGRs 2 days before HSC
activity, suggests both temporally and spatially distinct emergent sources
of CFU-S(11). (Blood. 2000;96:2902-2904