Definitive hematopoietic commitment within the embryonic vascular endothelial-cadherin(+) population.

Objective: The aim of this study was to assess the potential of FLK1(+) and vascular endothelial (VE)-cadherin(+) populations from different stages of embryonic development to generate hematopoietic cells ex vivo and to contribute to the hematopoietic systems of recipient mice.

Methods: FLK1(+) of VE-cadherin(+) cells were isolated from 7.5- to 9.5-dpc concepti and cultured ex vivo on OP9 stromal cells and hematopoietic development examined. VE-cadherin(+)CD45(-) cells from 8.5- and 9.5-dpc concepti were injected intrahepatically into newborn busulfan-treated SCID recipients and engraftment monitored.

Results: VE-cadherin(+) cells from 7.5- and 8.5-dpc concepti can readily generate hematopoi-etic cells ex vivo compared to FLK1(+) VE-cadherin(-) cells. Similar hematopoietic potential can be found in the VE-cadherin(+) cells from the 8.5-dpc yolk sac. When VE-cadherin(+)CD45(-) cells were injected into SCID recipients, long-term engraftment, particularly within the lymphoid system, was observed. This potential was observed in VE-cadherin(+)CD45(-) cells from 9.5-dpc embryo or yolk sac but from tissues from younger concepti.

Conclusions: FLK1(+)VE-cadherin(-) cells, possibly representing the lateral plate mesoderm, are not as effective at generating hematopoietic cells compared to similarly staged VE-cadherin(+) cells. VE-cadherin(+)CD45(-) cells can also contribute to the hematolymphoid system of intrahepatically injected newborn SCID recipients, suggesting that cells bearing an endothelial phenotype are capable of generating long-term hematopoietic precursors.