Angioblasts are the precursors of endothelial cells.

At 6.5 dpc an extraembryonic population of dispersed TAL1 and Flk1 immunopositive cells (panels A and B, arrowheads) are detected in regions where vascular networks form. Extraembryonic hematopoietic stem cells are TAL1⁺/Flk1⁻—TAL1 (panel A) and Flk1 (panel B) immunofluorescence associated with the 6.5-dpc blood islands (arrows) differ in immunostaining intensity. The absence of TAL1 (panel A, bracket) and Flk1 (panel B, bracket) immunostaining in intraembryonic regions indicates that vasculogenesis in the embryo proper is not initiated prior to 6.5 dpc. Magnification bar = 100μm.