Pre-HSC activity is associated with extraembryonic arteries, but not veins. Arteries and veins subdissected from UC and VV were cultured separately for 4 days with GFs (IL-3 + SCF + Flt3l) and transplanted into irradiated recipients (1 e.e. per mouse). Donor engraftment was assessed 12 to 14 weeks posttransplantation (n = 5 independent experiments).