Pre-HSC potential is revealed in explant cultures of UC and VV. (A-B) Direct transplants. E11.5 UC or VV were separated on dissection and transplanted individually into irradiated recipients, and the level of donor-derived PBL chimerism (%) was determined 14 weeks posttransplantation. Only 5 out of 30 UCs transplanted into recipients and 5 out of 22 VV recipients were repopulated. (C-D) Transplantation of explant cultures. UC or VV explants were cultured for 4 days with GF (IL-3 + SCF + Flt3l) and transplanted separately into irradiated mice as follows: 1 UC explant per 2 recipients (8 explants in total) and 0.2 to 0.5 VV explants per 1 recipient. Note that after culture all UC and VV explants contained HSC activity. M, recipient mouse.