Expansion of HSC and progenitor numbers in the presence of GFs. (A) In the absence of GFs, HSCs are only maintained in UC explants but show 56-fold increase with GFs (IL-3 + SCF + Flt3L). Addition of IL-3 alone was found to be sufficient to support HSC expansion; 0.05 to 1 e.e. was injected per recipient, as indicated, and engraftment was assessed 12 to 14 weeks posttransplantation in PBL. (B) VV or YSs were cultured for 4 days in the presence of the 3 GFs, with IL-3 only, or without GFs and injected into irradiated recipients (at 0.5 VV per mouse and 1 YS per mouse). Engraftment was assessed 12 to 14 weeks posttransplantation in PBL (n = 5 independent experiments). (C) CFU-C progenitor numbers increase after 4 days of explant culture in the presence of 3 GFs (n = 2-5 independent experiments). Values refer to fold increase in CFU-C in explants compared with their uncultured counterparts. (D) Levels of IL-3, SCF, and Flt3l expression were assessed by qRT-PCR relative to expression of the TBP housekeeping gene (n = 3-8 independent RNA preparations, representing up to 10 pooled tissues each).