Colony-forming activity of fetal hematopoietic progenitors. (A) Western blot of cell extracts from the stromal cell lines PA6 (lefthand lane) and S17 (righthand lane), separated on a 6% SDS gel under reducing conditions, transferred, and probed with the same anti–Jagged 1 antibody used in Fig 3. Note that while PA6 expresses Jagged 1, S17 cells do not. (B) Western blot of cell extracts from three independent pools (lanes 1 through 3) of S17 cells stably transfected with a human Jagged 1–containing expression vector. (C and D) Colony production by fetal CD34⁺, c-kit⁺ hematopoietic progenitor cells from d11 AGM (C) or fetal liver (D) after culture for 1 week on irradiated wild-type or Jagged 1–transfected S17 cells. CD34⁺, c-kit⁺ cells from AGM and fetal liver were sorted and cultured on irradiated S17 cells or pool 1 of the Jagged 1–transfected S17 cells (S17 Jagged). The numbers of cells generated after 1 week of stromal culture were as follows: AGM on S17 = 10⁴; AGM S17 Jagged = 10⁴; fetal liver on S17 = 6 × 10³; fetal liver on S17 Jagged = 4 × 10⁴. All cells derived from each stromal culture were passaged into methylcellulose suspension cultures as described in the text. Colonies of over 50 cells were counted after 1, 2, 3, and 4 weeks in culture. The mean number of colonies formed per dish from triplicate dishes at each time point is shown; error bars show the standard deviation.