Gastrulating embryos contain Sox7-responsive hematopoietic precursors. (A) Cells from E7.5 embryos were tested in clonogenic replating assay for hematopoietic progenitors with or without doxycycline (+dox, −dox). For each embryo, 1/10 of the cells was used for genotyping and the remaining divided equally in −dox and +dox replating conditions. Genotype for rtTA and iSox7 transgenes is shown below the graph. (B) Picture of a blastic colony from replating with doxycycline (×20 magnification). (C) Flow cytometric analysis of pooled colonies grown for 6 days in secondary replating with doxycycline. (D) 5 individual iSox7⁺ colonies from primary replating of 2 different embryos were replated in secondary clonogenic assay with doxycycline. (E) After May-Grünwald Giemsa staining, erythrocyte and myeloid cells were scored from individual iSox7⁺ colonies grown for 5 days in liquid hematopoietic conditions without doxycycline. A total of 100 cells were counted per colony. (F) Cells from individual embryos were tested in clonogenic replating assay for hematopoietic progenitors with or without doxycycline. For each embryo, 1/10 of the cells was used for genotyping and the remaining equally divided in −dox and +dox replating conditions. HF indicates headfold. The average numbers for each colony type are presented below the graphs; (n) represents the number of embryo analyzed at each stage of development. For blastic colonies, only embryos positive for rtTA and isox7 transgenes are depicted. All data are representative of at least 3 independent experiments.