The +23GFP reporter mouse line marks emerging HSCs and progenitor cells. (A) Schematic of the construct used to generate transgenic mice. (B) Whole-mount images of E8.5 (panels i and ii) and E10.5 (panels iii and iv) +23GFP transgenic embryos. Panels i and iii: bright field images. Panel ii shows GFP expression in the YS blood islands (arrowhead) of the embryo shown in panel i. Panel iv shows GFP expression in the YS, vitelline artery (VA), and FL of the embryo shown in panel iii. Expression in the dorsal aorta is masked by the ectopic expression in somites (S). (C) Flow cytometric analysis of GFP expression in cell suspensions of E11 AVU and E11 FL. Plots are representative of 3 independent experiments and mean percentage of positive cells (± SD) is indicated. AVU and FL cells were sorted on the basis of their GFP expression (sort gates as indicated; purities ranged from 92% to 99%) and assayed for the presence of hematopoietic stem and progenitor cells. (D) CFU-C assays were performed in methocult supplemented with appropriate growth factors (M3434). In both the AVU and FL, the large majority of CFU-C was found to reside in the GFP⁺ cell population. To examine HSC activity, cells were transplanted into adult irradiated recipients. PB was analyzed by genomic PCR for GFP at 8 weeks after transfer. *Data show the number of reconstituted mice of the number that received a transplant (Tx). n.d. indicates not done.