p57Kip2 deletion expands HSC numbers in the AGM and the early FL. Single-cell preparations of E11 AGM (0.5-1 embryo equivalent [ee]) (A), E12 AGM (1ee) (B), E11 FL (1ee) (C), E12 FL (0.05ee) (D), E11 placenta (PL, 2ee) (E), E12 PL (1ee) (F), E11 yolk sac (YS, 2ee) (G), and E12 YS (1ee) (H) from wild-type (WT) and p57Kip2-null (knockout [KO]) embryos were transplanted into irradiated recipients and donor chimerism determined after 4 months by flow cytometry. Data points represent individual recipients with the number of repopulated (>5% chimerism as indicated by the dashed line) recipients/total recipients indicated underneath each graph together with the percentage of repopulated mice. Solid line represents mean. (I) In situ hybridization with a p57Kip2 RNA probe on transverse cryosections of E11 placenta showing high expression of p57Kip2 in trophoblast cells of the placental labyrinth. (J) Percentage of CD45⁺ hematopoietic cells from E11 p57Kip2 WT and KO embryos in the different cell cycle stages. 4′,6-Diamidino-2-phenylindole (DAPI)–stained CD45⁺ E11 AGM cells were analyzed by flow cytometry. *P < .05, **P < .01, Mann-Whitney U test.