Fetal liver hematopoietic progenitors experience hypoxia. (A) Hypoxyprobe staining of E14.5 FL preparations. LSK cells (red) display enhanced staining relative to Lin⁺ differentiated cells (blue). (B) Representative LSK-SLAM FACS plots from E14.5 FLs. (C) Quantifications showing that KO animals have fewer LT-HSCs compared with controls and a trend toward more LSK cells (n = 9 ctrl, n = 4 KO). (D) Methylcellulose colony-forming assays on E14.5 FL cells show enhanced colony-forming activity in KO mice. (E) Distribution of colony subtypes from colony-forming assay (n = 9 ctrl, n = 4 KO). (F) Annexin V staining of control and Arnt KO E14.5 FL preparations. (G-H) Enhanced reconstitution of irradiated recipient peripheral blood by KO FL progenitors (G) but diminished contribution to the LT-HSC compartment (H). (I) qRT-PCR analysis for metabolic and survival factors of FACS-sorted LT-HSCs from control and Arnt KO E14.5 FL. *P < .05, **P < .01.