Early hematopoietic inactivation of Srf causes perinatal lethality and lack of bone marrow cellularity. (A) Embryos or animals were genotyped at the indicated stages and proportion of SrfKO^H (vav-iCre; Srf^f/f) scored; n = total embryos/animals genotyped. (B) Hemorrhage into skin, bladder, or eye in newborn SrfKO^H compared with Srf⁺ (Srf^f/f, Srf^f/+) animals. (C) P1 femurs stained with hematoxylin and eosin (H&E) (left) or endomucin (right). (D) Colony-formation assays with cells from the P1 femur of SrfKO^H animals. Data are from 3 Srf⁺ and 3 SrfKO^H animals, each assay was performed in triplicate. See supplemental Figure 1A. (E) Left, reduced colony-formation activity in hind-limb long bones of E18.5 SrfKO^H animals. Data are from 5 Srf⁺ and 3 SrfKO^H embryos; each assay was performed in triplicate. Right, Gr-1⁺ cellularity in E18.5 long bones. Data are from 6 embryos of each genotype (P < .0001; unpaired Student t test).