Decreased number of fetal liver macrophages in Mekk1^ΔKD embryos. (A) Fetal liver cell suspensions from wt and Mekk1^ΔKD E13.5 embryos were stained with macrophage-specific anti-F4/80 antibodies and analyzed by flow cytometry. The dot plots represent the log fluorescence intensities of live cells. Numbers indicate the percentage of F4/80-positive cells. (B) Immunohistochemical staining of E13.5 fetal liver sections from wt and Mekk1^ΔKD embryos with PE-conjugated anti-F4/80. (C) Immunohistochemistry of “native” erythroblast-FLM clusters from wt and Mekk1^ΔKD E13.5 embryos. Cell clusters were stained for F4/80 (red) and Ter119 (green). (D) E13.5 FLMs were isolated from Jnk2^–/–, Mekk1^ΔKD/+Jnk2^–/–, and Mekk1^ΔKDJnk2^–/– embryos and offered an excess of latex beads in an in vitro phagocytosis assay. Macrophages were counterstained with hematoxylin and the number of phagocytosed beads per cell was counted by phase-contrast microscopy. A minimum of 3 FLs was examined per genotype.