Palld^−/−macrophage, but notPalld^−/−erythroblast, has an intrinsic defect in the formation of an erythroblastic island. (A) Reconstituted, homologous (wt macrophages/wt erythroblasts or mutant (mt) macrophages/mt erythroblasts), and heterologous (wt macrophages/mt erythroblasts or mt macrophages/wt erythroblasts) erythroblastic islands were immunostained with F4/80 (green) and TER119 (red) antibodies. The result shows that Palld^−/− fetal liver macrophage has an intrinsic defect in the formation of an erythroblastic island. Images were viewed with an Olympus BX51 microscope with an Olympus UPlanFl 40×/0.75 objective, captured with a SPOT RTKE cooled color CCD camera (Diagnostic Instruments), and imported into SPOT software (Diagnostic Instruments). FITC (F4/80⁺ cells) and PE (Ter119⁺ cells) fluorescence are shown. (B) Percentage of F4/80⁺ cells that bound erythroblasts in erythroblastic island reconstitution assay. (C) Erythroblasts bound per macrophage in erythroblastic island reconstitution assay. For each combination, 20 macrophages were analyzed. M indicates macrophage; E, erythroblast. Error bar represents plus and minus a SEM; **P < .01. Original magnification, × 400 (A).