Cellular composition of spleen and bone marrow in mice segregating the splenomegaly phenotype. Monocellular suspensions from spleen (A) and bone marrow (B) from C57BL/6J and BXH-2 parental controls as well as from (BALB/cJ × BXH-2)F2 mice showing presence (high SI) or absence (low SI) of splenomegaly were used. Cells were analyzed by flow cytometry for the expression of various cell surface markers, the most informative ones being Thy1.2 (T lymphocytes), Mac1 (macrophage/granulocyte), GR1 (granulocyte), Ter119 (erythroid lineage), and CD11c (dendritic cells), as indicated at the top of each panel. Cells were prepared and stained with either one marker or with 2 markers in combination using different fluorophors (fluorescein isothiocyanate [FITC], phycoerythrin [PE]) and as described in “Materials and methods.” The fraction (percentage) of singly or doubly positive cells found in each quadrant is indicated. BXH-2 mice and F2 animals with splenomegaly (high SI) show important increase in the proportion of Mac1/GR1 doubly positive cells in bone marrow and spleen. In these mice, Ter119⁺ cells are increased in the spleen (extramedullary erythropoiesis) and reduced in bone marrow when compared with C57BL/6J controls or with F2 mice lacking splenomegaly.