Histology of representative leukemias arising in MSCV-FL mice. Hematoxylin and eosin-stained sections are shown for mouse 196 with B-lymphoid leukemia (a through d) and for mouse 199 with biphenotypic leukemia (e through h). (a) The bone marrow architecture of mouse 196 is effaced by a blast cell infiltrate that extends into the adjacent muscle (original magnification × 185). (b) At higher magnification, blast cells in the bone marrow of mouse 196 can be seen to be predominantly medium-sized with round to oval nuclei, moderately basophilic cytoplasm, and occasionally with prominent nucleoli; mitotic figures are abundant (original magnification × 1,115). (c) Lung of mouse 196 showing diffuse interstitial and nodular peribronchial parenchymal infiltrates (original magnification × 270). (d) Liver of mouse 196 showing perivascular accumulations of blast cells as well as some blast cells around bile ducts (original magnification × 370). (e) Bone marrow of mouse 199 showing blast cells of variable size occupying the hematopoietic space; scattered erythroblasts and megakaryocytes are present (original magnification × 185). (f) At higher magnification, blast cells in the bone marow of mouse 199 of variable shape with irregular nuclei and one to three distinct nucleoli can be seen to be intermixed with granulocytic cells of various maturational stages (original magnification × 1,115). (g) Lung of mouse 199 showing diffuse interstitial infiltrates of blast cells and granulocytes (original magnification × 270). (h) Liver of mouse 199 showing polymorphous tumor infiltrates, sometimes with pronounced granulocytic differentiation, mainly in periportal areas (original magnification × 370).