Gross appearance and histology of spleen from affected mice. A portion of the spleen from affected animals was fixed in 10% phosphate-buffered formalin, and histologic examination was performed. (A) Gross appearance of a typical animal including focal tumors in the skin and pleura/peritoneum (arrows), dilated blood vessels (arrowheads), and subcutaneous hematomas (chevrons). (B) Typical spleen from an affected animal. (C) Hematoxylin and eosin stain of spleen from line 6468. (D) Higher magnification of panel C showing infiltration of granulocytes (arrows) in the spleen. (E) Giemsa stain of spleen in panel C showing diffuse and focal infiltrates of cells containing basophilic granules consistent with mast cells. (F) Mast cell tryptase immunohistochemistry of the spleen from an affected animal. Original magnifications: (C) × 4, (D) × 40, (E) × 10, (F) × 40.